{"id":20425,"date":"2026-10-07T15:21:34","date_gmt":"2026-10-07T08:21:34","guid":{"rendered":"https:\/\/www.vietstock.org\/?p=20425"},"modified":"2026-10-05T15:36:06","modified_gmt":"2026-10-05T08:36:06","slug":"avian-leukosis-poultry","status":"publish","type":"post","link":"https:\/\/www.vietstock.org\/en\/industry-news\/avian-leukosis-poultry\/","title":{"rendered":"Avian Leukosis in Poultry: Signs, Transmission, and Prevention"},"content":{"rendered":"<p><span style=\"font-weight: 400;\">Avian leukosis is a group of neoplastic diseases caused by <\/span><b>Avian Leukosis Virus (ALV)<\/b><span style=\"font-weight: 400;\">. Chickens are the natural host and the species of greatest importance in commercial poultry production.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Unlike some acute infectious diseases, avian leukosis does not usually cause sudden mass mortality over a short period. Its impact often develops more gradually through cumulative mortality, reduced weight gain, lower egg production, and increased culling rates.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">One of the most important epidemiological characteristics of ALV is its ability to spread <\/span><b>vertically from infected hens to their offspring through the egg<\/b><span style=\"font-weight: 400;\">. Disease control must therefore focus primarily on <\/span><b>breeder-flock and chick-source health status<\/b><span style=\"font-weight: 400;\"> rather than only removing birds that have already developed tumours.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">There is currently no specific treatment or widely used effective vaccine for controlling avian leukosis.<\/span><\/p>\n<h2><b>4 key points to remember about avian leukosis<\/b><\/h2>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>The disease often develops gradually:<\/b><span style=\"font-weight: 400;\"> Avian leukosis does not usually cause sudden mass mortality. Instead, mortality may rise progressively, while birds become weak, show poorer weight gain, reduced egg production, and higher culling rates. Lymphoid leukosis is generally more apparent in chickens from approximately 14\u201316 weeks of age onward.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Breeding stock is a critical control point:<\/b><span style=\"font-weight: 400;\"> ALV can be <\/span><b>transmitted from infected hens to offspring through the egg<\/b><span style=\"font-weight: 400;\">. Chickens infected during embryonic development may carry and shed the virus for prolonged periods without obvious clinical signs.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>The presence of tumours alone does not confirm avian leukosis:<\/b><span style=\"font-weight: 400;\"> Differential diagnosis should include <\/span><b>Marek\u2019s disease, reticuloendotheliosis, and other neoplastic diseases<\/b><span style=\"font-weight: 400;\">. A positive PCR result alone is also not sufficient to establish ALV as the cause of a tumour.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Control should begin with breeder flocks:<\/b><span style=\"font-weight: 400;\"> There is currently no specific treatment or widely used effective vaccine for ALV. Prevention relies mainly on controlled breeding sources, reduction of vertical transmission, biosecurity, and monitoring of flock-performance data.<\/span><\/li>\n<\/ul>\n<h2><b>1. What is avian leukosis?<\/b><\/h2>\n<figure id=\"attachment_20411\" aria-describedby=\"caption-attachment-20411\" style=\"width: 835px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-20411\" src=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-1152697189-612x612-1.jpg\" alt=\"Close-up of a chicken illustrating avian leukosis in poultry\" width=\"845\" height=\"563\" srcset=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-1152697189-612x612-1.jpg 612w, https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-1152697189-612x612-1-300x200.jpg 300w\" sizes=\"(max-width: 845px) 100vw, 845px\" \/><figcaption id=\"caption-attachment-20411\" class=\"wp-caption-text\">Avian leukosis is a group of neoplastic diseases caused by Avian Leukosis Virus (ALV), with chickens being the most important host in commercial poultry production.<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400;\">Avian leukosis is a group of neoplastic diseases caused by viruses belonging to the avian leukosis\/sarcoma virus group.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">ALV belongs to:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">The family <\/span><i><span style=\"font-weight: 400;\">Retroviridae<\/span><\/i><span style=\"font-weight: 400;\">.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">The genus <\/span><i><span style=\"font-weight: 400;\">Alpharetrovirus<\/span><\/i><span style=\"font-weight: 400;\">.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">The virus can integrate its genetic material into host cells. Under certain conditions, this process may promote abnormal cellular transformation and proliferation, eventually resulting in tumour formation.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Disease expression depends on multiple factors, including:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Viral subgroup.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Chicken line.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Age at infection.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Genetic susceptibility.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Immune status.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Route of infection.<\/b><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Chickens are the most extensively documented and studied host.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Other bird species may carry related viruses, but in commercial poultry production, the economic importance of avian leukosis is primarily associated with chickens.<\/span><\/p>\n<h2><b>2. Common forms of avian leukosis<\/b><\/h2>\n<h3><b>2.1. Lymphoid leukosis<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">This is the classical and most commonly discussed form of avian leukosis.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Tumours originate mainly from <\/span><b>B lymphocytes<\/b><span style=\"font-weight: 400;\"> and are commonly associated with the bursa of Fabricius.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Tumours are generally not obvious when birds are very young.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Lymphoid leukosis is more commonly recognised in chickens:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">From approximately 14\u201316 weeks of age onward.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Near or after sexual maturity.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Tumours may occur in:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Bursa of Fabricius.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Liver.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Spleen.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Kidneys.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Other organs.<\/span><\/li>\n<\/ul>\n<h3><b>2.2. Myeloid leukosis<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Myeloid leukosis involves abnormal proliferation of cells of the myeloid lineage and has been particularly associated with certain ALV infections, with <\/span><b>ALV subgroup J<\/b><span style=\"font-weight: 400;\"> being of particular importance.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Lesions may occur in:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Liver.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Spleen.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Bone marrow.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Bone.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Other tissues or organs.<\/span><\/li>\n<\/ul>\n<h3><b>2.3. Less common forms<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Other neoplastic conditions within the leukosis\/sarcoma group may include:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Erythroblastosis.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Myelocytomatosis.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Certain forms of sarcoma.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">The disease form cannot be classified reliably from external appearance alone.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Where confirmation is required, diagnosis should combine:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Post-mortem examination.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Histopathology.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Appropriate laboratory testing.<\/span><\/li>\n<\/ul>\n<h2><b>3. Common clinical signs and lesions<\/b><\/h2>\n<figure id=\"attachment_20413\" aria-describedby=\"caption-attachment-20413\" style=\"width: 830px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-20413\" src=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2233404219-612x612-1.jpg\" alt=\"Veterinary professional in protective clothing examining a chicken on a poultry farm\" width=\"840\" height=\"549\" srcset=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2233404219-612x612-1.jpg 612w, https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2233404219-612x612-1-300x196.jpg 300w\" sizes=\"(max-width: 840px) 100vw, 840px\" \/><figcaption id=\"caption-attachment-20413\" class=\"wp-caption-text\">Chickens affected by avian leukosis may show progressive weight loss, reduced feed intake, pale combs, abdominal enlargement, poor growth, and reduced egg production.<\/figcaption><\/figure>\n<h3><b>3.1. External signs<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Affected chickens may show:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Progressive weight loss.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reduced feed intake.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Wasting.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Pale comb and wattles.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Abdominal enlargement.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Poor feather condition.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reduced activity.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Poor weight gain.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reduced or absent egg production.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">These signs usually develop gradually and are not specific to ALV.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">At flock level, farmers may notice:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Mortality increasing gradually over several weeks.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reduced egg production.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Poorer growth performance.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Increased culling.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Flock productivity below expected levels.<\/span><\/li>\n<\/ul>\n<h3><b>3.2. Liver and spleen<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">The liver may be:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Enlarged.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Affected by multiple white or greyish nodules or masses.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Involved in diffuse or localised neoplastic lesions.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">The spleen may be:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Enlarged.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Structurally abnormal.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Affected by infiltrative areas or tumours.<\/span><\/li>\n<\/ul>\n<h3><b>3.3. Kidneys and other organs<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">The kidneys may:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Become enlarged.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Be infiltrated by neoplastic cells.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Develop abnormal masses.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">In rare cases, a large renal tumour may cause compression and result in abnormal movement or locomotor signs.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">However, peripheral nerve lesions are not a typical feature of lymphoid leukosis.<\/span><\/p>\n<h3><b>3.4. Bursa of Fabricius<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">In lymphoid leukosis, the bursa of Fabricius is diagnostically important.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Possible findings include:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Abnormal enlargement.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Altered follicles.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Nodular or diffuse tumours.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">However, gross examination of the bursa alone is not sufficient to confirm the disease.<\/span><\/p>\n<h3><b>3.5. Ovaries<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">In laying hens, tumours or neoplastic infiltration may involve the ovaries and contribute to reduced reproductive performance.<\/span><\/p>\n<h2><b>4. How is avian leukosis transmitted?<\/b><\/h2>\n<h3><b>4.1. Vertical transmission through the egg<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">This is one of the most important epidemiological routes of ALV transmission.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">An infected hen may <\/span><b>transmit the virus through the egg to the developing embryo<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Chicks infected during embryonic development may:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Carry the virus from hatch.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Remain clinically normal for prolonged periods.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Develop persistent viraemia.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Shed virus.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Become a source of infection for other birds.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Congenitally infected chickens may fail to develop an effective neutralising antibody response because of immune tolerance.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">This is why ALV control must begin with parent flocks and breeding establishments.<\/span><\/p>\n<h3><b>4.2. Horizontal transmission<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">ALV can also spread between birds after hatching.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Horizontal transmission is particularly relevant when young chicks are exposed early to:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Infected or virus-shedding birds.<\/b><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Virus-containing secretions or excretions.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Contaminated equipment.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">However, ALV is less stable outside the host than several other important poultry viruses and can be relatively readily inactivated through appropriate cleaning and disinfection.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">The farm environment therefore contributes to horizontal transmission, but <\/span><b>it should not be regarded as a more important risk source than infected breeding stock<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<h3><b>4.3. Equipment and husbandry procedures<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Shared equipment may contribute to pathogen transmission if it is not properly cleaned.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Particular attention should be paid to:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Husbandry equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Feeders and drinkers.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Veterinary equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Needles.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Avoiding reuse of needles between multiple birds remains an appropriate biosecurity principle, even though this is not considered the primary transmission route for ALV.<\/span><\/p>\n<h2><b>5. Risk factors and the importance of chick source<\/b><\/h2>\n<figure id=\"attachment_20415\" aria-describedby=\"caption-attachment-20415\" style=\"width: 966px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-20415\" src=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2185465262-612x612-1.jpg\" alt=\"Chickens housed together at a poultry facility for flock and breeding-source management\" width=\"976\" height=\"650\" srcset=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2185465262-612x612-1.jpg 612w, https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2185465262-612x612-1-300x200.jpg 300w\" sizes=\"(max-width: 976px) 100vw, 976px\" \/><figcaption id=\"caption-attachment-20415\" class=\"wp-caption-text\">Breeding source is a critical factor in ALV control because infected hens can transmit the virus vertically to their offspring through the egg.<\/figcaption><\/figure>\n<h3><b>5.1. Chick sources without an ALV control programme<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">This should be one of the first risk factors assessed.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Farms should avoid or carefully evaluate chicks from:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Untraceable sources.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Breeding establishments without a clear ALV-control programme.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Parent flocks with a history of tumours or abnormal productivity that has not been investigated.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">With ALV, a chicken that appears healthy is not necessarily free of infection.<\/span><\/p>\n<h3><b>5.2. Congenitally infected chicks<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Chicks infected through the egg may become long-term virus shedders.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Removing only birds that develop tumours later in life is therefore insufficient to eliminate ALV from a flock when the original source of infection is the breeding population.<\/span><\/p>\n<h3><b>5.3. Horizontal spread within the flock<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Conditions that increase contact between birds may facilitate horizontal transmission, including:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Mixing birds from multiple sources.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Sharing equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Inadequate cleaning between areas.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Uncontrolled mixing of flocks.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Stocking density, ventilation, and stress may affect general flock health and management, but they should not be described as specific causes of avian leukosis.<\/span><\/p>\n<h3><b>5.4. Failure to monitor flock data<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">ALV-associated losses may develop gradually.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Farms should monitor:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Weekly mortality.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Egg production.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Weight gain.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Culling rate.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Post-mortem findings.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Long-term trends over several weeks are more informative than observations from only a few affected birds.<\/span><\/p>\n<h2><b>6. Prevention and control of ALV<\/b><\/h2>\n<h3><b>6.1. Control at breeder-flock level<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">The most important strategy is reducing the introduction of virus through breeding stock.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Priority should be given to chicks from systems with:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Traceable parent flocks.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">ALV surveillance programmes.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Virus-eradication or control programmes.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Appropriate breeder-flock health records.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">In more specialised breeding programmes, testing methods may include:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Virus detection.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Antigen testing.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Detection of <\/span><b>p27 antigen in egg albumen<\/b><span style=\"font-weight: 400;\">.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Other virological or molecular techniques.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">The objective is to identify hens that are shedding virus and reduce transmission to offspring.<\/span><\/p>\n<h3><b>6.2. Management of newly introduced flocks<\/b><\/h3>\n<p><b>Separation and monitoring of newly introduced birds<\/b><span style=\"font-weight: 400;\"> remains useful within an overall biosecurity programme, but it does not replace breeder-source control.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">During the observation period:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Keep new birds separated.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Use dedicated equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Monitor mortality.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Record abnormal signs.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Review source documentation.<\/span><\/li>\n<\/ul>\n<h3><b>6.3. Cleaning and disinfection<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Between production cycles:<\/span><\/p>\n<ol>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Remove manure and litter.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Dispose of poultry carcasses appropriately.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Clean feeders and drinkers.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Clean housing surfaces.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Apply an appropriate disinfectant.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Clean equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Control movement of people and equipment between areas.<\/span><\/li>\n<\/ol>\n<p><span style=\"font-weight: 400;\">ALV can be relatively readily inactivated outside the host when cleaning and disinfection are performed correctly.<\/span><\/p>\n<h3><b>6.4. Record management<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Farms should retain records of:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Chick source.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Incoming batch.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Date of placement.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Mortality.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Production.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Post-mortem findings.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Laboratory results where available.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">If multiple batches from the same source develop similar problems, these records can be important for tracing the risk.<\/span><\/p>\n<h2><b>7. What should be done when avian leukosis is suspected?<\/b><\/h2>\n<figure id=\"attachment_20417\" aria-describedby=\"caption-attachment-20417\" style=\"width: 928px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-20417\" src=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2213697459-612x612-1.jpg\" alt=\"Poultry worker monitoring and recording the condition of a chick flock inside a poultry house\" width=\"938\" height=\"527\" srcset=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2213697459-612x612-1.jpg 612w, https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2213697459-612x612-1-300x169.jpg 300w\" sizes=\"(max-width: 938px) 100vw, 938px\" \/><figcaption id=\"caption-attachment-20417\" class=\"wp-caption-text\">When avian leukosis is suspected, the whole flock should be assessed by reviewing mortality trends, productivity, chick source, and the number of birds with tumour-like lesions. Pasted markdown<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400;\">Attention should not focus only on a few visibly affected birds.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">ALV may be present in birds without obvious clinical disease.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">When chickens show:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Progressive wasting.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Abdominal enlargement.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Pale combs.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reduced egg production.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Internal tumours at post-mortem examination.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">the following steps should be considered.<\/span><\/p>\n<h3><b>Step 1: Record and assess the whole flock<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Review:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Flock age.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Chick source.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Mortality rate.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Mortality trends over time.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Production performance.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Weight gain.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Number of birds with neoplastic lesions.<\/span><\/li>\n<\/ul>\n<h3><b>Step 2: Manage affected birds<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Weak birds or birds selected for sampling may be separated to facilitate monitoring and examination.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">However, isolating a few affected birds <\/span><b>does not mean ALV has been controlled within the flock<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<h3><b>Step 3: Conduct post-mortem examinations on appropriate birds<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Record lesions involving:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Liver.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Spleen.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Kidneys.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Bursa of Fabricius.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reproductive organs.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Peripheral nerves.<\/span><\/li>\n<\/ul>\n<h3><b>Step 4: Contact a veterinarian or diagnostic laboratory<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Professional investigation is particularly appropriate when:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Mortality continues to rise.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Multiple visceral tumours are found.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">ALV cannot be distinguished from Marek\u2019s disease.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Similar problems occur in consecutive flocks.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Abnormalities are suspected in breeding stock.<\/span><\/li>\n<\/ul>\n<h3><b>Step 5: Review the breeding source<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">If ALV is confirmed or strongly suspected, review:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Chick source.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Parent flock.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Other batches from the same source.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">History of ALV control.<\/span><\/li>\n<\/ul>\n<h2><b>8. Diagnosis and sample collection<\/b><\/h2>\n<figure id=\"attachment_20419\" aria-describedby=\"caption-attachment-20419\" style=\"width: 806px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-20419\" src=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2270248133-612x612-1.jpg\" alt=\"Laboratory professional preparing poultry tissue samples for diagnostic testing\" width=\"816\" height=\"544\" srcset=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2270248133-612x612-1.jpg 612w, https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-2270248133-612x612-1-300x200.jpg 300w\" sizes=\"(max-width: 816px) 100vw, 816px\" \/><figcaption id=\"caption-attachment-20419\" class=\"wp-caption-text\">Diagnosis of avian leukosis should combine flock age, clinical signs, gross lesions, histopathology, and appropriate laboratory testing; a positive PCR result alone is not sufficient. Pasted markdown<\/figcaption><\/figure>\n<h3><b>8.1. Do not diagnose avian leukosis by PCR alone<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">This is particularly important.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">ALV may be detected in chickens <\/span><b>that do not have tumours caused by avian leukosis<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Therefore:<\/span><\/p>\n<p><b>A positive PCR result or detection of ALV alone is not sufficient to confirm that a tumour is lymphoid leukosis.<\/b><\/p>\n<p><span style=\"font-weight: 400;\">Diagnosis should integrate:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Flock age.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Clinical signs.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Gross lesions.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Histopathology.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Virological or molecular testing where appropriate.<\/span><\/li>\n<\/ul>\n<h3><b>8.2. Role of histopathology<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Histopathology is highly valuable for:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Confirming the nature of the tumour.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Differentiating types of lymphoma.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Supporting differentiation between avian leukosis, Marek\u2019s disease, and other neoplastic conditions.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">In lymphoid leukosis, neoplastic cells are generally relatively uniform and predominantly of <\/span><b>B-cell origin<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<h3><b>8.3. Common samples<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Depending on the diagnostic objective, samples may include:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Tumour tissue.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Liver.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Spleen.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Bursa of Fabricius.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Kidney.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Other abnormal tissues.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">The diagnostic laboratory should be contacted before sample collection because:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Histopathology samples usually require fixation.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Samples for virus detection or PCR may require different storage conditions.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">All samples should not automatically be placed into the same preservative before the intended test has been determined.<\/span><\/p>\n<h2><b>9. Differentiating avian leukosis from Marek\u2019s disease and other neoplastic diseases<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">Marek\u2019s disease is one of the most important differential diagnoses because both diseases can cause visceral tumours.<\/span><\/p>\n<table>\n<tbody>\n<tr>\n<td><b>Feature<\/b><\/td>\n<td><b>Avian leukosis \u2013 lymphoid leukosis<\/b><\/td>\n<td><b>Marek\u2019s disease<\/b><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Typical age<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Usually over 14\u201316 weeks<\/span><\/td>\n<td><span style=\"font-weight: 400;\">May occur from approximately 3 weeks of age<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Neoplastic cell origin<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Predominantly B lymphocytes<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Predominantly T lymphocytes<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Bursa of Fabricius<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Often enlarged and may contain tumours<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Often atrophied; bursal tumours may still occur<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Peripheral nerves<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Usually not enlarged<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Nerve enlargement is an important finding<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Leg\/wing paralysis<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Not typical<\/span><\/td>\n<td><span style=\"font-weight: 400;\">More common<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Skin\/muscle tumours<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Less typical<\/span><\/td>\n<td><span style=\"font-weight: 400;\">May occur<\/span><\/td>\n<\/tr>\n<tr>\n<td><span style=\"font-weight: 400;\">Vaccine<\/span><\/td>\n<td><span style=\"font-weight: 400;\">No widely used effective vaccine<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Effective protective vaccines are available<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><span style=\"font-weight: 400;\">This table is intended only to <\/span><b>support differential diagnosis<\/b><span style=\"font-weight: 400;\"> and does not replace professional diagnostic investigation.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">In particular:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Tumours in the bursa of Fabricius may support lymphoid leukosis but <\/span><b>do not completely exclude Marek\u2019s disease<\/b><span style=\"font-weight: 400;\">.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Enlargement of peripheral nerves strongly supports Marek\u2019s disease.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Marek\u2019s disease may occur in substantially younger birds than lymphoid leukosis.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">In addition to Marek\u2019s disease, <\/span><b>reticuloendotheliosis<\/b><span style=\"font-weight: 400;\"> should also be considered in some cases because it can produce similar lymphoid tumours.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">When lesions are atypical, histopathology and specialised laboratory testing are preferable to diagnosis based solely on gross post-mortem findings.<\/span><\/p>\n<h2><b>10. Is there treatment, a vaccine, or a risk to humans?<\/b><\/h2>\n<h3><b>10.1. Is there a treatment for ALV?<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">There is currently no specific treatment that eliminates ALV from infected chickens.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Antibiotics:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Do not kill ALV.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Do not make tumours disappear.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Should not be used simply because avian leukosis is suspected.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Antibiotics may only be considered by a veterinarian where there is an appropriate <\/span><b>secondary bacterial infection<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Disease management instead focuses on:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Breeding-source control.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Flock monitoring.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Diagnostic confirmation.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Management of affected birds.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Reduction of horizontal transmission.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Management of infected breeder flocks.<\/span><\/li>\n<\/ul>\n<h3><b>10.2. Is there a vaccine against avian leukosis?<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">There is currently no widely used effective vaccine for controlling ALV comparable with vaccination against Marek\u2019s disease.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Control therefore depends mainly on:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Eliminating or reducing virus in breeder flocks.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Controlling vertical transmission.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Selecting monitored breeding sources.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Biosecurity.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Epidemiological surveillance.<\/span><\/li>\n<\/ul>\n<h3><b>10.3. Can ALV infect humans?<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">There is currently <\/span><b>no convincing evidence that ALV naturally causes infection or disease in humans<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">ALV is therefore not regarded as an important zoonotic disease.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Nevertheless, people handling affected birds or poultry carcasses should still:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Use appropriate protective equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Wash their hands.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Clean equipment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Dispose of carcasses appropriately.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">These are general biosecurity principles because poultry flocks may carry multiple pathogens at the same time.<\/span><\/p>\n<h2><b>11. Checklist for prevention and management of suspected ALV<\/b><\/h2>\n<figure id=\"attachment_20421\" aria-describedby=\"caption-attachment-20421\" style=\"width: 878px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-20421\" src=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-1323860611-612x612-1.jpg\" alt=\"Poultry health professional using a tablet to monitor flock health and farm records\" width=\"888\" height=\"592\" srcset=\"https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-1323860611-612x612-1.jpg 612w, https:\/\/www.vietstock.org\/wp-content\/uploads\/2026\/10\/istockphoto-1323860611-612x612-1-300x200.jpg 300w\" sizes=\"(max-width: 888px) 100vw, 888px\" \/><figcaption id=\"caption-attachment-20421\" class=\"wp-caption-text\">Prevention of avian leukosis should begin with controlled breeding sources and combine biosecurity, flock-performance monitoring, traceability, and appropriate diagnostic investigation. Pasted markdown<\/figcaption><\/figure>\n<h3><b>Before flock placement<\/b><\/h3>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Prioritise chick sources with an ALV-control programme.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Check parent-flock traceability.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Review breeder-flock health records where available.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Complete cleaning of the housing environment.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Prepare a separate observation area for new birds where appropriate.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Avoid mixing birds from multiple sources with unknown health status.<\/span><\/li>\n<\/ul>\n<h3><b>During production<\/b><\/h3>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Monitor weekly mortality.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Monitor egg production.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Monitor weight gain.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Track culling rates.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Record birds showing wasting, abdominal enlargement, or pale combs.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Do not reuse needles between birds.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Clean equipment appropriately.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Maintain traceability for each incoming batch.<\/span><\/li>\n<\/ul>\n<h3><b>When tumours or suspicious cases are detected<\/b><\/h3>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Record flock age.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Assess the entire flock.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Perform post-mortem examinations on appropriate birds.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Record organs affected by tumours.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Contact a veterinarian.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Prepare samples according to laboratory requirements.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Do not diagnose ALV based only on a positive PCR result.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Differentiate from Marek\u2019s disease and reticuloendotheliosis.<\/span><\/li>\n<\/ul>\n<h3><b>After laboratory results are available<\/b><\/h3>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Assess the extent of the problem.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Manage affected birds according to professional guidance.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Review the chick source.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Evaluate other flocks from the same source.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Increase surveillance where appropriate.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Update flock-health records.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Adjust breeder-flock control programmes if necessary.<\/span><\/li>\n<\/ul>\n<h2><b>Proactive Avian Leukosis Control from the Breeding Source at VIETSTOCK 2026<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">With avian leukosis, removing individual birds that have already developed tumours is not sufficient to control the risk to the entire flock. Because ALV can be transmitted vertically through the egg, breeder-source selection, parent-flock monitoring, mortality and productivity surveillance, and differential diagnosis from other neoplastic diseases are particularly important for long-term disease control.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">At <\/span><b>VIETSTOCK 2026 \u2013 Vietnam\u2019s Premier International Feed, Livestock &amp; Meat Industry Show<\/b><span style=\"font-weight: 400;\">, livestock producers, veterinarians, and technology providers can exchange professional knowledge and practical experience across the livestock and animal health value chain. The event is expected to bring together <\/span><b>300 brands<\/b><span style=\"font-weight: 400;\">, over <\/span><b>10,000 sqm of exhibition area<\/b><span style=\"font-weight: 400;\">, and <\/span><b>13,000 trade visitors from 40 countries<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><b>For farms prioritising disease control at the breeding-source level and early detection of flock abnormalities, participants can explore solutions in areas such as:<\/b><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Breeding and traceability:<\/b><span style=\"font-weight: 400;\"> exploring solutions that support source-flock monitoring, breeding information, and data used to manage upstream disease risk.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Veterinary diagnostics:<\/b><span style=\"font-weight: 400;\"> learning about technologies that support pathogen detection, lesion assessment, and differentiation between diseases with similar presentations.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Biosecurity and hygiene:<\/b><span style=\"font-weight: 400;\"> exploring solutions that help control pathogen transmission between farm areas, equipment, and production cycles.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Flock health and productivity monitoring:<\/b><span style=\"font-weight: 400;\"> accessing tools that support tracking of mortality, production, weight gain, and long-term flock-performance changes.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Farm technology:<\/b><span style=\"font-weight: 400;\"> exploring solutions that help standardise data collection and improve early detection of abnormal flock-level trends.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">Producers and businesses interested in system-level disease prevention can also follow the <\/span><b>One Health Forum on 22 October 2026<\/b><span style=\"font-weight: 400;\">. The programme includes topics related to <\/span><b>integrating One Health approaches into commercial livestock-production systems<\/b><span style=\"font-weight: 400;\">, <\/span><b>linking animal health, human health, and environmental outcomes<\/b><span style=\"font-weight: 400;\">, as well as approaches relating to <\/span><b>biosecurity and farm management<\/b><span style=\"font-weight: 400;\">. These discussions help broaden the focus from managing individual disease cases to building more sustainable systems for disease prevention and flock surveillance.<\/span><\/p>\n<p><b>Date:<\/b><span style=\"font-weight: 400;\"> 21\u201323 October 2026<\/span><span style=\"font-weight: 400;\"><br \/>\n<\/span> <b>Venue:<\/b><span style=\"font-weight: 400;\"> Saigon Exhibition and Convention Center (SECC), 799 Nguyen Van Linh Street, Tan My Ward, Ho Chi Minh City, Vietnam<\/span><span style=\"font-weight: 400;\"><br \/>\n<\/span> <b>Event website:<\/b><a href=\"https:\/\/www.vietstock.org\/en\/\"> <span style=\"font-weight: 400;\">https:\/\/www.vietstock.org\/en\/<\/span><\/a><\/p>\n<p><strong>Register to visit now<\/strong>: <a href=\"http:\/\/Register to visit now: https:\/\/ers-vn.informa-info.com\/vs26\">https:\/\/ers-vn.informa-info.com\/vs26<\/a><\/p>\n<p><span style=\"font-weight: 400;\">\ud83d\udc49<\/span><a href=\"https:\/\/www.vietstock.org\/en\/event-features\/vietstock-2026-pre-registration-is-now-open-connect-with-the-future-of-livestock-production-and-animal-health-2\/?\"> <b>VIETSTOCK 2026 Visitor Registration Information<\/b><\/a><\/p>\n<p><span style=\"font-weight: 400;\">\ud83d\udc49<\/span><a href=\"https:\/\/www.vietstock.org\/en\/event-features\/one-health-forum-at-vietstock-2026-advancing-sustainable-livestock-development\/\"> <b>Learn more about the One Health Forum<\/b><\/a><\/p>\n<p><b>Contact information:<\/b><\/p>\n<p><b>Exhibiting:<\/b><span style=\"font-weight: 400;\"> Ms. Sophie Nguyen \u2013 Sophie.nguyen@informa.com<\/span><span style=\"font-weight: 400;\"><br \/>\n<\/span> <b>Visitor Support:<\/b><span style=\"font-weight: 400;\"> Ms. Phuong \u2013 Phuong.C@informa.com<\/span><span style=\"font-weight: 400;\"><br \/>\n<\/span> <b>Marcom Support:<\/b><span style=\"font-weight: 400;\"> Ms. Anita Pham \u2013 Anita.pham@informa.com<\/span><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Avian leukosis is a group of neoplastic diseases caused by Avian Leukosis Virus (ALV), primarily affecting chickens. Learn about its clinical signs, vertical and horizontal transmission, tumour lesions, differential diagnosis with Marek\u2019s disease, and prevention strategies focused on breeder-flock control.<\/p>\n","protected":false},"author":3,"featured_media":20426,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[45],"tags":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v15.9 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Avian Leukosis in Poultry: Signs, Transmission &amp; Prevention<\/title>\n<meta name=\"description\" content=\"Learn about avian leukosis in poultry, including clinical signs, ALV transmission through eggs, diagnosis, differential diagnosis and prevention strategies.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" 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