Avian Leukosis Virus (ALV) is a retrovirus that causes cancerous tumors in chickens, broadly classified as avian leukosis. Of the ten identified viral subgroups, labeled A through J, six are known to infect chickens. The virus leads to tumors on internal organs, impacting a bird’s health and productivity. While ALV is found in poultry worldwide, its effects on a flock can range from unnoticeable to severe.
Identifying ALV Infection
A significant number of birds infected with ALV may never show outward signs of illness, carrying a subclinical infection. When symptoms do appear, they are often associated with the most common form of the disease, Lymphoid Leukosis. This form affects chickens that are four months of age or older. The clinical signs are nonspecific and develop gradually, including a loss of appetite, weakness, diarrhea, dehydration, and progressive weight loss.
As tumors grow on internal organs, owners might notice a chicken has a pale or sometimes bluish comb and wattles. An enlarged abdomen can also be a sign, which is detectable through careful palpation. Internally, Lymphoid Leukosis is characterized by nodular tumors on the liver, spleen, and the bursa of Fabricius, a specialized immune organ in birds. The involvement of the bursa is a consistent finding with this disease.
Less common manifestations of ALV can present with different symptoms. One such form, osteopetrosis, results in abnormally thick and dense leg bones, which can cause lameness. Another form, myelocytomatosis, is associated with the J subgroup and causes soft, creamy-white tumors on the head, mouth, or shanks. This form can be seen in birds as young as five weeks old, and once a bird becomes visibly ill, the disease often progresses rapidly.
Transmission Pathways
Avian Leukosis Virus spreads through two primary routes. The main pathway is vertical transmission, where an infected hen passes the virus to her offspring through the egg. The virus is shed by the hen into the yolk or albumen, infecting the embryo as it develops. Chicks hatched from these eggs are congenitally infected and often fail to develop an effective immune response, shedding the virus for life.
The second method is horizontal transmission, which occurs from bird to bird. Infected chickens shed the virus in their feces, saliva, and dander, and other birds can become infected through direct contact or a contaminated environment. This route is less efficient than vertical transmission because the virus does not survive for long outside a host and is susceptible to common disinfectants.
Exposure to the virus at a very young age is particularly impactful. Chicks infected horizontally right after hatching are more likely to develop persistent infections. Birds that encounter the virus when they are older may experience a temporary infection before their immune system clears it, and these birds rarely develop tumors. Infected roosters do not pass the virus to offspring through semen but can spread it to hens through direct contact.
Diagnosis and Prevention
Confirming an ALV infection based on symptoms alone is not possible because the signs are vague and resemble other diseases. A definitive diagnosis requires laboratory analysis. Veterinarians may perform a post-mortem examination to identify tumors, but further testing of blood or tissue samples is needed to distinguish ALV from other viruses. Common diagnostic tools include the ELISA test to detect antibodies and the PCR test to detect the virus’s genetic material.
Since there is no treatment or vaccine for Avian Leukosis Virus, prevention is centered on biosecurity and eradication strategies. The most effective approach is to source chicks from breeding flocks certified as ALV-free. For existing flocks, a program of testing and culling is used to eliminate the virus. This involves testing breeding birds and removing any that are positive, thereby preventing transmission.
Strict biosecurity measures help protect a flock. This includes quarantining any new birds before introducing them and maintaining high levels of sanitation in housing and on equipment. Because the virus is easily inactivated by disinfectants, regular cleaning can reduce the risk of horizontal spread. In hatcheries, separating eggs from different flocks can prevent cross-contamination.
Differentiating from Marek’s Disease
Confusion often arises between ALV and Marek’s disease, as both cause tumors in chickens. However, they are caused by two distinct types of viruses; ALV is a retrovirus, while Marek’s disease is caused by a herpesvirus. This difference leads to variations in how the diseases present and are managed.
One practical distinction for flock owners is the age of onset. ALV-related tumors are common in birds over 16 weeks of age, while Marek’s disease affects younger chickens. A clinical sign of Marek’s disease is nerve inflammation, which often leads to paralysis in the legs and wings, a symptom not associated with ALV. Management also differs, as a highly effective vaccine is available for Marek’s disease, but not for ALV.