Lymphoid leukosis is a viral infection that causes tumors in chickens globally. This neoplastic disease involves abnormal tissue growth. While it primarily affects chickens, its frequency has been significantly reduced in commercial flocks due to control programs.
Causes and Spread
Lymphoid leukosis is caused by the avian leukosis virus (ALV), a retrovirus belonging to the leukosis/sarcoma group. Several ALV subgroups exist, with A and B historically prevalent in Western countries, and subgroup J emerging later. The virus can be transmitted through both vertical and horizontal routes.
Vertical transmission occurs when the virus passes directly from an infected hen to her offspring through the egg. Chicks infected at hatching may shed the virus throughout their lives. Horizontal transmission can happen through contact with infected birds or contaminated environments, often via the fecal-oral route. The virus does not survive for extended periods outside the bird and is readily inactivated by disinfectants. Lymphoid leukosis has a long incubation period, typically four to six months, allowing for silent spread before clinical signs appear.
Identifying Affected Birds
Chickens with lymphoid leukosis often display general signs of illness. These can include reduced feed intake, weakness, diarrhea, dehydration, and weight loss. Affected birds may also appear depressed or listless before death.
More specific indicators relate to tumor formation. Palpation of affected birds may reveal an enlarged bursa of Fabricius, a lymphoid organ, and sometimes an enlarged liver. Tumors can also develop in other internal organs like the spleen, kidneys, lungs, ovaries, testicles, and bone marrow, leading to an enlarged abdomen or physical deformities such as paralysis or lameness if nerves are affected. Diagnosis is typically confirmed through post-mortem examination, where distinct nodular tumors are observed, particularly in the bursa of Fabricius. Laboratory tests, such as testing blood samples or cloacal swabs for the virus or viral antigens, can also aid in diagnosis.
Control and Prevention Strategies
There is currently no effective treatment or vaccine for lymphoid leukosis once chickens are infected. Therefore, control and prevention strategies focus on reducing and eventually eradicating the causative virus from poultry flocks. A primary method involves sourcing chicks from breeding flocks certified free of the avian leukosis virus.
Strict biosecurity measures are also important for preventing disease spread. This includes maintaining rigorous hygiene in poultry facilities, isolating new birds before introducing them to an existing flock, and controlling biting pests like red mites, flies, and lice, which can transmit the virus. Infected birds or those suspected of having the disease should be culled to prevent further transmission. The virus is susceptible to ambient temperatures and most disinfectants, assisting in environmental control.