Lymphoid leucosis: A vast viral poultry disease

 Lymphoid leucosis is a neoplastic disease of birds produced by the avian leucosis virus worldwide. The disease is categorized by B cell lymphoma occurred in birds around 16 weeks of age. It may produce a tumor in the bursa of Fabricius and metastasize to other tissue and lead to widening and swelling of the abdomen. At 5-8 months of age, birds are more susceptible to this disease.


The incubation period of this disease is 4 to 6 months. It is caused by a sarcoma or leucosis group of avian retroviruses. These viruses are mainly known as avian leucosis viruses belonging to subgroup A, B, C, D, E, and J. Subgroup E is categorized as a non-oncogenic endogenous virus. Until the appearance of J subgroup in western countries, subgroup A and B have been more dominant. While in the 1990s, in meat-type chicken, myeloid leucosis was predominant.


The International Committee on Taxonomy of viruses has nominated avian leucosis virus within the genus Alpharetrovirus belonging to the family Retroviridae. This RNA virus has a common group-specific antigen. Lymphoid leucosis occur naturally in birds and produced tumor mortality in birds about 1-2% and infrequently causes round about 20% losses in the poultry industry.


The occurrence of tumors in lymphoid leucosis is usually low ≤4%, even in highly infected bird flock. However, in naturally infected commercial broiler breeder flock with subgroup J avian leucosis virus, 1.5%/week mortality has been documented.  


Sign and symptom:


§  Enlargement of bursa and abdomen

§  Emaciation, weakness, and depression

§  Reduced weight gain

§  Reduced feed intake and egg production

§  dehydration

§  Greenish diarrhea at the terminal stage


Transmission of lymphoid leucosis disease:



The virus has been transmitted horizontally (bird to bird) as well as vertically (hen to chick). Subclinically infected hen typically shed viral antigen or virus into the egg. Infected chicks shed the virus throughout their life. Horizontal spread may occur through the fecal-oral route. Previously the infection leads to tolerance, persistent, viremia, and tumor. Other factors that are involved enhancing the liability of chickens to horizontal infection are the presence of endogenous retrovirus and the absence of maternal antibodies, particularly those related too the late feathering K gene.


Tumors usually occur in congenital infection than horizontal. In contrast, chickens are mainly infected horizontally then congenitally. Usually, the rate of embryo transmission is 1-10%. Almost all the chicks in the infected flock are exposed by contact. Late infection, usually 12 to 20 weeks of age, leads to virus shedding.


Avian leucosis virus infection categorized into four classes in matured chickens:


§  No viremia and antibody (-V-A)

§  No viremia with antibody (-V+A)

§  Viremia with antibody (+V+A)

§  Viremia with no antibody (+V-A)


Infection free flock falls into the category (-V-A); in an infected flock, genetically susceptible birds fall into the other 3 categories mostly are categorized (–V+A), and only ≤10% are categorized into (+V-A).



Pathogenesis of lymphoid leucosis:


It is a clonal malignancy of the bursal dependent lymphoid system. Alteration consistently happens usually as early as 4 to 8 weeks post-infection in the intact bursa. At the same time, the tumor is particularly detectable at 14 weeks of age. Mortality is infrequent before 14 weeks of age and more frequent during sexual maturity. The infection can be control even though up to 5 months of age through the treatment for the destruction of the bursa. The initiation of lymphoid leucosis tumors may be increased in chickens that coinfected with serotype 2 Marek's disease virus, a common vaccine virus.


In subclinical infection, no tumor formation and categorized by decreased egg production leads to economic losses. Chicken with subclinical infection typically shed the virus in egg albumin.


Lesions of lymphoid leucosis disease in poultry:


Enlarged liver and bursa

Diffuse or nodular lymphoid tumors are commonly seen in the liver, bursa, and spleen might be present in kidney, mesentery, and gonads.

Many strains of leucosis virus produced the nonlymphoid tumors such as sarcomas, myeloblastosis, erythroblastosis, hemangiomas, osteopetrosis, and nephroblastomas.

Myelocytomas appear around the orbit of eyes, lead to hemorrhages and blindness; hemangiomas can appear in the skin as a blood blister that might be rupture and bleed.

Renal tumor leads to paralysis due to pressure on the sciatic nerve

Microscopically: uniform and enlarged lymphoid cells



Diagnosis of lymphoid leucosis:

§  

Lymphoid leucosis is diagnosed by history, clinical signs, gross pathology (tumor of the liver, bursa, and spleen formed ≥14 weak of age), and histopathology (lymphoid cells are uniform and large having IgM and B cell).

§  Serological test (ELISA for subtype A, B and J are available commercially)

§  Immunohistochemistry

§  Standard and quantitative PCR

§  Virus isolation

Through gross lesion and histopathology, lymphoid leucosis can be easily differentiated from Marek's disease also through a molecular technique that determines the clonal integration of proviral DNA to tumor cell genome along with related interruption of c-myc oncogene.

Treatment control and prevention of lymphoid leucosis:

§  For lymphoid leucosis, vaccination and treatment are not available commercially. So from breeding flock elimination of avian leucosis, viruses are more effective for control of the disease.

§  The virus is rapidly inactivated at ambient temperature and exposure to most disinfectant

§  Shedders hens egg are discarded so that the infection in newborn chicks particularly declined

§  Chicks obtained from virus-free breeder flock and rearing birds in isolation with suitable ventilation.

§  Good management and biosecurity to avoid the stress and control other secondary bacterial infection.

§  Dead bird disposal through composting, incineration and deep burial

§  Breeders flock is assessed for virus shedding through viral antigen testing in egg albumin by enzyme immunoassays or through biologic assay for infectious virus.

§  For tumor prevention, no vaccination available, but the recombinant vaccine has been developed that may produce the antibodies in chicks and might be helpful in the elimination program.


Dr. Maria Jameel, Dr. Muhammad Kashif Saleemi, Dr. Aisha Khatoon
(UAF)

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