22. Diseases of the bursa of Fabricius- aetiology, symptoms, diagnostics, differential diagnostics, therapy and prevention

What is the Bursa of Fabricius and what is its function?

The Bursa of Fabricius is an epithelial and lymphoid organ found only in birds. It develops in young birds and atrophies around 6 months of age. It plays a critical role in the development of antibody-mediated immunity.

What is infectious bursal disease (IBD)?

Infectious bursal disease (also known as Gumboro disease) is an acute, contagious, viral disease affecting young chickens. It is caused by Infectious Bursal Disease Virus (IBDV) and characterized by inflammation followed by atrophy of the Bursa of Fabricius and varying degrees of immunosuppression.

What are the two serotypes of IBDV and how do they differ?

There are two serotypes of IBDV, but only serotype 1 is pathogenic. Additionally, virus strains can be divided into classical and variant strains.

How is infectious bursal disease transmitted in birds?

Infectious bursal disease is highly contagious and is shed in the feces of infected birds. Transmission can be direct (bird-to-bird through droppings) or indirect (through contaminated clothes or equipment).

What are the clinical signs of clinical IBD in chickens?

Clinical signs of IBD typically appear 2-3 days after infection. Infections in birds younger than 3 weeks may be subclinical, while older birds (3-6 weeks) may exhibit a sudden onset of tremors, depression, anorexia, ruffled feathers, drooping appearance, diarrhea, dehydration, and vent picking.

What are the economic consequences of subclinical IBD?

Even without causing mortality, subclinical IBD can lead to permanent immunosuppression in chickens, making them more susceptible to secondary infections and reducing their response to vaccinations. This can result in poor performance, lower weight gains, and higher feed conversion ratios.

What are the post-mortem lesions of infectious bursal disease?

In chickens with clinical IBD, the cloacal bursa will be swollen, edematous, yellowish, and occasionally hemorrhagic. In subclinical disease, the bursa will be atrophied without inflammation.

How is infectious bursal disease diagnosed?

Infectious bursal disease can be diagnosed based on clinical signs, necropsy findings, ELISA testing, virus isolation, or PCR.

How is infectious bursal disease treated?

There is no treatment for infectious bursal disease.

How can infectious bursal disease be prevented?

Vaccination of breeder hens (to induce high maternal immunity) and young chicks is the primary method of preventing infectious bursal disease. Strict biosecurity measures are also important.

What are the differential diagnoses for infectious bursal disease?

Sulfonamide poisoning, aflatoxicosis, and pale bird syndrome (vitamin E deficiency) can be confused with infectious bursal disease.

What is lymphoid leukosis and what causes it?

Lymphoid leukosis is a neoplastic disease affecting chickens over 14-16 weeks of age. It is caused by Avian Leukosis Virus (ALV).

How is lymphoid leukosis transmitted in chickens?

Lymphoid leukosis can be transmitted vertically through eggs or horizontally between young birds. The virus is shed in saliva and feces.

What are the clinical signs of lymphoid leukosis?

Lymphoid leukosis causes non-specific signs like inappetence, weakness, diarrhea, dehydration, and emaciation. Death rarely occurs before 14 weeks of age.

What are the post-mortem lesions of lymphoid leukosis?

Lymphoid leukosis is characterized by lymphomas in the bursa of Fabricius, which may metastasize to other organs like the liver, spleen, and kidneys. Tumors can be nodular, miliary, or diffuse. Hemangiomas may also be present. Microscopically, the tumors originate from bursal lymphocytes.

How is lymphoid leukosis diagnosed?

Lymphoid leukosis can be diagnosed based on history, clinical signs, histopathology, virus isolation, and examination of the bursa (to distinguish it from Marek's disease).

How is lymphoid leukosis treated?

There is no treatment for lymphoid leukosis.

How can lymphoid leukosis be prevented?

Lymphoid leukosis can be prevented by destroying the bursa of Fabricius in chicks up to 5 months old.

What is the differential diagnosis for lymphoid leukosis?

Marek's disease can be confused with lymphoid leukosis, but Marek's disease involves central nervous system (CNS) signs.

What is Marek's disease and what causes it?

Marek's disease (also known as avian leukosis virus type 2) is a highly contagious, oncogenic, neoplastic disease caused by Marek's disease virus (MDV). It causes T-cell lymphoma in various tissues of chickens.

What are the clinical signs of Marek's disease?

Marek's disease can cause various syndromes depending on the affected tissues. Here are some of the common signs:

Classical (neurolymphomatosis): Asymmetric paralysis of limbs, difficulty breathing, crop dilation (due to vagus nerve infection), lymphomatous infiltration of skin, muscle, and visceral organs.
Acute (death): Depression, paralysis, and death (within days).
Cutaneous (lesions at feather follicles): Round, firm lesions at the feather follicles.
Atherosclerosis (thickening/hardening of arteries).
Immunosuppression: Impaired T-lymphocytes, leading to B-cell shutdown.
Ocular lymphomatosis: Lymphocyte infiltration of iris (grey discoloration), unequal pupil sizes, blindness.

What are the post-mortem lesions of Marek's disease?

Gross lesions of Marek's disease can vary depending on the affected organ system (visceral, neural, ocular, skin). In some cases, no gross lesions may be observed. Here are some potential lesions:

Visceral: Lymphomatosis (lymphosarcoma) with tumors in the liver, spleen, kidney, lungs, ovary, muscles, or other tissues. General organ enlargement (hepatomegaly, splenomegaly) due to diffuse proliferation of white blood cells. Livers may have light grey to white nodules (tumors with soft centers).
Neural: Non-suppurative encephalitis (inflammation of the brain) and neuritis (inflammation of nerves). Neurolymphomatosis with swollen lumbosacral plexus and sciatic nerve. Involvement of the gizzard nerve can lead to a very small gizzard and intestine, causing the bird to waste away.
Ocular: Inflammatory and degenerative changes of the optic nerve.
Skin: Skin neoplasia (tumors).

How is Marek's disease diagnosed?

Marek's disease can be diagnosed through histological examination of nerve and visceral lesions (looking for lymphocytic proliferation), virus isolation, PCR, agar gel immunodiffusion, or indirect fluorescent antibody test.

How is Marek's disease treated?

There is no treatment for Marek's disease.

How can Marek's disease be prevented?

Vaccination (through injection into the egg or subcutaneously shortly after hatching) is the primary method for preventing Marek's disease. It prevents the development of tumors and paralysis but does not stop birds from getting infected.

What is the differential diagnosis for Marek's disease?

Lymphoid leukosis can be confused with Marek's disease. However, Marek's disease is a T-cell lymphoma affecting young chickens (10-12 weeks) before sexual maturity, while lymphoid leukosis is a B-cell lymphoma affecting mature chickens (after sexual maturity).

What is reticuloendotheliosis and what causes it?

Reticuloendotheliosis is a rare disease caused by a retrovirus.

What are the clinical signs of reticuloendotheliosis?

Reticuloendotheliosis can cause abnormal feathering, anemia, weight loss, and occasional paralysis. Chronic lymphomas may cause depression before death. Chicks infected within the first week of life may develop runting syndrome, characterized by severe stunting and abnormal feathers.

What are the post-mortem lesions of reticuloendotheliosis?

Runting syndrome caused by reticuloendotheliosis can lead to severe atrophy of the thymus and bursa of Fabricius. Some birds may have enlarged nerves, proventriculus (muscular stomach), enteritis (intestinal inflammation), and anemia. Chronic lymphoma syndrome can cause tumors in the liver and spleen, although these are not common in the bursa.

How is reticuloendotheliosis diagnosed?

Reticuloendotheliosis can be diagnosed based on clinical signs, PCR testing.

How is reticuloendotheliosis treated?

There is no treatment for reticuloendotheliosis.

How can reticuloendotheliosis be prevented?

Avoiding contamination with live vaccines can help prevent reticuloendotheliosis.

What are the differential diagnoses for reticuloendotheliosis?

Lymphoid leukosis, Marek's disease, infectious bursal disease, and chicken infectious anemia can be confused with reticuloendotheliosis.

What can infect the bursa of Fabricius in birds younger than 6 months and the oviduct in birds older than 6 months?

Prostogonimus worms can infect the bursa of Fabricius in young birds and the oviduct in mature birds.