Infectious bursal disease
Infectious bursal disease (popularly known as Gumboro disease) is an acute contagious disease of young chicks caused by an RNA virus, a member of family Birnaviridae. The virus mainly affects chickens of 3 to 6 weeks of age. Very young chicks up to 10-12 days of age and laying birds do not show signs. Older birds and turkeys may also get the infection. There are two serotypes of the virus, serotype-1 vary in virulence from mild to highly pathogenic (very virulent) causing mortality upto 80% in chickens whereas turkeys and ducks (but not chickens) show infection with serotype-2. In addition to the direct effect of the clinical disease, the damage caused to the immune system results in increased susceptibility to other opportunists and pathogens to cause significant losses. The virus is comparatively resistant and persists for a long period in the contaminated pens/materials. The disease is highly contagious. Mealworms and litter mites may harbour the virus for 8 weeks, and affected birds excrete large amounts of virus for about 2 weeks post-infection. There is no vertical transmission.
Symptoms and lesions: Incubation period is 2 to 3 days. In the acute form of the disease, clinical signs start with anorexia, ruffled feathers, watery diarrhea, and depression. The birds may die of dehydration in 1 to 3 days of the illness. The morbidity may be as high as 100% and mortality varies between 20-80%. The gross lesions observed are edematous, enlarged, hemorrhagic bursa of Fabricius and hemorrhages in the skeletal muscles particularly thigh muscles. The bursa may contain necrotic cheesy materials. Enlargement of the spleen and uretic deposits in the renal tubules/ureters are also observed in some birds. Hemorrhages are also seen at the junction of the gizzard and proventriculus. The bursa may return to normal size but continue to atrophy rapidly. The histopathology of the bursa reveals severe bursitis characterized by edema, cellular exudates and necrosis.
Diagnosis: Clinical symptoms, gross lesions, virus isolation, detection of viral antigens in bursa/spleen/kidney/liver by agar gel diffusion/ immunoperoxidase/ fluorescent antibody tests, and detection of specific antibodies by agar gel diffusion/ ELISA tests are used for diagnosis of the disease.
Prevention and control: Strict biosecurity, proper hygiene and good management practices are necessary for prevention of IBD. Live vaccines are given in drinking water during the third week to provide protection as maternal immunity wanes. Breeding hens are vaccinated with attenuated live virus vaccine between 4 and 10 weeks of age and then given an inactivated adjuvanted vaccine 6 to 8 weeks later.