Synonyms
Infectious bursitis, Avian nephrosis, gumboro disease.
Economic importance of the disease
This disease is illustrated in two ways:
Clinical disease and mortality occur in chicken 3 weeks of age and older.
Severe and prolonged immune suppression of chicken at early stage which can lead to increase susceptibility to Newcastle disease, infectious bronchitis, E. coli infection and vaccine failures.
Aetiology
The disease is caused by infectious bursa disease virus which is a Birnavirus belonging to family Birnaviridae. It is a very stable virus capable of withstanding high temperature for extended period. It has two distinct serotypes: IBD serotype 1 and IBD serotype 2.
Serotype 1 is pathogenic whereas serotype 2 is non-virulent or apathogenic. In the serotype 1 group, there is considerable antigenic variation with variation in virulence or pathogenicity. In the serotype 1 group, there can be a pathogenic strain to very virulent strain that cause up to 50% mortality.
Host
The clinical disease is seen in chicken of ages 3-6 weeks of age. It causes subclinical infection of bird less than 3 weeks of age resulting in severe immunosuppression.
Transmission
This viral disease is very contagious and the virus persist in an environment for long period in time. Their appearance in this disease in successive of chicken in a farm once it has appeared is one of the traits of the infection.
Feral birds, meal worms, litter mites, rodents and humans could act as mechanical vectors of the disease. The virus presence in the faeces is the main source of the virus. The disease may also be spread by inhalation and cutaneous lesions.
Clinical signs
The severity of signs is dependent on age, breed of chicken as well as virus virulence. The incubation period is short about 2-3 days. One of the earliest sign of the infection in the flock is tendency of the bird to peck at their own vent. Other signs observed are trembling, soiled vent feathers, depression, whitish and watery diarrhea, anorexia, ruffled feathers, severe prostration and death. In fully susceptible flock, there is high morbidity usually approaching 100%. Mortality usually begins at 3 days post infection and will peak within a period of 5-7 days. The striking feature of this disease is a sudden high rate of morbidity, spiking mortality curve and rapid flock recovery. Most outbreaks are associated with about 30% mortality occasionally reaching 50%. Morbidity ranges from 10-100%
Post-mortem lesions
Dehydration result in darkening/ discoloration of the pectoral muscle. Haemorrhage is observed in muscle and thigh. There is excess mucous in the intestine. Renal changes include enlargement of the kidney, distention of renal tubules and ureters with accumulated urates leading to nephrosis.
The bursa is enlarged, inflamed and edematous and cream coloured and after about 3-8 days, it atrophies. Haemorrhages may be seen in the internal and serosal surface of the bursa. Haemorrhages is also present in thigh muscles.
Diagnosis
This is based on clinical signs and post-mortem lesions. Positive diagnosis is based on post-mortem lesion especially by examination of the bursa for the characteristic gross lesion. Confirmatory diagnosis can also be based on virus neutralization test and agar gel precipitation test in which masserated bursa is used as an antigen against a known positive and serum obtained from a reference laboratory for example NVRI Vom.
Control
There is no therapeutic or supportive treatment for this disease due to harden nature of the virus that causes the disease.
Good animal husbandry and hygiene should be practiced.
Disinfection of poultry house and equipment with formaldehyde and iodophores reduces the viral load.
Vaccination is the principal method used to control the disease in the chicken.
The parent stock are vaccinated at 4-10 weeks of age with live vaccine and boosted with oiled adjuvanted inactivated vaccine at age of 16 weeks (16 weeks is at age of laying birds to pass immunity to new chicks). In the progeny or chicks, the maternally derived antibody level wanes with time protecting against virulent challenge up to 2-3 weeks of age. These chicks are then vaccinated at 2-3 weeks of age.