Aetiology
It is a corona virus belonging to the family Coronaviridae. Infectious bronchitis virus has a number of antigenically distinct strains which include mass strain, conn strain, hohe and grey strain and Australian-T strain. Mass strain and conn strain are associated with respiratory tropism while Hohe and grey strain and Australian-T strain are associated with kidney tropism. Predisposing factors are intercurrent infections like Newcastle disease, infectious laryngotracheitis, mycoplasmosis, infectious coryza and E. coli. Cold air can predispose to the disease.
Host range
Domestic chicken is the natural host. All ages of chicken are susceptible to the disease but the clinical respiratory disease occurs in chicks and growers especially between 4 and 10 weeks of age.
Transmission
The virus is excreted in nasal exudates such that spread from bird to bird or flock to flock which is airborne.
Clinical signs
Incubation period is short between 18-24 hours.
Clinical signs of the disease in chicks are rales, dyspnea, sneezing, watery nasal discharge which may be accompanied by excessive lacrimation causing matting of the face feathers and facial swelling. Uncomplicated disease runs the course of about 10-14 days with low mortality. The renal form presents few clinical signs but may be accompanied by excessive urate deposits in the faeces leading to pasting on the vent.
The disease in adults affects egg production but occasionally present respiratory signs. The effects on in egg production depends on fowl age at infection and include delay onset of laying, delay onset of peak production, reduction to percentage peak and sudden reduction in laying. Reduction in egg production may be as low as 5% or as high as 50%. Apart from this quantitative aspect, there are qualitative defects in egg production which are loss of shell pigmentation, thin shelled eggs and loss of shell formation. Others are watery albumen and broken chalaza.
Post-mortem lesions
In respiratory form, there is catarrhal inflammation of the tracheal mucosae and sinuses. In severe cases, mucoid plugs cause asphyxiation in young chicks.
In reproductive form, there is reduction in size and weight of the oviduct. When infection occurs in chicks or growers, there is agenesis of the oviduct which may be reduced to vestiges and is non-patent.
In uric or kidney form, the kidneys are swollen and inflamed in early stages or pale in advanced stages. Renal tubules are distended with whitish crystals of urate and there may be visceral gout with deposits of urates in liver, heart and kidneys.
Diagnosis
Tentative diagnosis can be reached by a combination of epizootiological signs like age at infection, clinical signs and post=mortem lesions. Confirmation is reached by isolation and recognition of the virus. It causes dwarfing and curling in embryo. Diagnosis can be done using ELISA and agar gel precipitation test.
Prevention and control
This is based on management and hygiene practices and the control of intercurrent diseases like Newcastle disease, chronic respiratory disease, infectious laryngotracheitis and vaccination using attenuated vaccines applied in drinking water and sprays or eyedrops according to manufacturer’s direction. Birds are vaccinated at first week of age and booster doses given at 4-6 weeks. Plurality of strains implies that a given vaccine may not be preventive against challenge from heterologous strain.