Infectious bronchitis (IB) is an acute, highly contagious respiratory disease of chickens, caused by the infectious bronchitis virus (IBV), a type of coronavirus. There are several different serotypes and strains of the IBV, which are found worldwide.
IBV infection initially occurs in the respiratory tract, with the trachea being the primary target. It remains there for about a week, where it replicates and causes varying amounts of damage to the epithelium, predisposing birds to secondary infections with pathogenic bacteria, especially E. coli
Depending on the strain of the virus, it may migrate from the trachea to other areas of the chicken's body, most commonly, the kidneys, reproductive system (hen's oviduct and rooster's testes), or alimentary tract.
IB virus is acquired through inhalation or direct contact with infected birds or indirectly through exposure to a contaminated environment. Droplets containing the virus are expelled during coughing or sneezing, and shed in the feces or on the eggs, from infected birds. The virus is not transmittable from hen to chick through the egg. Once recovered, chickens may remain carriers and shed the virus for up to 15-20 weeks.
IB has a short incubation period. In an affected flock, all the birds will typically develop clinical signs within 24 to 48 hours. Clinical disease will usually last for 7 days.
Clinical signs include watery or frothy exudate in the eyes, coughing, tracheal rales, nasal discharge, and sneezing. Young chicks usually appear depressed and huddle near a heat source. In some cases they may develop severe respiratory distress. Infection in very young chicks may result in the development of cystic oviducts.
Adult hens actively laying eggs often develop a sudden drop in egg production lasting for 10 to 14 days. They may also lay thin, wrinkled or roughened egg shells with thin, watery albumen. Loss of pigment in brown-shelled eggs is common.
Uncomplicated cases of the respiratory disease usually resolve within a couple weeks. However, the damage to the respiratory system caused by the infection often results in complicated co-infections with opportunistic bacteria. In addition, depending on the strain of the virus, after the virus replicates in the trachea, it will spread to other organs. If it spreads to the kidneys, affected birds often develop urolithiasis and renal failure. When it spreads to the reproductive organs, it can cause egg laying related complications and disorders.
During the first week of infection, a trachea swab from the affected bird can be taken and sent to a veterinary diagnostic lab for confirmation of the IBV. After the first week, cloacal swabs are the preferred location. If the chicken has died, samples can be taken from the trachea, cecal tonsils, lung, kidney, testes or oviduct.
There are several types of vaccines available against IBV, which are widely used by the commercial poultry industry. Laying hens and broiler breeders initially receive the live attenuated vaccine administered to young chicks en masse through sprays or via drinking water. Followed by the inactivated vaccine as a booster in 4-6 weeks.