Infectious bronchitis is an acute, rapidly spreading, viral disease of chickens usually characterized by respiratory signs, although decreased egg production and poor egg quality is sometimes observed in breeders and layers. Some strains of the causative virus, infectious bronchitis virus (IBV), are nephropathogenic. The latter strains produce interstitial nephritis resulting in significant mortality. Infectious bronchitis is of major economic importance to commercial chicken producers worldwide.
Etiology and Epidemiology
IBV, a coronavirus, is worldwide in distribution and has numerous serotypes. Two or more serotypes may be seen simultaneously in one geographic region. IBV is shed by infected chickens in respiratory discharges and feces. The highly contagious virus is spread by airborne droplets, ingestion of contaminated feed and water, and contaminated equipment and clothing of caretakers. Naturally infected chickens and those vaccinated with live IBV may intermittently shed virus for many weeks or even months. Virus infection in layers and breeders occurs cyclically as immunity declines or on exposure to different serotypes.
Signs normally occur after a very short incubation period (18–48 hr). Spread to other birds is rapid, and morbidity may be nearly 100%. The nature and severity of the disease are influenced by the age and immune status of the flock and virulence of the causal strain. Young chickens cough, sneeze, and have tracheal rales for 10–14 days. Wet eyes and dyspnea may be seen, and facial swelling may also occur occasionally, particularly with concurrent bacterial infection of the sinuses. In broiler chickens, IBV infection is a major cause of poor feed conversion, reduced growth rate, and condemnation of meat at processing. In most outbreaks mortality is 5%, although higher mortality rates are observed when secondary bacterial infections complicate te original respiratory viral infection. Nephropathogenic strains can produce interstitial nephritis with high mortality (up to 60%) in young chickens.
In layers, egg production may drop 5–50%, and eggs are often misshapen, thin-shelled, and contain watery albumen. Egg production and egg quality generally return to near normal levels in most birds on recovery.
Respiratory tract lesions include mucoid exudate in the trachea and bronchi, generally without hemorrhage. Caseous plugs may be found in the trachea of young birds. Air sacs are thickened and opaque. Secondary bacterial infections in meat-type birds, especially with coliform bacteria, produce caseous airsacculitis, perihepatitis, and pericarditis. Nephropathogenic strains produce swollen, pale kidneys, with tubules and ureters distended with urates. In layers, urolithiasis is associated with virus infection and certain dietary factors.
Diagnosis cannot be based on clinical signs because of similarities to mild respiratory forms of other respiratory agents such as Newcastle disease, avian metapneumovirus, laryngotracheitis, mycoplasma, and infectious coryza. Seroconversion or a rise in IBV antibody titer shown by ELISA, hemagglutination inhibition, or virus neutralization tests can be used for diagnosis given a history of respiratory disease or reduced egg production. A definitive diagnosis is generally based on virus isolation and identification. Virus can be isolated by inoculation of bacteria-free tissue homogenates of trachea, cecal tonsils, and kidneys into 9- to 11-day-old chicken embryos. Several blind passages of the virus may be necessary for isolation of some field strains. The virus produces embryo stunting, curling, and urate deposits in the mesonephros, with variable mortality.
Because the virus exhibits great antigenic variation, the serotype should be identified if possible. Serotypes are conventionally identified with the aid of known serotype-specific chicken antisera in the virus neutralization test. However, the virus neutralization test is expensive, time consuming, and not readily available; therefore, it is not commonly used. A limited number of serotype-specific monoclonal antibodies (MAb) have been developed for serotyping purposes. However, direct application of MAb-based immunohistochemical procedures for detection of viral antigen in infected chicken tissues is not considered dependable because of the low concentration of the antigen in the tissues. The MAb have been best used after the virus is propagated by passage in chicken embryos, in which case the virus can be detected in the cells associated with the chorioallantoic membranes by immunofluorescence or immunoperoxidase staining, or in the allantoic fluid by ELISA.
Analyses of the viral genome are commonly used to identify the virus serotype. These methods are based on the application of reverse transcriptase PCR (RT-PCR), using specific oligonucleotide primers, to produce DNA copies of IBV genes, usually of the S1 part of the spike glycoprotein gene. Subsequently, the RT-PCR product is subjected to restriction fragment length polymorphism (RFLP) or analyzed by nucleotide sequencing, which is currently the most frequently used test to genotype strains of IBV. For RFLP, the RT-PCR product is digested with a set of specific restriction endonucleases, and the digested nucleic acid fragments are separated by gel electrophoresis. The specific pattern of their separation in the gel is compared with those of the standard strains for identification.
No available medication alters the course of the disease, although antibiotic therapy may reduce mortality due to secondary infections. During cold weather, increasing the temperature in the poultry house and under the hover by 5–10°F (3–5°C) may lower mortality.
Attenuated vaccines used for immunization may produce mild respiratory signs. Live vaccines are initially given to chicks 1–14 days old by spray, drinking water, or eye drop. Revaccination is common. Live or adjuvanted killed vaccines are sometimes used in breeders and layers to prevent egg production losses.
Many serotypes are recognized, and a number of new or variant serotypes have been reported, which pose problems in immunization and diagnosis. If possible, selection of vaccine should be based on knowledge of the prevalent serotype(s) on the premises. The most commonly used live vaccines in the USA contain strains of IBV serotypes Massachusetts, Connecticut, and Arkansas. Vaccination with selected variant serotypes is practiced in some areas. Outbreaks with mortality due to nephritis have been associated with several variant strains in Australia and the USA. Infection with standard as well as variant serotypes has been associated with egg production losses in vaccinated layer flocks.
Last full review/revision March 2012 by Pedro Villegas, DVM, MS, PhD, DACVM, DACPV