Infectious bronchitis virus (IBV) is an avian gammacoronavirus that only causes disease in chickens, although the virus has also been found in pheasants and peafowl, which may be subclinically infected. The virus is worldwide in distribution, and there are many antigenic types that can cocirculate in a given region. Some IBV types are widespread, whereas others are regional.
IBV is shed by infected chickens in respiratory discharges and feces, and it can be spread by aerosol, ingestion of contaminated feed and water, and contact with contaminated equipment and clothing. Naturally infected chickens and those vaccinated with live IBV may shed virus intermittently for up to 20 weeks after infection. The incubation period is generally 24–48 hours, with the peak in excretion of virus from the respiratory tract lasting 3–5 days after infection.
The severity of disease and the body systems involved are influenced by:
In addition, coinfection with Mycoplasma gallisepticum, M synoviae, Escherichia coli, and/or Avibacterium paragallinarum can exacerbate disease.
Morbidity for flocks affected by infectious bronchitis is typically 100%. Chicks may cough, sneeze, and have tracheal rales for 10–14 days. Conjunctivitis and dyspnea may be seen, and sometimes facial swelling, particularly with concurrent bacterial infection of the sinuses. Chicks may appear depressed and huddle under heat lamps. Feed consumption and weight gain are reduced. Infection with nephropathogenic strains can cause initial respiratory signs, then later depression, ruffled feathers, wet droppings, greater water intake, and death.
In layers, egg production may drop by as much as 70%, and eggs are often misshapen, with thin, soft, wrinkled, rough, and/or pale shells, and can be smaller and have watery albumen. Egg production and egg quality can return to normal, but this may take up to 8 weeks. In most outbreaks, mortality is approximately 5%, although mortality rates can be as high as 60% when disease is complicated by concurrent bacterial infection or when nephropathogenic strains induce interstitial nephritis in chicks. Infection of chicks may cause permanent damage to the oviduct, resulting in layers or breeders that never reach normal levels of production, so-called false layer syndrome.
In the respiratory tract, the trachea, sinuses, and nasal passages may contain serous, catarrhal, or caseous exudates, and the air sacs a foamy exudate initially, progressing to cloudy thickening. If complicated by infection with E coli, there may be caseous airsacculitis, perihepatitis, and pericarditis. Birds infected when very young may have cystic oviducts, whereas those infected while in lay have an oviduct of reduced weight and length and regression of the ovaries. Infection with nephropathogenic strains results in swollen, pale kidneys, with the tubules and ureters distended with urates; in birds with urolithiasis, the ureters may be distended with urates and contain uroliths, and the kidneys may be atrophied.
Laboratory confirmation is required for diagnosis of respiratory forms of infectious bronchitis because of similarities to mild forms of disease caused by agents such as Newcastle disease virus, avian metapneumovirus, infectious laryngotracheitis virus, mycoplasmas, A paragallinarum, and Ornithobacterium rhinotracheale. Demonstration of seroconversion or a rise in antibody titer against IBV by ELISA, or hemagglutination inhibition or virus neutralization tests can be used for diagnosis when there is a history of respiratory disease or reduced egg production.
Definitive diagnosis is generally based on virus detection and identification. Virus can be isolated by inoculation of homogenates of tracheal, cecal tonsil, and/or kidney tissue into 9- to 11-day-old SPF chicken embryos, with growth of IBV indicated by embryo stunting and curling and by deposition of urates in the mesonephros, with variable mortality. Alternatively, IBV may be isolated in tracheal organ cultures, with growth of virus indicated by cessation of cilial motility. Several blind passages of the virus may be necessary for isolation of some field strains. Diagnosis is commonly achieved using reverse transcriptase PCR assays to detect viral RNA in nucleic acid extracts of tracheal, cecal tonsil, or kidney tissue.
Typing viruses is extremely important to diagnose outbreaks caused by serotypes distinct from those of the vaccines used in a flock. Serotypes have been identified using sera from SPF chickens inoculated with known serotypes in virus neutralization tests. However, because this is expensive and time-consuming, it is not readily available. The S1 region of the spike glycoprotein can be used to determine the genetic type of the virus, which correlates with the virus serotype. RT-PCR products derived from this region can be analyzed by nucleotide sequencing, then the deduced amino acid sequence compared to sequences in GenBank to determine its relatedness to known strains.
No medication alters the course of IBV infection, although antimicrobial therapy may reduce mortalities caused by complicating bacterial infections. In cold weather, increasing the ambient temperature may reduce mortalities, and reducing the protein concentrations in feed and providing electrolytes in drinking water may assist in outbreaks caused by nephropathogenic strains.
The live-attenuated vaccines used for immunization may produce mild respiratory signs. These vaccines are initially given to 1- to 14-day-old chicks by spray, drinking water, or eye drop, and birds are commonly revaccinated approximately 2 weeks after the initial vaccination. Revaccination with a different serotype can induce broader protection. Attenuated or adjuvanted inactivated vaccines can be used in breeders and layers to prevent egg production losses as well as to pass protective maternal antibodies to progeny.
There are many distinct types of IBV, and new or variant types, which are not fully controlled by existing vaccines, are identified relatively frequently. Variant viruses historically arise from mutations accumulating over time as the virus replicates (genetic drift). However, recombination can occur in coronaviruses and may result in unique viruses that may or may not cause disease. Selection of vaccines should be based on knowledge of the most prevalent virus type(s) in the area. The correlation between IBV type and protection is imperfect, and selection of the most appropriate vaccine, or combination of vaccines, may require experimental assessment in vivo.
The most commonly used live vaccines worldwide contain derivatives of the Massachusetts strain (Mass41, H120 and H52). In addition, there are a number of different IBV vaccine types licensed for use in various countries as well as live and killed autogenous vaccines specific for the variant virus in the region.
Infectious bronchitis is caused by an avian coronavirus.
The ability of the virus to quickly mutate requires constant surveillance to identify IBV types circulating in a specific region.
Different antigenic types do not cross-protect, making it extremely important to choose the appropriate vaccine(s) for protection.