Overview of Infectious Bronchitis in Poultry
Infectious bronchitis is an acute, highly contagious disease of major economic importance in commercial chicken flocks throughout the world. It is usually characterized by respiratory signs, although decreased egg production and poor egg quality are sometimes seen in breeders and layers. Some strains of the etiologic agent, infectious bronchitis virus (IBV), are nephropathogenic, causing interstitial nephritis, particularly in chicks. Associations with myopathy and proventriculitis have also been reported.
IBV is a coronavirus that only causes disease in chickens, although some other birds may be subclinically infected. Some serotypes are geographically restricted, but multiple serotypes commonly cocirculate in one geographic region. In recent years, a novel IBV genotype, the QX strain, has become increasingly common in Asia and Europe. 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 wk after infection. The incubation period is generally 24–48 hr, 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 the strain of the virus; the age, strain, immune status, and diet of the chickens; and cold stress. In addition, coinfection with Mycoplasma gallisepticum, Mycoplasma synoviae, Escherichia coli, and/or Avibacterium paragallinarum can exacerbate disease.
Morbidity is commonly close to 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, rough, and/or pale shells, and can be smaller and have watery albumen. In most cases, egg production and egg quality return to normal, but this may take up to 8 wk. In most outbreaks mortality is 5%, although mortality rates are higher when disease is complicated by concurrent bacterial infection. Nephropathogenic strains can induce interstitial nephritis with high mortality (up to 60%) in young chicks. Infection of young chicks may cause permanent damage to the oviduct, resulting in layers or breeders that never reach normal levels of production.
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 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 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. More rapid diagnosis may be achieved using reverse transcriptase-polymerase chain reaction (RT-PCR) assays to detect viral RNA in nucleic acid extracts of tracheal, cecal tonsil, or kidney tissue.
Typing viruses can help distinguish vaccine and field strains and may help 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. A restricted range of serotype-specific monoclonal antibodies (MAb) have been developed for serotyping, but direct detection viral antigen using these MAbs to immunohistochemically stain tissue sections from diseased birds is of limited value because of the low concentration of antigen in tissues. The MAbs have been best used after propagation in chicken embryos, to detect viral antigen in the chorioallantoic membranes by immunofluorescence or immunoperoxidase staining, or in the allantoic fluid by ELISA. Analyses of the products of RT-PCR assays are now commonly used to identify the virus serotype and to identify individual strains within serotypes. The S1 region of the spike glycoprotein gene determines the serotype, and RT-PCR products derived from this region can be subjected to restriction fragment length polymorphism analysis, analyzed by nucleotide sequencing, or compared with reference strains using high-resolution melting curve analysis. Genotype determination based on the S1 region can be complemented by analyzing other regions of the viral genome, including the nucleocapsid gene and the 5' untranslated region. These analyses can also aid in rapid detection of novel recombinant IBVs.
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 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. Revaccination with a virus from a distinct serotype can induce broader protection. Attenuated or adjuvanted inactivated vaccines can be used in breeders and layers to prevent egg production losses.
There are many distinct serotypes of IBV, and new or variant serotypes, which are not fully controlled by existing vaccines, are identified relatively frequently. Some variants may be derived from recombination between existing field strains and vaccine strains, whereas others result from point mutations in existing strains. Selection of vaccines should be based on knowledge of the most prevalent serotype(s) on the premises. The correlation between serotype and protection is imperfect, and definition of the most appropriate vaccine, or combination of vaccines, may require experimental assessment of several combinations of vaccines to identify the most effective regimen. The most commonly used live vaccines in the USA contain derivatives of the Massachusetts, Connecticut, and Arkansas strains, whereas in Australia, where the most prevalent serotypes are distinct from most other countries, vaccines are based on derivatives of the VicS and Armidale strains. In Europe, vaccines incorporating derivatives of the 4/91 strain and those derived from QX-like viruses are available. Vaccination with selected variant serotypes may be of use when these variants are the dominant strain in flocks, although regulatory authorities in some countries only permit use of vaccines derived from the Massachusetts strain.