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Infectious bronchitis is an acute, rapidly spreading, viral disease of chickens characterized by infection of respiratory, urogenital, and GI tract tissues.
Etiology and Epidemiology: The causal coronaviruses are found worldwide and exist as numerous serotypes. Two or more serotypes may occur simultaneously in one geographic region. The virus is present in respiratory discharges and feces and on contaminated eggshells. It is spread by droplets through the air; by ingestion of contaminated feed and water; and by contact with infected chickens, contaminated equipment, and clothing of caretakers. Chickens infected with some strains excrete virus in the feces for =1 mo after clinical recovery. Virus infections in layers and breeders occur cyclically as immunity declines, or on exposure to different serotypes.
Clinical Findings: Signs occur after an incubation period of 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 have coughing, sneezing, and tracheal rales for 10-14 days. Wet eyes and dyspnea may be seen. Facial or head swelling is seen occasionally, particularly in concurrent coliform infections of the sinus and turbinate mucosa. Nephrogenic strains can produce interstitial nephritis with high mortality (up to 60%) in young chickens. In most outbreaks, however, mortality is <5%, although secondary bacterial infections cause higher losses in meat-type chickens.
In layers, egg production may drop 5-50%, and eggs are often misshapen, thin-shelled, and contain watery albumen.  Egg production and quality generally return to near normal levels in most birds on recovery. Poor-producing birds are culled.
Lesions: Respiratory tract lesions include mucoid exudate in the trachea and bronchi, generally without hemorrhage. A caseous plug may be found in the trachea of a young bird. Air sacs are thickened and opaque. Secondary bacterial infections in meat-type birds produce caseous airsacculitis, perihepatitis, and pericarditis. Nephrogenic 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: Diagnosis cannot be based solely on clinical signs because of similarities to mild respiratory forms of Newcastle disease, laryngotracheitis, and infectious coryza. Seroconversion or a rise in infectious bronchitis virus 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 isolation can be made 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 (MAbs) have been developed for serotyping purposes. Direct application of MAb-based immunofluorescence or immunoperoxidase 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 MAbs 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 for the purpose of identifying the virus serotype have also been introduced. One such method that is now available for viral serotyping is restriction fragment length polymorphism analysis. In this procedure, the spike glycoprotein gene of a viral isolate is first amplified by reverse transcriptase polymerase chain reaction (RT-PCR) using specific oligonucleotide primers. Subsequently, the RT-PCR product is digested with a set of specific restriction enzymes, and the digested nucleic acid fragments are separated by gel electrophoresis. The specific pattern of their separation in the gel is used to identify the serotype of the virus.
Control: No available medication alters the course of the disease, although antibiotic therapy may reduce mortality due to secondary infections. Increasing the temperature in the house and under the hover by 5-10°F (3-5°C) may lower mortality.
Attenuated vaccines used for immunization may produce relatively mild respiratory signs. Live vaccines are initially given to chicks 1-14 days old by spray, drinking water, or eyedrop. 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 the knowledge of the prevalent serotype(s) on the premises. 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 have been associated with egg production losses in vaccinated layer flocks.
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