Infectious Bronchitis

Infectious bronchitis( seen worldwide) is an acute, rapidly spreading, viral disease of chickens identified by respiratory signs, decreased egg production, and poor egg quality so it is of major economic importance . Some strains of the causative virus, infectious bronchitis virus (IBV), are nephro-pathogenic resulting in significant mortality.

Etiology and Epidemiology:

IBV, a coronavirus has numerous serotypes. Simultaneously  Two or more serotypes may be seen in one geographic area. 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 periodically shed virus for many weeks or even months. Virus infection in layers and breeders is seen when 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%. Young chickens cough, sneeze, and have tracheal rales for 10-14 days. Wet eyes and dyspnea may be seen, some tiems  facial swelling may be there, along 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. Nephropathogenic strains can produce interstitial nephritis with high mortality (up to 60%) in young chickens. In most outbreaks generally mortality is 5%, except cases with secondary bacterial infections.

 
Infectious bronchitis, wrinkled egg shells

  Infectious bronchitis, airsacculitis, chicken

     
 
     
In layers, egg production may drop 5-50%, and eggs are often misshapen, thin-shelled, and contain watery albumen, On recovery, Egg production and egg quality generally return to near normal levels.

  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:

Diagnosis can not 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 IBV antibody titer shown by ELISA, hemagglutination inhibition, or virus neutralization tests  in flocks with 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(It is impractical because it is expensive, time consuming,not easily available). A limited number of serotype-specific monoclonal antibodies (MAb) have been developed for serotyping purposes which again has limitation because of the low concentration of the antigen( mandatory requirement) 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 for the purpose of identifying the virus serotype are now commonly used.

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 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. Re-vaccination 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. Vaccination with selected variant serotypes is practiced in some areas. Outbreaks with mortality due to nephritis have been associated with several variant strains. Infection with standard as well as variant serotypes have been associated with egg production losses in vaccinated layer flocks.

Reference :- Merck Veterinary Manual

 
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