Download Infectious Bronchitis Infectious bronchitis is an acute, rapidly

Infectious Bronchitis
Infectious bronchitis is an acute, rapidly spreading, viral
disease of chickens characterized by respiratory signs,
decreased egg production, and poor egg quality. 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 clothng 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 .
Clinical Findings :
Signs occur after an incubation period of18-48 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 ccasionally, 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. Nephropathogenic strains can produce
interstitial nephritis with high mortality (up to 60%) in young
chickens. In most outbreaks, however, mortality is5%,
although secondary bacterial infections may cause higher
losses .
In layers, egg production may drop5-50%, 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 .
Lesions :
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 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 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 for the purpose of identifying
the virus serotype are now commonly used. 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 S 1part of the spike
glycoprotein gene. Subsequently, the RT-PCR product is
subjected to restriction fragment length polymorphism (RFLP)
or analyzed by nucleotide sequencing. 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 .
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 5-10F (3-5°C) may lower
mortality .
Attenuated vaccines used for immunization may produce mild
respiratory signs. Live vaccines are initially given to chicks 114 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 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
have been associated with egg production losses in
vaccinated layer flocks .