167 Diseases of Poultry, Fourteenth Edition. Editor-in-chief David E. Swayne. © 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc. Summary Agent, Infection, and Disease. Infectious bronchitis is caused by the avian coronavirus, infectious bronchitis virus (IBV), which is found worldwide. Infections, depending on the strain, may cause an acute upper‐respiratory tract disease, drops in egg production, decreased egg quality, and nephritis. The virus is transmitted by inhalation or direct contact with contaminated objects and morbidity is usually 100%; whereas, mortality can vary depending on a variety of host factors and the strain of the infecting virus. Chickens are the primary host but the virus has also been found in pheasants and peafowl. There exist many different antigenic types of the virus and due to a high rate of mutation, new antigenic variants constantly emerge. As a general rule, different antigenic types and variants have little to no cross‐protection. Diagnosis. The preferred diagnostic test is molecular detection of the viral spike (S1) gene or virus isolation in embryonating chicken eggs. Multiple antigenic types of the virus are identified by sequence analysis of the S1 gene or by the virus neutralization test using serotype‐ specific antibodies. The disease can also be diagnosed by demonstrating rising antibody titers against IBV between preclinical and convalescent sera. Intervention. Attenuated live and killed vaccines are used in an attempt to control the disease. However, multiple different antigenic types and constantly emerging new types that do not cross‐protect make it difficult to prevent transmission and disease. Introduction Definition. Avian infectious bronchitis (IB) is an economically important, highly contagious, acute, upper‐respiratory tract disease of chickens and other fowl, caused by the avian gammacoronavirus infectious bronchitis virus (IBV). The virus is found worldwide and is transmitted by inhalation or direct contact with infected birds or contaminated litter, equipment, or other fomites. Vertical transmission of the virus within the embryo has not been reported, but virus may be present on the shell surface of hatching eggs via shedding from the oviduct or gastrointestinal tract. A poor viral polymerase proofreading mechanism resulting in genetic mutations and genome recombination events can lead to the emergence of new serotypes of the virus, which do not cross protect, complicating control by vaccination. Avian coronaviruses similar to IBV also have been found in pheasants (Phasianus colchicus) and peafowl (Galliformes) and IBV‐like viruses have been isolated from turkeys, teal (Anas crecca), geese (Anserinae), pigeons (Columbiformes), guinea fowl (Numida melea- gris), partridge (Alectoris), and ducks (Anseriformes) (28). The turkey coronaviruses (TCoV) are described in Chapter 12. Gammacoronaviruses similar to IBV have also been detected in wild birds (71, 92, 138). Economic Significance The disease is characterized by respiratory signs, reduced weight gain, and reduced feed efficiency in meat‐type broiler chickens infected with the virus. Infection also predisposes broilers to secondary opportunistic bacte- rial infections that can result in airsacculitis, pericarditis, and perihepatitis. Morbidity is almost always 100%, but mortality can vary depending on the age and immune status of the birds, the strain of the virus, and if second- ary bacterial or viral pathogens are involved. Some strains of IBV are nephropathogenic and can cause high mortality due to kidney failure in susceptible birds. In layer and breeder chickens, infection may result in reduced egg production of up to 70% and declines in egg- shell quality. The virus can replicate in the oviduct and cause permanent damage in young hens resulting in lim- ited egg production over a prolonged period of time and birds that fail to come into production (false layers). Eggs 4 Infectious Bronchitis Mark W. Jackwood and Sjaak de Wit