Infectious Bronchitis in Chickens

Infectious bronchitis virus (IBV) is an avian gammacoronavirus that causes disease only in chickens; however, the virus has also been found in pheasants and peafowl, which can be subclinically infected. IBV is worldwide in distribution, and many antigenic types can co-circulate in a given region. Some IBV types are widespread; others are regional.

IBV is shed by infected chickens in respiratory discharge and feces. It can be transmitted by aerosol, ingestion of contaminated feed and water, or contact with contaminated equipment and clothing. Naturally infected chickens and those vaccinated with live IBV can shed virus intermittently for up to 20 weeks after infection. The incubation period is generally 24–48 hours, and the peak in excretion of the virus from the respiratory tract lasts 3–5 days after infection.

The severity of disease and the body systems involved are influenced by the following factors:

• strain of the virus

• age, strain, immune status, and diet of the chicken

• ventilation, ammonia concentrations, temperature, and other environmental factors

In addition, coinfection with Mycoplasma gallisepticum, Mycoplasma synoviae, Escherichia coli, and/or Avibacterium paragallinarum can exacerbate disease.

The morbidity rate for flocks affected by infectious bronchitis is typically 100%. Because of excessive mucus in the trachea (see mucus image), chicks can cough, sneeze, and have tracheal rales for 10–14 days. Conjunctivitis and dyspnea can occur, sometimes with facial swelling, particularly when there is concurrent bacterial sinus infection.

Excessive mucus in trachea, chicken

Image

Courtesy of Dr. Pedro Villegas.

Infected chicks can appear listless and huddle together for warmth. Feed consumption and weight gain are decreased. Infection with nephropathogenic strains can lead initially to respiratory signs, then later to lethargy, ruffled feathers, wet droppings, polydipsia, and death.

In layers with infectious bronchitis, egg production can drop by up to 70%, and eggs are often misshapen, with thin, soft, wrinkled, rough, and/or pale shells. Eggs can also be smaller and have watery albumen. Egg production and egg quality can return to normal; however, recovery can take up to 8 weeks. In most outbreaks, the mortality rate is approximately 5%, but it can be as high as 60% when disease is complicated by concurrent bacterial infection or when nephropathogenic strains induce interstitial nephritis.

False layer syndrome, cystic oviduct, layer

Image

Courtesy of Dr. Elise Myers.

Infection of chicks can lead to permanent damage to the oviduct, resulting in cystic oviduct formation and layers or breeders that never reach normal egg production —so-called false layer syndrome (see false layer syndrome image). The oviduct is unable to capture an ovum, even though the ovary is normal and the bird still ovulates.

Lesions

In the respiratory tract of chickens with infectious bronchitis, the trachea, sinuses, and nasal passages can contain serous, catarrhal, or caseous exudates. In addition, air sacs can contain a foamy exudate that leads to cloudy air sac thickening (see airsacculitis image).

Airsacculitis, 4-week-old broiler

Image

Courtesy of Dr. Brian Jordan.

If IBV infection is complicated by infection with E coli, there can be caseous airsacculitis, perihepatitis, and pericarditis. Birds infected when very young can have cystic oviducts, whereas those infected while laying have an oviduct of decreased weight and length and regression of the ovaries.

Nephritis with urate deposition, 5-week-old broiler

Image

Courtesy of Dr. Don Ritter, Poultry Business Solutions, LLC.

Infection with nephropathogenic IBV strains results in swollen, pale kidneys and distention of the tubules and ureters with urates (see nephritis image). Urate deposition results from kidney damage and dehydration and can lead to increased morbidity and mortality rates. In birds with urolithiasis, the ureters can be distended with urates and contain uroliths, and the kidneys can be atrophied.

• Detection of rising antibody titers by ELISA or hemagglutination inhibition (HI) testing

• Virus detection and typing using RT-PCR and sequencing or using RT-qPCR

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, the respiratory form of infectious bronchitis requires laboratory confirmation for diagnosis. Demonstration of seroconversion or an increase in antibody titer against IBV by ELISA, hemagglutination inhibition, or virus neutralization tests can be used for diagnosis when there is a history of respiratory disease or decreased egg production.

Definitive diagnosis of infectious bronchitis is generally based on virus detection and identification. IBV can be isolated by inoculation of homogenates of tracheal, cecal tonsil, and/or kidney tissue into 9- to 11-day-old SPF chicken embryos, and growth of IBV is indicated by embryo stunting and curling and by the deposition of urates in the mesonephros, with variable mortality rates.

Alternatively, IBV can be isolated in tracheal organ cultures, with growth of the virus indicated by cessation of cilial motility. Isolation of some field strains can require several blind passages (sequential rounds of cultures) of the virus. Diagnosis is commonly achieved via reverse transcription PCR (RT-PCR) assays to detect viral RNA in nucleic acid extracts of tracheal, cecal tonsil, or kidney tissue.

Typing viruses is extremely important to diagnose IBV infection outbreaks that are due to 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, this method of serotyping is expensive and time-consuming, so it is not readily available.

Coronaviruses have several structural proteins, the largest of which is called the spike glycoprotein. The S1 region of the spike glycoprotein can be used to determine the genetic type of the virus, which correlates with the virus serotype. RT-PCR products derived from this region can be analyzed by nucleotide sequencing, and the deduced amino acid sequence can then be compared to sequences in GenBank to determine its relatedness to known strains.

Many laboratories now do reverse transcription quantitative real-time PCR (RT-qPCR) typing as well, using assays developed against the S1 gene region of the spike glycoprotein of known strains of IBV circulating in a given geographical region. This method is more rapid, sensitive, and cost-effective than are traditional typing methods.

• Antimicrobials for secondary infections

• Supportive care (eg, adjusting ambient temperature, decreasing protein content in feed, adding electrolytes to water)

• Attenuated live or killed (inactivated) vaccines

No medication alters the course of IBV infection; however, antimicrobial treatment can lower the mortality rate exacerbated by complicating bacterial infections. In cold weather, increasing the ambient temperature also can decrease the mortality rate. In addition, decreasing the protein concentrations in feed and providing electrolytes in drinking water can assist in outbreaks that are due to nephropathogenic strains.

The live, attenuated vaccines used for immunization against IBV can produce mild respiratory signs. These vaccines are initially administered to 1- to 14-day-old chicks by spray, drinking water, or eye drops, and layers and breeders are commonly revaccinated approximately 2 weeks after the initial vaccination. Revaccination with a different serotype can induce broader protection. Additional live, attenuated or killed, adjuvanted vaccines can be used in breeders and layers to prevent egg production losses, as well as to pass protective maternal antibodies to progeny.

There are many distinct types of IBV, and new or variant types that are not fully controlled by existing vaccines are identified relatively frequently. Variant viruses historically arose from mutations accumulating over time as the virus replicated (genetic drift). However, recombination in coronaviruses can result in unique viruses that may not cause disease.

Vaccine selection should be based on knowledge of the most prevalent virus type(s) in a particular geographic area. The correlation between IBV type and protection is imperfect, and selection of the most appropriate vaccine, or combination of vaccines, can require experimental assessment in vivo.

The most commonly used live IBV vaccines worldwide contain derivatives of the Massachusetts-type strains M41, H120, and H52. In addition, several IBV vaccine types, as well as live and killed autogenous vaccines, specific for the variant virus in a given geographical region are licensed for use in various countries.

• American Association of Avian Pathologists: Respiratory Diseases Committee

• Gast RK, Porter RE. Salmonella infections. In: Swayne DE, ed. Boulianne M, Logue CM, McDougald LR, Nair V, Suarez DL, assoc eds. Diseases of Poultry. 14th ed. Wiley-Blackwell; 2020:717-753.

• Brugère-Picoux J, Vaillancourt J-P, Shivaprasad HL, Venne D, Bouzouaia M, eds. Manual of Poultry Diseases. AFAS; 2015.