Infectious bronchitis virus (IBV) is an avian coronavirus that has had significant impacts on the poultry industry since the 1930’s. While primarily replicating and causing disease in the upper respiratory tract of chickens, IBV can also affect kidneys and gonads. Infected chickens shed the virus in both feces and respiratory droplets for up to 20 weeks (Alexander and Gough, 1977). It can remain in the environment for up to 56 days where it can infect more birds via ingestion of contaminated feed, water, and litter (Cavanagh, 2008). Similarly to other coronaviruses, IBV is highly transmissible and rapidly mutates. This results in the continuous formation of new IBV strains, or variants, making it very difficult to achieve sufficient protection within a flock. Some of these variants are worldwide in distribution while others remain regional.

Clinical signs can vary depending on the specific virus strain as well as age, breed, and immune status of the chicken. Once introduced to a flock, morbidity typically reaches 100% with respiratory symptoms lasting for up to 2 weeks (Ignjatović and Sapats, 2000). Initial respiratory signs include coughing, tracheal rales, and sneezing while appearing depressed with ruffled feathers. Feed consumption then falls with subsequent reduction in weight gain, increased water intake, and potential death. Mortality ranges from 5-60% depending on secondary bacterial infections, E. coli being the most common, due to a weakened immune system (Bhuiyan et al., 2021). In layers and breeders, egg production drops significantly and is accompanied by poor egg quality. Egg shells are often wrinkled, very thin, misshaped, and discolored. 

In addition to respiratory disease, some strains of IBV can have a significant impact on the kidneys as well as gonad formation e.g., ovary and testes. This is particularly a problem in layers and breeders where we rely on their ability to produce eggs. Young chickens infected with this unique strain of IBV, DMV/1639, can develop nephritis and accumulate urate deposits in the kidneys (Jordan, 2022). This kidney inflammation leads to flushing, dehydration, and predisposition to secondary bacterial infections. Early infection can also damage the oviduct leading to a condition called false layer syndrome (FLS) once sexually mature (Jordan, 2022). Hens with FLS have large, fluid filled cysts in the oviduct resulting in layers never coming into production.

Presumptive diagnosis can be made based on clinical signs and necropsy findings, but further diagnostics are required due to similarities with other respiratory diseases. Commonly used diagnostics include viral isolation, serology, and molecular techniques (Legnardi et al., 2020). Viral isolation is not routinely performed due to cost and lengthy turnaround time, but it is considered the gold standard. Serology tests, such as ELISA, are a great monitoring tool to track antibodies in flocks over time but must be taken at multiple time points for accurate results. Molecular techniques, such as RT-PCR, are most commonly used because of their high sensitivity and quick turnaround time but cannot differentiate between past or present challenge. Each diagnostic test has its strengths and weaknesses and must be carefully chosen based on the results desired.

Prevention measures are key to controlling the spread of IBV due to its highly infectious nature. Vaccines are widely available and commonly used across all sectors of the poultry industry. IBV vaccines can be administered to chicks 1-14 days of age via spray, drinking water, or eye via droplet and typically revaccinated weeks later (Davelaar and Kouwenhoven, 1980). In general, the vaccine must be the same strain as the field challenge to provide proper immunity. Most vaccines offer limited cross protection, so it is important to choose the correct vaccine type depending on the most prevalent strain in the area.

In conclusion, IBV is a highly contagious respiratory disease impacting poultry health across the globe. Just like other viruses in the coronavirus family, new variant strains are still emerging due to its high capacity for mutation and recombination. This unique quality makes IBV particularly difficult to control via standard biosecurity practices and vaccine programs. Surveillance and identification of the different variants are important for the long-term success in managing this disease.