Infectious bronchitis virus (IBV), a highly contagious upper respiratory tract disease, has a devastating impact on flock health and the economics of commercial poultry producers, noted Brian Jordan, PhD, associate director of outcomes research at Zoetis, during an American Association of Avian Pathologists webinar about the challenges and promises of IBV serotyping with quantitative polymerase chain reaction (qPCR) and sequencing.
T.J. Gaydos, DVM, owner of GTS Health, and Rodrigo Gallardo, DVM, PhD, DVM, professor of population health and reproduction at the University of California-Davis School of Veterinary Medicine, also served as panelists for the webinar.
Vaccines are available and effective, Jordan said, but IBV surface proteins vary widely, requiring specific vaccines and antibodies.
To design an effective strategy to fight IBV, performing surveillance and identifying specific strains are necessary. Serotyping and the use of diagnostic testing like reverse transcriptase qPCR (RT-qPCR) are used to identify specific IBV variants and ensure vaccination programs are effective.
IBV symptoms
Chickens with IBV can exhibit nephritis, decreased egg production and egg quality, as well as respiratory symptoms.
IBV causes mucus to accumulate in the birds’ trachea. Chicks can have trouble breathing and develop tracheal rales. Coughing and sneezing are also common. Infected chicks eat and drink less, are often listless, have ruffled feathers and huddle together for warmth.
Other symptoms include conjunctivitis, inflammation and/or infection of the outer membrane of the eyeball and inner eyelid and even facial swelling, especially with secondary bacterial sinus infections.
Jordan noted that egg production in layers with IBV can drop by 70%. Eggs can be smaller and are often misshapen with wrinkled, rough, soft, thin or pale shells. Although egg quality and production can return to normal, recovery can require 8 weeks. Unfortunately, IBV infection in chicks can damage the oviducts permanently so that normal egg production is never achieved.
The morbidity rate for flocks affected by infectious bronchitis is typically 100%. In most outbreaks, the mortality rate is approximately 5%, but it can be as high as 60% when the disease is complicated by a concurrent bacterial infection or when nephropathogenic strains induce interstitial nephritis, Jordan noted.1
Infected chickens shed the virus through respiratory discharge and feces. The virus can be transmitted by aerosol, ingestion of contaminated feed and water, and contact with contaminated equipment.
Chickens vaccinated with live IBV and those with naturally occurring infections can intermittently shed the virus for up to 20 weeks post-infection, accounting for the virus being widely transmissible.
Jordan pointed out that disease symptoms and severity are influenced by several factors:
- Virus strain
- Age
- Immune and nutritional status of birds
- Environmental factors, such as ventilation, ammonia concentrations and temperature
- Coinfections
IBV diagnosis
The Merck Veterinary Manual states that, “Because of similarities to mild forms of disease caused by agents such as Newcastle disease virus, avian metapneumovirus, infectious laryngotracheitis virus, mycoplasmas, Avibacterium paragallinarum and Ornithobacterium rhinotracheale, the respiratory form of infectious bronchitis requires laboratory confirmation for diagnosis.”2
When there is a history of respiratory disease or decreased egg production, detecting rising antibody titers by enzyme-linked immunosorbent assay or hemagglutination inhibition testing can indicate IBV. However, “diagnosis is commonly achieved via reverse transcriptase PCR (RT-PCR) assays to detect viral RNA in tracheal and cecal tonsil swabs or kidney tissue,” Jordan said.
He cautioned that the virus is fragile and easily destroyed by disinfectants, sunlight, heat and other environmental factors. And, control of this variable virus hinges on surveillance and the identification of specific variants, which is then used to develop an effective vaccination strategy.
Roles of surveillance, serotyping
The goal of surveillance is to provide enough information to design preventive programs, Jordan said. Real-time PCR is the gold standard for gene quantification, but it’s used for more than virus detection. PCR is also used to measure vaccine effectiveness by identifying viruses present after vaccination.
The percentage of infection can also be determined by taking multiple samples in a house and in complexes. Gaydos pointed out that qPCR is useful in field monitoring because it can identify the specific viruses present in an area and determine how vaccinations affect the virus prevalence. Once a base of information is established, he explained, surveillance can alert us to changing disease pressures.
IBV treatment and control
There are no medications that alter the course of IBV. However, decreasing the number of chickens that die from IBV is possible.
Treatment and control revolve around utilizing effective vaccines and implementing supportive care measures such as adding electrolytes to water, decreasing protein concentration in feed, adjusting the ambient temperature of houses and eliminating secondary infections.3
IBV is particularly challenging to vaccinate against due to new variants appearing and the wide variety of distinct types of IBV. Jordan suggests that selecting a vaccine, or a combination of vaccines, should be based on the most prevalent virus types in a geographic region.
PCR pros and cons
PCR testing plays a significant role in helping control IBV and other diseases, but it also has drawbacks.
RT-qPCR pros:
- Very sensitive
- Very specific – detects only the target
- Rapid – results in under 24 hours
- Scalable equipment – able to test 96 to 384 samples
- Can be automated
- Relatively inexpensive
- Able to build databases of information
RT-qPCR cons:
- Will not detect novel or mutated sequences
- A new assay must be designed for each new virus, increasing cost and time
- Detects the genome but doesn’t necessarily indicate live or disease-causing virus
- Is only semi-quantitative – data trends can be compared across data sets or labs but not individual cycle-threshold values (Ct)
- Ct values must be interpreted as trends and in context
- Requires a well-equipped lab
- Interpretation requires experience, skill and correct data
Data interpretation is key to arriving at the correct conclusions. Although RT-qPCR is quicker, easier, less costly and can detect viruses that are otherwise undetectable with other methods, Jordan pointed out that the data needs to be taken with a grain of salt.
“Single reports or positives may indicate something, but trends over time and across tests are needed to analyze the results. And even experienced diagnosticians can be fooled without all or incorrect context. ”
However, RT-qPCR is a great tool to build a database of information on poultry complexes and help us combat difficult diseases like IBV, he said.
References
- Infectious bronchitis in chickens. https://www.merckvetmanual.com/poultry/infectious-bronchitis/infectious-bronchitis-in-chickens
- Ibid.
- Ibid.
