Pathogenecity of the dominant serotype 8 FAdV of Ontario, Part 2

Fowl adenoviruses (FAdVs) are common in the poultry environment. Although adenoviruses have been associated with numerous clinical conditions, including inclusion body hepatitis (IBH), their primary role in disease is still not unequivocal. To learn more about this adenovirus, Dr. Éva Nagy and her research team at the University of Guelph decided to study the pathogenesis of a serotype 8 virus (FAdV-8). This virus was isolated by the Animal Health Laboratory (AHL) at the University of Guelph from an outbreak that occurred at an Ontario broiler farm and was diagnosed as IBH.

An experimental trial in specific pathogen-free chickens (SPF) was conducted using oral and intramuscular (im) inoculations, the latter being the normal and widely accepted route for virus administration in research settings. Because horizontal spread of the virus among chickens is mainly related to the oral-fecal contact, the researchers decided to perform oral inoculation as well, to mimic the more natural way of infection.

Their findings? Clinical signs or pathologic changes well known for IBH were not seen in the inoculated chickens. Intranuclear inclusion bodies (the classical characteristics of IBH in liver cells) were not present in any of the infected chickens. Despite the lack of clinical signs and pathological changes, the virus was present in tissues and cloacal swabs of all of the inoculated chickens. Quantitative real-time PCR (described in Part 1, found in the April 2010 issue of Canadian Poultry) was employed to establish viral copy numbers in liver, bursa of Fabricius and cecal tonsil.

Viral DNA was detected in 95.1 per cent of the collected tissues, the 4.9 per cent negative samples originated from the bursa of Fabricius. The cecal tonsil was the organ with the highest number of viral copies through the entire study period in both, orally and intramuscularly inoculated chickens. The presence of virus in cloacal swabs was also identified, and virus shedding occurred throughout the entire study period – 28 days in both, oral and im groups – although at later days the percentage of shedders decreased. The orally inoculated chicks shed the virus in higher titer than chicks inoculated im.

Since the disease was not reproducible in SPF chickens and under experimental conditions, the primary role of FAdVs remains non-equivocal. Other conditions, such as environment, diet and co-infection with other agents, might compromise the health status of the birds.

Overall, the very long virus shedding in feces as determined in this study is a continuous source of virus in the barn environment. In addition to the hardy nature of FAdVs, this maintained viral transmission in and among barns and flocks makes FAdV eradication from commercial flocks very difficult, if not impossible.

Enhancing Immune Response
Lactic acid-producing bacteria (LAB) are a group of microbes that can be found in the intestines of mammals and avian species. Several LAB, including lactobacilli, have been found to have probiotic activities. Some lactobacilli are normal members of the intestinal community of microbes and confer upon their host health benefits, including control of disease-causing enteric microbes and enhancement of host immune response.

In the poultry industry, some practices, such as the prophylactic use of antibiotics, alter the composition of the intestinal microbial community and may limit the ability of commensal microbes to control disease-causing microbes, such as Salmonella, a costly foodborne illness.  Therefore, it has become an urgent matter for the poultry industry to develop strategies for control of transmission of foodborne pathogens from poultry to humans.

Probiotics containing lactobacilli offer one such strategy. Probiotics help to control enteric pathogens, and lactobacilli possess immunomodulatory activities. However, the immune-enhancing mechanisms of probiotics in the chicken have not been studied in depth. More importantly, a very limited number of probiotic products are currently available with proven immune-enhancing capabilities in chickens.

Dr. Shayan Sharif and his research team at the University of Guelph conducted a series of studies to screen commensal lactobacilli present in the chicken gut microbiota, to identify isolates that have superior immunomodulatory activities. First, they examined changes in intestinal microbes after treatment of chickens with antibiotics that are used routinely as growth promotants; namely, bacitracin and virginiamycin. They then looked into whether changes in gut microbial composition by treatment with antibiotics can affect generation of antibodies in chickens. Finally, they collected isolates of L. reuteri, L. salivarius and L. acidophilus, further characterized them, and used them in a cell culture model to gain a better understanding of the way they can affect immune response of chickens.

Their findings? Lactobacillus salivarius was significantly inhibited by a high dose of virginiamycin, but enriched by a low dose of virginiamycin or bacitracin. Lactobacillus acidophilus and Lactobacillus reuteri were not significantly affected by antibiotic therapy. Virginiamycin, and potentially L. salivarius, were implicated in enhancing antibody responses to some antigens in chickens. It was also demonstrated that L. acidophilus is effective at inducing a pro-inflammatory response, while L. salivarius induces an anti-inflammatory response.

Moreover, when chickens were treated with the above lactobacilli alone or in combination, it was discovered that L. salivarius or L. reuteri treated birds had significantly more serum antibodies to a selected group of antigens compared to the birds that were not treated with probiotics. Cell-mediated immune responses were also induced more significantly in chickens that received L. reuteri.

Overall, these studies have identified a few promising lactobacilli strains that may be used for enhancement of immune response in chickens. These findings should lead to the development of defined and rational probiotic products that have superior immune-enhancing activities and are devoid of undesirable properties, such as carrying antimicrobial resistance genes.