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Beware of new reovirus infection
The infection, which has been identified on poultry operations throughout the world, is due to a unique reovirus serotype identified in recent years. It’s called enteric reovirus strain (ERS) because it is generally found in broilers with malabsorption syndrome or wet litter. Recently, ERS has also been isolated from layers with wet litter and decreased production.

“If you see signs of reovirus in flocks that have been vaccinated with traditional reovirus vaccines, suspect ERS,” says Dr. Rik Koopman, a veterinarian with Intervet/Schering-Plough Animal Health, Boxmeer, the Netherlands. “In fact, think of ERS in all cases of malabsorption syndrome.”

Besides high mortality in affected flocks, as well as malabsorption syndrome that causes retarded growth and poor flock uniformity, ERS has been associated with lameness (tenosynovitis), viral arthritis, hepatitis and, more recently, neurological signs, he says. “ERS is very costly for producers because of the high mortality, poor flock performance and increased condemnation rates,” he adds.

ERS was first isolated after serious disease outbreaks in the late 1990s among broiler flocks on different farms in Poland, Koopman explains. The outbreaks were surprising because, in most cases, the parent stock had been well-vaccinated for reovirus with both live and inactivated reovirus vaccines.

Affected birds had difficulty walking and exhibited signs of malabsorption, such as slow growth and poor uniformity. Mortality was high — up to 70% in some flocks — and occurred at a young age, he says.

Novel strain identified

Intervet/Schering-Plough Animal Health took swift action and sent samples to its diagnostic lab, where Dr. Adriaan van Loon and colleagues characterized the virus with a panel of monoclonal antibodies. They found that the Polish isolate had a distinct panel pattern when compared to other known reovirus strains described in the literature.

In addition, the scientists could not neutralize the novel strain with antibodies against well-known strains such as those included in inactivated commercial reovirus vaccines. Interestingly, antibodies induced by the newly discovered ERS could neutralize well-known reovirus strains, van Loon and associates reported in a published article (The Veterinary Quarterly, 2001; 23(3)129-33).

When the scientists challenged 1-day-old, specific-pathogen-free birds with the reovirus, mortality was 100%. Older birds that were challenged had lower mortality, demonstrating that resistance to the disease increases with age and that younger birds are more susceptible, van Loon and colleagues wrote. Table 1 shows the percentage of mortality among birds of various ages that were infected with ERS by various routes.

The scientists also found that experimental infection with ERS resulted in slower growth. Although they were able to isolate the virus from many different organs for 3 to 7 days after infection, ERS seemed to prefer the intestines and pancreas.

After the initial outbreak of ERS in Poland, Koopman says, scientists have used field screening to monitor the strain, which so far has also been found in other countries in Latin America, the Middle East, Southeast Asia, Africa and throughout Europe.

For example, in Belgium between August 2001 and October 2006, ERS was diagnosed on 21 of 70 farms that had submitted chicks for necropsy because they had disease problems, Peter De Herdt and colleagues of Intervet/Schering-Plough Animal Health say in a published paper (The Flemish Veterinary Journal, 2008; 77:171-176).

Of the 21 broiler flocks that appeared to be infected with ERS, 19 were from breeders that had been vaccinated with currently available reovirus vaccines.

ERS isolates from the Belgian flocks were consistently obtained from the liver, pancreas and/or intestinal tract. Clinical signs were similar in all affected flocks; the primary complaint was retarded growth, followed by wet litter and lameness. Increased mortality occurred in over 50% of the flocks, and of the 21 infected farms, the clinical signs had been showing up in multiple, successive rounds on 17 of the farms and for over 1 year on nine of them, the investigators write.

Concurrent infections found

An interesting finding in Belgium was the presence of concurrent infections with other pathogens in 10 of the 21 ERS-infected flocks. These infections included the bacterium Escherichia coli and fowl adenoviruses, which manifested primarily as bacterial polyserositis and septicemia.

“This may indicate that the increased mortality in ERS infections under field conditions is especially important in flocks that at the same time are suffering from concurrent infections,” De Herdt and colleagues reported in their article. Furthermore, most of the Belgian flocks with ERS came from parent stock vaccinated against reovirus, which indicates that current reovirus vaccines cannot sufficiently protect against ERS.

Koopman says that ERS may be involved in cases of malabsorption syndrome that occur in US flocks, a possibility that is under investigation at the University of Georgia, Athens.

It’s quite possible that ERS is present in many more countries around the world, but it’s easy to miss the diagnosis if you don’t have the proper test reagents,” he says.

To establish the diagnosis of ERS, laboratory testing must be conducted and should focus on intestinal samples. At one time, Intervet/Schering-Plough Animal Health had the only lab that could confirm ERS, since it conducted the initial research on this viral serotype, but today there are many laboratories that can confirm the presence of ERS or “ERS-like viruses,” Koopman says.

ERS vaccine development
To prevent ERS infection and its costly consequences, Intervet/Schering-Plough Animal Health patented this unique reovirus strain and with it developed an inactivated oil-emulsion vaccine called Nobilis Reo ERS.

Testing results with the vaccine, which were presented in 2005 at the World Veterinary Poultry Congress, Istanbul, by Dr. Saskia Van de Zande of Intervet/Schering-Plough Animal Health, indicated that “Progeny from vaccinated breeders were protected against ERS challenge.” There was no mortality and the weight decrease was significantly lower compared to chicks that came from unvaccinated breeders challenged with ERS.

Birds from vaccinated parents also had no signs of central nervous system problems, such as tremor, which Van de Zande and an associate have linked to ERS (Veterinary Microbiology, 2007; 120, Issues 1-2:42-49).

Nobilis Reo ERS is administered to breeders, which pass along immunity against ERS to their offspring, Koopman says. Broiler chicks appear to be vulnerable to ERS at a very young age or from egg transmission and need to acquire immunity against ERS as soon as possible. The vaccine is labeled for protection against weight loss caused by ERS and for reduction of virus replication in the target organs. Initial experience with the vaccine in Poland has yielded good results.

The standard protocol for vaccinating with Nobilis Reo ERS is two doses of the vaccine administered intramuscularly in the thigh or breast muscle at least 11 weeks apart but no later than 4 weeks before the onset of lay.

The safety profile of the new vaccine is good and similar to other Intervet/Schering-Plough Animal Health reovirus vaccines, Koopman says.

“Since reovirus problems cause significant economic losses to the poultry industry, preventing the ravages of reovirus infection is crucial. Vaccination is an important tool for controlling the damage,” he says.

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