Since late 2004, broiler companies throughout the United States had increasing numbers of cases of a runting and stunting syndrome (RSS) particularly in the winter and spring, according to Dr Holly Sellers, head researcher at the College of Veterinary Medicine. She is Associate Professor in the Department of Population Health at Poultry Diagnostic & Research Center at the University of Georgia's College of Veterinary Medicine.

She explained that RSS is a transmissible disease affecting young broilers between one and two weeks of age. Most notably, RSS causes severe weight suppression during the first few weeks of age, lack of flock uniformity, diarrhoea, distention of the intestines and a significant increase in the feed conversion ratio. Cystic enteropathy is consistently observed by histopathological examination of small intestine samples from affected flocks.

Although descriptions of RSS date back to the 1970s, the aetiologic agent(s) had not been identified although evidence suggests this is a multifactorial disease. It can be reliably reproduced using filtered intestinal homogenates from clinically affected broilers, thus implying viral aetiologic agents.

Numerous viruses were isolated and characterised at PDRC using classical and molecular techniques. Many of the viruses isolated were determined to be the vaccine viruses following characterisation.

Two common denominators in many RSS cases were novel reoviruses and chicken astroviruses, according to Dr Sellers. Molecular characterisation of these viruses indicates they are novel compared to other known astroviruses and reoviruses. Additional in vivo studies are necessary to determine the biological role of these viruses as inciting agents in RSS in an effort to rule-in or rule-out either the reovirus or astrovirus.

Her latest research at the University of Georgia – sponsored by US Poultry & Egg Association – had the objective to determine the pathogenicity of novel chicken astrovirus (CAstV) and reovirus isolates in vivo to elucidate their roles in the aetiology of RSS.

Summarising her results, Dr Sellers said: "Novel astroviruses and reoviruses isolated from flocks experiencing RSS caused significant body weight suppression and mild to moderate enteritis by 12 days of age in commercial broilers inoculated at day of age.

"However, histological evaluation of the gastrointestinal tract did not reveal cystic enteropathy, a hallmark lesion in RSS affected birds."

Dr Sellers explained that her work provides useful information on the biological role of novel astroviruses and reoviruses in RSS. Challenge with either virus alone or in combination resulted in significant body weight suppression in commercial broilers and mild to moderate enteritis.

Furthermore, birds inoculated with the CAstV (either alone or with reovirus) had damage to the kidneys as evidence by histological examination. Leghorns appeared to be less affected by challenge with either virus alone or in combination.

These results are consistent with previous RSS challenge studies in leghorns as body weight suppression and clinical signs and lesions were minimal. Of the tissues examined by histopathology, the intestines and kidneys were most affected by challenge with either or both viruses.

"While challenge with virus isolates did not result in the full clinical manifestation of RSS as evidenced by lack of cystic enteropathy, bird health and body weight gain was compromised to a degree that would be unacceptable at the commercial level," Dr Sellers concluded.

She and her team are now evaluating inactivated experimental vaccines to both of these viruses to determine if maternally derived antibodies to either virus can mitigate the effects of RSS.

Reference

Sellers H.S. Identification and characterization of novel astro- and reoviruses associated with Runting Stunting Syndrome and the development of reagents for diagnostic detection in the field.

September 2009