Research Enhances the Understanding of Runting and Stunting Syndrome of Broilers14 December 2013
Three new studies at the University of Georgia shed new light on the causes of Runting-Stunting Syndrome (RSS) in chickens and the possibilities of vaccination as a means of preventing the disease.
Runting and Stunting Syndrome (RSS) is an enteric disease that has been reported in the US and other countries, according to Professor Holly S. Sellers of the University of Georgia's College of Veterinary Medicine.
Although descriptions of RSS date back to the 1970s, she reports that the aetiologic agent(s) has yet to be identified. The disease is reliably reproduced using filtered intestinal homogenates from RSS affected broilers, implying a viral aetiology. Although several novel viruses have been isolated, the clinical disease has not been reproduced.
Metagenomic studies in our laboratory have identified viral sequences belonging to the Astroviridae and Parvoviridae virus families present only in RSS+ birds, as well as sequences of unknown origin. In addition, three astroviruses, ANV-1, ANV-2 and a novel chicken astrovirus, were detected in the intestines of RSS affected chickens by in-situ hybridisation. Specifically, only CkAstV-specific signals were detected in the cystic lesions, implying a close association of this virus with the hallmark lesion. Amplicons encoding viral capsid proteins of avian astroviruses and a chicken parvovirus were cloned into eukaryotic expression systems and purified for use as experimental vaccines.
Working with Dr Egbert Mundt, DVM, of Boehringer Ingelheim's Veterinary Poultry Diagnostic and Research Center in a study sponsored by the US Poultry and Egg Association, they explained that the three specific objectives of their research.
Objective 1: To generate experimental vaccines based on recombinant proteins cloned and expressed in an established baculovirus expression system
Purified immunogenic proteins of a new chicken astrovirus (CAstV), avian nephritis virus-like virus (ANV-1), avian nephritis virus-2 like (ANV-2) and a new chicken parvovirus (CParvo) will be prepared for use as a polyvalent vaccine.
The coding region for the immunogenic proteins, encoded by the capsid genes of CAstV, ANV-1, ANV-2 and CParvo were amplified, cloned and expressed in baculovirus and purified. The purified capsid proteins were mixed in equal volumes and emulsified in an oil adjuvant for use as a polyvalent recombinant protein vaccine. In addition, sufficient quantities of the purified proteins were prepared for use in the individual capsid ELISAs for serological evaluation.
Objective 2: To immunise broiler breeders with a combination of the CAstV, ANV-1, ANV-2 and CParvo purified proteins and evaluate progeny protection
Seroconversion to the individual capsid proteins was observed in breeders vaccinated at 12 and 18 weeks of age with the recombinant protein/oil adjuvant vaccine.
Eggs from vaccinated and unvaccinated hens were set and hatched for a total of seven placements. Progeny from vaccinated breeders had positive levels of antibodies, at day-of-hatch, to each of the capsid proteins used in the vaccine for all seven placements.
Comparisons of bodyweights taken at five and 12 days post challenge revealed significant differences between the RSS challenged and unchallenged groups, irrespective of vaccination. However, in progeny sets 1 and 3, bodyweight suppression in the vaccinated/RSS challenged birds was less than that observed for the unvaccinated/challenge birds at 12 dpc.
The mean number of cystic crypt lesions for all progeny placements, excluding progeny set 3, were higher in the MCAP/RSS challenged groups compared to the NEG/RSS challenged groups, suggesting an enhance pathological response in vaccinated chicks. However, progeny set 3 from the MCAP/RSS challenged groups had 40 per cent fewer cystic lesions than did the NEG/RSS challenge group, suggesting some mitigation of the presence of cystic lesions.
Based on previous studies, antibodies to the astrovirus capsid protein can, at best, mitigate the disease by reducing total number of lesions and decreasing the level of bodyweight suppression.
The addition of ANV-1, ANV-2 and CParvo capsid proteins to the astro capsid did not result in an improved mitigation of clinical disease and in some cases appeared to enhance pathology following challenge.
Objective 3: In-ovo vaccination of commercial broiler embryos with preparations of purified recombinant proteins
The following preparations of purified recombinant proteins were used:
- CAstV/ANV-1/CParvo and
Protection was not observed in either 14- or 18-day in-ovo vaccinated groups. In fact, a significant reduction of hatchability was observed in embryos in-ovo injected at 14 days of embryonation in both the ccCkAstV and sterile PBS groups, suggesting that the reduced hatchability may be related to the age of inoculation.
Since the Intelliject machine was set up for injecting 18-day eggs, it may be that some modification of the mechanical injection needs to be made.
The ccCkAstV alone does not appear to affect bodyweights out to 21 days of age following in-ovo injection at 14 or 18 days. However, when in-ovo is injected at 14 day, 11 per cent of the birds had cystic lesions by five days post challenge in the unchallenged group, suggesting the virus is capable of causing pathological changes in the intestinal villi.
Attenuation of the ccCkAstV may be necessary to reduce the pathology in unchallenged birds; however, it is not clear whether this would mitigate clinical disease.
The mechanism of immunity to astroviruses is not understood. This virus is detected in the intestinal villi of chickens exposed to RSS contaminated litter in as little as six hours post placement, thus limiting traditional vaccination strategies.
Protective immunity against RSS may not be primarily humoral but rather mucosal and cell-mediated.
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