Picornavirus Infections in Poultry
Members of the Picornaviridae family are very small, nonenveloped RNA viruses that replicate in cytoplasm of infected cells. Picornavirus infections in birds are known to cause, or be associated with, a range of diseases, including avian encephalomyelitis in chickens, hepatitis in ducks and turkeys, malabsorption in chickens and turkeys, proventriculitis in chickens, and keratin disorder in chickadees. In this topic, only enterovirus-like viruses (ELVs) associated with enteritis and malabsorption syndrome are discussed.
ELVs have been found in a range of bird species, including commercial poultry and wild birds from several countries around the globe. ELVs have been linked with enteritis and stunting syndrome, but a number of reports have also found them in clinically healthy birds. Therefore, the pathogenicity of EVLs requires further investigations. Pathogenesis of ELVs is believed to be due to their predilection for propagation in epithelial lining and resultant destruction, leading to malabsorption and enteritis. As a reflection of their replication in the intestinal mucosa, ELVs are predominantly transmitted by the fecal-oral route, although vertical transmission is also suspected.
Clinical signs of an enterovirus-like virus are frequently reported in the first few weeks of life and include diarrhea, stunted growth, and mortality. Affected birds may also show depression and a pasted vent.
Gross lesions include intestinal dilation, attenuation of the intestinal wall, and presence of foamy intestinal/cecal contents. Microscopic lesions include shortening of the villi, elongation of crypts, hyperplasia of enterocytes, and varying degrees of inflammatory cell infiltration into the lamina propria of the intestinal tract.
Enterovirus-like viruses often poorly grow in cell culture and often need inoculation into neonatal birds or embryonated eggs from the same bird species for propagation. Other challenges associated with isolation and propagation of ELV include outgrowth of other intestinal viruses, especially reoviruses. Intestinal contents or feces can be used for sampling.
Transmissible electron microscopy of the affected tissues, intestinal contents, fecal materials, or propagated viruses in the laboratory can be used for diagnosis. Care should be taken for interpretation of results, because birds/eggs used for propagation (such as SPF birds) may already be infected with ELV.
Further characterization of the viruses require specific reagents (such as antisera) for known viruses with similar morphology, but these reagents are not readily available in conventional diagnostic laboratories. With availability of advanced molecular techniques such as next generation sequencing and metagenomics, characterization of ELVs may become feasible as a routine practice in the future.
Serologic assays have been described for the detection of antibodies to some ELVs, but these assays are not routinely used because the true pathogenicity and distribution of ELVs in commercial poultry have not been fully investigated as yet.