IDENTIFICATION OF NONCYTOPATHIC EQUINE RHINOVIRUS-1 AS A CAUSE OF ACUTE FEBRILE RESPIRATORY-DISEASE IN HORSES

Equine rhinovirus 1 (ERhV1) is a recognized cause of acute febrile res piratory disease in horses, although the virus is rarely isolated from such animals, despite seroprevalence rates as high as 50% in some hor se populations, Recently, ERhV1 has been shown to be most closely rela ted to foot-and-mouth disease virus, raising questions as to its disea se associations in horses. We report that ERhV1 infection was the like ly cause of two separate outbreaks of severe febrile respiratory disea se which involved more than 20 horses, Attempts to isolate ERhV1 from nasopharyngeal swabs by conventional cell culture methods were unsucce ssful, in that cytopathology was not observed. Viral antigen was detec ted by immunofluorescence assay in the cytoplasm of cells infected wit h 10 of 15 nasopharyngeal swab samples, indicating the presence and pr esumably replication of ERhV1, A rise in serum neutralizing antibody t iter between acute- and convalescent-phase sera confirmed that ERhV1 w as causatively associated with one of the outbreaks, ERhV1 RNA was det ected in nasopharyngeal swabs collected from all horses during the acu te phase of disease by reverse transcription-PCR. Nucleotide sequencin g of amplified products showed that within each outbreak a single stra in of ERhV1 was involved but that distinct viruses were involved in ea ch outbreak. A retrospective study of samples from nine other outbreak s of respiratory disease in horses suggested ERhV1 etiology in at leas t two of these, We conclude that the relative importance of ERhV1 as a cause of acute febrile respiratory disease in horses has been underes timated due to failure in many instances to isolate virus by conventio nal cell culture methods.