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 respiratory disease in horse, although the virus is rarely isolated from such animals, despite seroprevalence rates as high as 50% in some horse populations. Recently, ERhV1 has been shown to be most closely related to foot-and-mouth disease virus, raising questions as to its disease associations in horses. We report that ERhV1 infection was the likely cause of two separate outbreaks of severe febrile respiratory disease which involved more than 20 horses. Attempts to isolate ErhV1 from nasopharyngeal swabs by conventional cell culture methods were unsuccessful, in that cytopathology was not observed. Viral antigen was detected by immunofluorescence assay in the cytoplasm of cells infected with 10 of 15 nasopharyngeal swab samples, indicating the presence and presumably replication of ERhV1. A rise in serum neutralizing antibody titer between acute- and convalescent-phase sera confirmed that ERhV1 was causatively associated with one of the outbreaks. ERhV1 RNA was detected in nasopharyngeal swabs collected from all horses during the acute phase of disease by reverse transcription-PCR. Nucleotide sequencing of amplified products showed that within each outbreak a single strain of ERhV1 was involved but that distinct viruses were involved in each outbreak. A retrospective study of samples from nine other outbreaks of respiratory disease in horses suggested ERhV1 etiology in at least two of these. We conclude that the relative importance of ERhV1 as a cause of acute febrile respiratory disease in horses has been underestimated due to failure in many instances to isolate virus by conventional cell culture methods.