Limited evidence for exposure of horses in the United Kingdom to influenza D virus

Background: Since it was first detected in 2011, antibodies to influenza D virus (IDV) have been found in several species, although cattle are the major host. In cattle, IDV is typically associated with mild respiratory signs, but it is clinically significant as a component of bovine respiratory disease complex. Although antibodies to IDV have been detected in equids, no association of IDV with naturally occurring respiratory disease in horses has yet been demonstrated.

Aims/objectives: The aim of this study was to determine whether IDV may be a causal agent of equine respiratory disease in the UK.

Methods: Nasopharyngeal swab extracts from horses with respiratory signs were screened by RT-qPCR for the presence of IDV RNA. Equine serum samples from apparently healthy horses were screened by haemagglutination inhibition (HI) test for the presence of IDV antibodies. Cattle and HI positive equine serum samples were screened by neutralisation test using an IDV pseudotyped virus (PVNT)

Results: None of the 264 equine respiratory samples were positive for IDV RNA. Of the 330 serum samples from 169 Thoroughbred brood mares and 105 Thoroughbreds in training yards, only one (from a 5-year-old brood mare) was positive for HI antibodies. This single positive sample was confirmed in the PVNT. In contrast, 103 of 145 bovine serum samples (71%) were positive by PVNT, confirming widespread circulation of IDV in cattle in Great Britain and Ireland.

Conclusion: This study found a low risk of horses being exposed to IDV in the UK, with no evidence that IDV contributes to equine respiratory disease.