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

Overview

• This study investigated whether influenza D virus (IDV), a virus commonly found in cattle, is present in horses in the UK and if it could cause respiratory disease in them.
• The researchers found minimal evidence of IDV exposure in horses and no indication that the virus contributes to respiratory illness in the equine population studied.

Background

• IDV was first discovered in 2011, and antibodies have been found in several animal species, with cattle being the primary host.
• In cattle, IDV often causes mild respiratory symptoms and is a significant factor in bovine respiratory disease complex.
• Previous studies detected antibodies to IDV in horses (equids), but there was no clear link between IDV infection and respiratory disease in naturally affected horses.

Objective

• The primary aim was to determine whether IDV is a causative agent of respiratory disease in horses in the United Kingdom.

Methods

• Respiratory samples (nasopharyngeal swabs) were collected from horses showing respiratory signs and tested for IDV RNA using RT-qPCR, a technique for detecting viral genetic material.
• Serum samples from apparently healthy horses were tested for IDV antibodies using the haemagglutination inhibition (HI) test, which detects immune response to the virus.
• Positive samples from horses and cattle were further evaluated using a pseudotyped virus neutralisation test (PVNT), which confirms the presence of neutralising antibodies specific to IDV.
• Samples included 264 respiratory swabs from sick horses, 330 sera from healthy Thoroughbred brood mares and racehorses, and 145 sera from cattle in Great Britain and Ireland.

Results

• No IDV RNA was detected in any of the 264 equine respiratory samples, indicating no active IDV infection in those horses with respiratory illness.
• Out of 330 horse serum samples, only one sample from a 5-year-old brood mare tested positive for IDV antibodies via HI and PVNT, indicating very low exposure in the equine population.
• In contrast, 103 out of 145 cattle serum samples (71%) tested positive for neutralising antibodies to IDV, demonstrating widespread virus circulation in these animals across Great Britain and Ireland.

Conclusions

• The findings suggest that IDV exposure is very rare in horses in the UK.
• There is no evidence from this study that IDV plays a role in respiratory disease in horses in the regions studied.
• The widespread presence of IDV antibodies in cattle confirms that cattle remain the primary host and reservoir for this virus in the UK and Ireland.
• These results reduce concerns about IDV as a contributing factor to respiratory illnesses in horses, shifting focus back to other pathogens for equine respiratory conditions.