Equine herpesvirus-5 virus linked to respiratory disease in horses in Australian study

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More work is required to learn about the clinical relevance of recently discovered equine parvoviruses, according to scientists.
EHV1 and EHV4, are well known causes of equine respiratory disease, but considerable mystery surrounds EHV2 and EHV5. (File image)

An Australian study adds to the mounting body of evidence linking equine herpesvirus-5 (EHV5) with respiratory disease in horses.

The equine alphaherpesviruses, EHV1 and EHV4, are well-known causes of equine respiratory disease. On the other hand, the gammaherpesviruses, EHV2 and EHV5, are often isolated from clinically healthy horses, despite a known association in some disease processes.

“The consequences of infection with these enigmatic viruses remains unknown,” Charles El‐Hage and his fellow researchers noted in the journal Animals.

This lack of clarity around the clinical importance of EHV2 and EHV5 may be attributed to the frequent detection of these gammaherpesviruses in horses with and without signs of disease, under both experimental and field conditions.

Researchers in the University of Melbourne study set out to determine the prevalence of, and any association between, herpesviruses EHV1, EHV2, EHV4 and EHV5 infection in horses with and without signs of respiratory disease.

Nasal swabs were collected from 407 horses in the Australian state of Victoria. The samples had been collected from horses during Australia’s equine influenza outbreak in 2007, in the phase when the nation was undertaking testing to provide proof of freedom from the flu.

All horses tested in Victoria were negative for the flu, meaning that the archived swabs were available to screen for other pathogens such as EHVs.

Molecular-based testing methods were used in the study.

Of the 407 horses sampled, 249 (61%) were clinically normal, while 120 (29%) had clinical signs of mild respiratory disease, and 38 (9%) horses had an unknown clinical history.

Of the three horses detected shedding EHV1, and the five shedding EHV4, only one was noted to have clinical signs linked to respiratory disease.

Of the 120 horses in the diseased group, 85 of them (70.8%) were infected with EHV5. This was a significantly higher percentage than that found in the clinically healthy horses: Among the 249 healthy animals, 137 were infected with the virus, equating to 55% of the animals.

The odds of an EHV5-positive horse showing clinical signs of respiratory disease were twice that of EHV5-negative horses, the study team reported.

No difference was found in the amount of average viral shedding between diseased and non-diseased horses.

“The clinical significance of respiratory gammaherpesvirus infections in horses remains to be determined,” the authors wrote. “However, this survey adds to the mounting body of evidence associating EHV5 with equine respiratory disease.”

Discussing their findings, the researchers said equine herpesvirus infections were common in samples from the respiratory tract, irrespective of clinical disease status.

About 40% of horses were shedding at least one herpesvirus at the time of sampling. In total, 67.9% of horses with no obvious clinical disease were shedding detectable levels of at least one herpesvirus.

The detection of the alphaherpesviruses in a small proportion of horses (8 horses, or 2%) contrasted markedly with the high frequency of shedding of the equine gammaherpesviruses (282 horses, or 69.3%).

The clinical significance of respiratory gammaherpesvirus infections in horses remains to be determined, the researchers concluded.

“The task of identifying a definitive role of the equine gammaherpesviruses as the cause of respiratory disease on a case‐by‐case basis remains challenging, since the precise role of both EHV2 and EHV5 and their relation to clinical disease is likely to be complex and remains to be elucidated for these enigmatic viruses.”

The study team comprised El‐Hage, Zelalem Mekuria, Kemperly Dynon, Carol Hartley and James Gilkerson, all with the University of Melbourne; and Kristin McBride, with the University of New South Wales.

El-Hage, C.; Mekuria, Z.; Dynon, K.; Hartley, C.; McBride, K.; Gilkerson, J. Association of Equine Herpesvirus 5 with Mild Respiratory Disease in a Survey of EHV1, -2, -4 and -5 in 407 Australian Horses. Animals 2021, 11, 3418. https://doi.org/10.3390/ani11123418

The study, published under a Creative Commons License, can be read here

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