Swiss study reveals how equine gammaherpesviruses readily circulate in horse population

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Equine herpesvirus-2 (EHV-2) is commonly shed by healthy horses in Switzerland, the findings of fresh research suggest.
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Equine herpesvirus-2 (EHV-2) is commonly shed by healthy horses in Switzerland, the findings of fresh research suggest.

Equid herpes viruses are important pathogens worldwide. They have an ability to establish latency, during which no infectious virions are produced, and the virus escapes the immune system.

Horses usually become infected with EHV early in life, with alpha herpesviruses establishing latency in regional ganglia and to some extent in leukocytes, while gammaherpesviruses are thought to establish latency in the lymphatic system only.

The virus may then be reactivated periodically throughout life during periods of immune suppression.

During reactivation, infectious virus particles are shed via site-specific secretions with or without accompanying clinical signs, and these virus-shedding horses serve as a source of infection for other animals.

There are two subfamilies of herpes viruses with importance in equine medicine — alphaherpesviruses (EHV-1, EHV-3, and EHV-4) and equid gammaherpesviruses (EHV-2, EHV-5, and EHV 7, or (=AHV-2).

Alpha herpes viruses are well studied, but less is known about equid gammaherpesviruses, which can cause clinical signs, such as respiratory problems in young horses.

Laura Scheurer and her fellow researchers at the veterinary school at the University of Zurich in Switzerland noted that the prevalence of equine gammaherpesvirus in healthy Swiss horses is unknown to date, but would be valuable in diagnosing clinical cases and formulating biosecurity recommendations.

Nasal swabs from 68 healthy horses from 12 Swiss stables and two stables near the Swiss border region in Germany were analyzed for herpesviruses. Positive samples were then sequenced to identify which type of herpes virus was detected.

Overall, the prevalence of equine gammaherpesviruses was found to be 59%. The prevalence for equine EHV-2 was 38%, EHV-5 was 12%, and asinine herpesvirus-5 (AHV-5), which circulates in donkeys as well as horses, was 9%.

Co-infections with multiple equine gammaherpesviruses were observed in 25% of the positive samples, the study team reported in the journal Viruses.

The odds of shedding EHV-2 decreased with age in the sample population, whereas the odds of shedding AHV-5 increased with age.

Breed, sex, region, or stable had no significant association with equine gammaherpesvirus shedding.

“As EHV-2 shedding was common in healthy horses, a positive PCR result must be interpreted with caution regarding the formulation of biosecurity recommendations and causal diagnosis,” the study team concluded.

“As EHV-5 and AHV-5 shedding was less common than EHV-2, a positive test result is more likely to be of clinical relevance. Shedding of multiple equine gammaherpesviruses complicates the interpretation of positive test results in a horse.”

Discussing their findings, the authors noted that their results correlated well with other studies around the world. For example, in New Zealand, the EHV-2 prevalence was estimated at 32% in healthy horses, as well as horses with respiratory disease. A recent prevalence study of herpesviruses involving 500 healthy Polish horses reported a 77% EHV-2 prevalence in respiratory tract samples.

They said investigations of the clinical relevance of equine gammaherpesviruses suggest that EHV-2 may induce or predispose equids to respiratory diseases. Previous studies have shown that horses shedding EHV-2 were three times more likely to display clinical respiratory disease compared to non-shedders, and EHV-2 was found more often in horses with lower airway inflammation compared to healthy horses in several studies.

The shedding rates identified indicate that these viruses readily circulate in the equine population and might be an incidental finding, the authors said. Similarly, biosecurity measures should not be implemented solely based on a positive test result. Instead, multiple factors such as epidemiological data, herd characteristics, case history, and clinical signs, as well as results from other diagnostic tests, should be considered.

The authors said although the sampled population was not representative of the entire Swiss horse population, it provides a first insight into the diversity and occurrence of equine gammaherpesviruses in Switzerland.

They said further investigations, including a larger sample size as well as repeated sampling over time of individual horses, would be helpful to shed more light on the epidemiology of equine gammaherpesviruses in Swiss horses.

The study team comprises Scheurer, Claudia Bachofen, Isabelle Hardmeier, Julia Lechmann and Angelika Schoster.

Scheurer, L.; Bachofen, C.; Hardmeier, I.; Lechmann, J.; Schoster, A. Prevalence of Nasal Shedding of Equid Gammaherpesviruses in Healthy Swiss Horses. Viruses 2021, 13, 1686. https://doi.org/10.3390/v13091686

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

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