Infectious Equine Diseases: EHV in the Horse Stable, at the Zoo, and Around the African Watering Hole

Here we are in the years where infectious diseases are rewriting their own DNA and science has the ability to track it as it happens. The course of equine infectious diseases has me writing headlines about diseases I had thought were part of history…but they are back in the news again.

Come to think of it, we thought that developing technology would give us the edge over infectious diseases. But viruses have kept pace with our efforts to control them. Some would say that they have out-smarted us, but at least we usually know what they’re up to.

Consider equine influenza. It popped up in the United Kingdom this year, and once again in Oregon. In Malaysia, over 100 horses were recently affected by an outbreak. And today, the South China News is reporting the cancellation of the premiere race meet hosted by the Hong Kong Jockey Club at Chengdu in Sichuan province on the Chinese mainland. 

The reason? Equine influenza, according to the news report, diagnosed in horses at another track that would have competed at Chengdu.

In the United States, the US Equestrian Federation is stepping up requirements for equine influenza vaccinations, and equine herpes virus as well, for competition horses at events beginning December 1.

A scare over glanders in Brazil threw equestrian sports into a mild panic over the upcoming 2016 Olympics in Rio. Will our horses be safe there, national organizers wondered?

This time of year, our thoughts turn to Equine Herpes Virus (EHV). Maybe it’s the colder weather. Maybe it’s the transfer of vast numbers of horses between summer and winter racetracks and showgrounds. Maybe it’s the congregation of large numbers of horses at season-end championship shows, or horses going to auctions so owners won’t have to feed them over the winter.

Or, maybe it’s all those things. Call it a perfect storm for a virus that is learning its way around our vaccine protocols, and taking advantage of the fact that some of us simply don’t vaccinate horses. If your horse never leaves your farm and rarely, if ever, contacts outside horses, your vet won’t recommend vaccination for EHV or influenza. But when you decide to sell that horse or send it to an auction, he goes into a risky pen with risky companions.

And did I mention strangles?

Some new research about EHV this week will make you stop and think the next time you read about an outbreak. Occasionally, The Jurga Report has published news about EHV in zoo animals. It’s heartbreaking to hear of a captive animal suffering from the disease, but it is also mystifying when a non-hooved animal suffers from it. We expect to hear about EHV in zebras and maybe in other ungulates, but a polar bear?

EHV is one of a range of viruses in a diverse and active group that science never tires of studying. These viruses can close a racetrack, as EHV-1 did in Pennsylvania last month, or they can paralyze a wild animal in Africa. They can also put a lone, valuable representative of an endangered species in a zoo at risk for its life. And one thing we know about these viruses is that they change their ways. 

Keeping an eye on equine herpes virus in all its forms can become an obsession.

We worry about it in our equine corner of the world, where only EHV-1 and EHV-4 have clinically relevant implications for our horses but researchers in Germany wondered about the broader risk of equine herpes virus to and between zoo and wild animals. Do the viruses work the same way in zoo animals that they do in the same species in the wild? And should we be paying attention to the way our horses’ cousins, the zebras, are affected by equine herpes virus?

EHV-9 is the newest virus in the family, and it was first discovered when it was determined to cause the death of a gazelle in a Japanese zoo. How did the gazelle contract the virus? In 2007, the San Diego Zoo reported to the Centers for Disease Control and Prevention (“CDC”) that a polar bear’s infection with EHV-9 was via transmission from Grevy’s Zebras. 

Japanese research showed that, excluding experimental studies, “neither titers to EHV-9 nor the virus itself have been found in domestic horses”, according to the CDC report. EHV-1, on the other hand, affects horses, domestic donkeys, zebras, onagers, and black rhinoceroses. Antibodies to EHV-1 have been found in white rhinoceroses.

The new research, published in the Open Access journal PLOS ONE, compared the same species of animals in zoos and in the wild, and found differences in the way that the captive animals were defending themselves against these particular viruses. The research title uses the terms “seroprevalence” which means simply the frequency with which the viruses were found in each species was determined by testing the blood. 

The researchers tested 30 species in 12 families and five orders. The team of scientists was equally diverse; in addition to Berlin’s IFW, a representative of France’s Réserve Africaine de Sigean, Namibia’s Bwabwata Ecological Institute, Switzerland’s Zoo Basel, and Egypt’s Kafrelsheikh University contributed to the project.

What did they find? First, none of the animals tested for the project showed clinical signs of infection with EHV. Let’s look at zebras, since they are the closest relatives of horses in the study. The prevalence of EHV-1 in the sampled wild zebras was significantly higher than in zoos, suggesting that captivity may reduce exposure to EHV-1. Furthermore, the seroprevalence for EHV-1 was significantly higher than for EHV-9 in zebras.

The rhinoceros was the second species to show interesting data. In contrast to the zebras, EHV-9 antibody prevalence was high in both captive and wild African rhinoceros species suggesting that they may serve as a reservoir or natural host for EHV-9. 

The authors speculated that, over time, the mingling of animals around African watering holes may have resulted in the mingling of viruses, as well, resulting in the rhino’s high numbers and the presence of the equine herpes virus in non-equine species.

Will the word “equine” be dropped from the virus name one day? To be sure, the more we can learn about how the virus works in zoos, where animals’ medical histories are close at hand and the addition of new animals is documented, the more we may learn about preventing EHV in domestic horses. 

Knowing that a species harbors a virus so differently in captivity than it does in the wild offers even more food for thought. EHV offers a feast for anyone hungry to unravel the mysteries of a virus that complicates our lives and endangers our horses.

Read the article:

Abdelgawad A, Hermes R, Damiani A, Lamglait B, Czirják GÁ, East M, et al. (2015) Comprehensive Serology Based on a Peptide ELISA to Assess the Prevalence of Closely Related Equine Herpesviruses in Zoo and Wild Animals. PLoS ONE 10(9): e0138370. doi:10.1371/journal.pone.0138370

To learn more:

New Hosts for Equine Herpesvirus 9 via Centers for Disease Control and Prevention




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