Equine Herpesvirus Reviewed

Equine Herpesvirus Reviewed

An electron microscope view of EHV-1.

Photo: Walid Azab, PhD

As equine herpesvirus (EHV) gains worldwide significance, international research groups are seeking a better understanding of the disease. Scientists' ultimate goal is to provide horse owners with an effective vaccine with which to protect their horses—and other species as well—in the relatively near future.

“Our knowledge about EHV is now greatly expanded, and we are putting more efforts into developing new strategies, vaccines, and/or antiviral drugs to control the virus,” said Walid Azab, PhD, of the Virology Institute at Freie University, in Berlin, Germany, and the Department of Virology, Faculty of Veterinary Medicine at Zagazig University, in Egypt.

In a recently published academic review, Azab and co-authors Ma Guanggang, PhD, and Nikolaus Osterreider, DrMedVet, described the history of EHV research up to 2013. Their focus is on EHV-1 and EHV-4, which they said are “arguably the most relevant herpesviruses affecting equids.”

“Recently we have accumulated more data about EHV,” Azab said. “EHV causes severe losses in horse populations all over the world, and it can jump species barriers and infect other wild animals. It’s therefore important to share our data with other scientists and shed more light on this important virus.”

In their review, the researchers analyzed how EHV-1 differs from EHV-4. Once thought to be different strains of the same virus, scientists now know that the two independent viruses have marked differences in their DNA, which could explain their different ways of spreading to other animals and the various clinical signs they cause. While EHV-4 typically only affects horses' respiratory system, for example, EHV-1 can spread systemically in horses and in other non-equid animals, such as llamas, Azab said. EHV-1 can also lead to a variety of clinical signs, including nervous system disease and abortion in addition to respiratory problems, Azab said.

A total of nine types of EHV affect equids, five of which naturally infect horses and ponies: EHV-1, -2, -3, -4, and -5. Two of these (EHV-2 and EHV-5) are part of the “gammaherpes” group. EHV-3 (also known as equine coital exanthema) is a sexually transmitted disease. EHV-1 and -4 are the most devastating to horse health and cause the most economic damage to the industry, Azab said. Equine herpesvirus-6, -7 (also a gammaherpes virus), and -8 affect donkeys, and the recently discovered EHV-9 can affect wild animals including zebras. It was originally isolated in gazelles (which are not equids), the team noted. While EHV-9 in zebras usually shows no signs, it can spread to other animals that become symptomatic. For example, veterinarians believe an equine herpesvirus, probably transmitted by zebras, killed two polar bears in 2010 in a German zoo.

Currently available vaccines provide suboptimal protection against disease in horses, Azab said. Vaccinated horses can still develop clinical signs, and at this time there is no vaccine effective against abortion. The only kinds of vaccines available on the market today are modified-live vaccines and inactivated vaccines. However, researchers are actively developing new vaccines based on modern technology and genetics.

“New generation” modified-live vaccines will be based on current virus strains, he said. Two safe, low-cost options currently in development are subunit/vector vaccines and DNA vaccines. Subunit vaccines hone in on only a select group of antigens, and vector vaccines work by sending protective DNA into the host’s cells via a “vector”—in this case, an attenuated (or weakened) virus. DNA vaccines create immunity at the cellular level by instructing cells to create antigens. However, these latest-generation vaccines might still have several years of work ahead of them before they're ready for commercial use.

“Currently we are working on different topics targeting virus entry, pathogenesis (what causes the disease), and immune evasion (how viruses escape the body’s natural immune system),” said Azab. “We have accumulated more data on these topics, which will help a lot in our understanding and the subsequent production of rational vaccines, which we hope to see available in the near future.”

But while vaccine development is critical, it isn’t the only goal, Azab said.

“The important thing for owners is to have their horses safe from dangerous diseases either by having strong vaccines or potent antiviral drugs; and this is our job,” he said. “But it is our responsibility to educate the owners about virus epidemiology and how they can protect their horses from different possible sources of infection.”

Specifically, he said, owners should understand that horses are infected through the upper respiratory tract route following contact with virus-infected secretions, tools and materials, or air-borne particles shed from infected horses. Azab also provided several tips for horse owners to follow:

  • Avoid gathering your horses with "non-certified horses" (those without health records), especially during competitions or shows;
  • Keep broodmares in good health and in low-stress conditions to avoid possible virus reactivation leading to infection of the foals; and
  • Keep vaccinations updated, which will help protect horses to large extent.

The study, "Equine herpesviruses type 1 (EHV-1) and 4 (EHV-4)-Masters of co-evolution and a constant threat to equids and beyond," was published in Veterinary Microbiology

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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