Five things you need to know about Equine Herpes Virus (EHV)

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Vaccinations can help horses generate additional antibodies and minimize the spread of disease.
Vaccinations can help horses generate additional antibodies and minimize the spread of disease, says Dr Peter Heidmann. © Palm Beach Equine Clinic

Following the recent European Equine Herpes Virus (EHV-1) outbreak, internal medicine specialist Dr Peter Heidmann from the Palm Beach Equine Clinic shares what horse owners need to know about the virus.

1. EHV can be present in any area where there are horses

EHV can be found in horses in every country around the world. The virus can have varying consequences and not every horse may be symptomatic. The duration of their immunity, their capability to shed viral particles, and whether they continue to carry the virus are all variables unique to the individual horse.

There are clusters of the disease reported all over the United States, although they are unrelated to the neurological strain of EHV-1 outbreak in Europe.

A few isolated cases reported in the US this March were found to be caused by the neurotropic form of the disease and some have been non-neurotropic. The concern is warranted, but cases are highly localized, mainly in areas with large equine populations. We see the disease cropping up because the virus may already be dormant in their bodies. To minimize the likelihood of outbreaks on the individual farm level, have a vaccination program with your veterinarian and if necessary take the temperatures of horses twice daily to catch any viral infection early.

2. There are multiple strains of EHV that lead to varying health defects

Most horses, by two years old, have been exposed to EHV. Nearly every horse over five years old has had EHV in a mild respiratory form, in which they have likely suffered from symptoms such as a runny nose or cough. The disease becomes gravely serious when there is a neurological form of EHV-1 that is spreading rapidly among horses.

Over the last century, the primary focus of EHV disease control was related to reproduction. EHV was a major cause of infectious abortions in mares. Since the advent of effective vaccines that protect against EHV abortions, the concern has shifted to the rare but serious occurrences of neurologic disease caused by EHV-1.

Certain strains of EHV can have more propensity to cause neurological disease, while others do not. It is important to understand that both “neurologic” and “non-neurologic” strains can cause neuro symptoms, and that some horses infected with “non-neurologic” strains may also develop the neurologic form of the disease. Symptoms of mild neurologic disease can include ataxia or hind end weakness and irregular gaits. When horses suffer mild neurologic symptoms, most will make a full recovery. If they suffer from severe symptoms, it can be increasingly difficult to treat, and some cases can be fatal.

3. Vaccination is key

There are currently no genetic explanations regarding whether a particular horse may be more likely to contract EHV-1. However, when a horse is stressed, whether it be from shipping, competing, or other causes, its immune system is more susceptible to illness. EHV-1 can live in the central nervous system, and during times of stress when endogenous corticoids are produced, the virus can start to replicate. This causes the horse to shed viral particles into the environment and explains why there are outbreaks of the disease. It does not take much for a stressed horse to start shedding viral particles into the environment and infecting non-stressed horses nearby.

Vaccinations can help horses generate additional antibodies and minimize the spread of disease. The vaccines available today are effective at preventing the respiratory and reproductive strains of EHV, but are not effective for the neurologic strain. It is just as important as ever to vaccinate because vaccines help to limit the spread of viruses by decreasing the shedding of viral particles from each individual horse.

Immunization is recommended every six months at a minimum. Most vaccine manufacturers suggest vaccinating against EHV as often as once every 90 days when travelling or competing. It is recommended to work closely with your veterinarian to tailor a vaccination program for your horses’ specific requirements.

4. Surveillance is crucial to preventing serious outbreaks

Following the recent outbreak in Europe, medical surveillance has increased. There are now extra layers of protection in the process of importing horses into the US. Imported horses have always been quarantined, but now horses are also receiving a nasal swab to look for the presence of EHV-1 in their nasal passages. There have been cases of EHV-1 in the US recently, but there is no evidence that these EHV-1 cases are related to the major outbreak in Europe, partially thanks to our increased level of surveillance on horses being brought into the country.

5. How cases are handled, post-European outbreak

Veterinarians have significantly shifted how show horses with any symptoms are being handled. If a horse had presented with a fever or runny nose before the European EHV-1 outbreak in early 2021, it would typically have been treated by its own veterinarian and left to return to full health. What is happening now is that each horse with a fever becomes a possible candidate for being the index case of EHV-1. This changes veterinarians’ responsibilities everywhere because of the potential for the disease to spread among the population in respective local areas. Horses with fevers must therefore be strictly quarantined and tested.

The fastest and most accurate test for diagnosing EHV-1 is a PCR test, which looks for the DNA of the infecting organism. The horse will test positive if it is actively shedding organisms. The biggest downside is that it typically takes 24 hours to produce results. Therefore, horses who have been tested are put under strict isolation until the test results come back. Often, by the time test results are ready, the horse’s symptoms have ceased. Yet that is the level of scrutiny necessary to prevent a serious outbreak.

Palm Beach Equine Clinic

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