EHV-1 remains a big threat to horses, say scientists, citing rise in neurological form

Most horses are infected with EHV-1 or the closely related EHV-4 virus before two years of age.
Most horses are infected with EHV-1 or the closely related EHV-4 virus before two years of age.

Equine herpesvirus-1 (EHV-1) remains a huge threat to the horse industry, a just-published review has concluded, with the scientists noting the rising number of neurological cases.

Fatai Oladunni and his colleagues at the University of Kentucky’s Maxwell H. Gluck Equine Research Center say that detailed information on factors around EHV-1 infection that give rise to neurological disease remain elusive.

EHV-1 is among the most important and prevalent viral pathogens of horses, Oladunni, David Horohov and Thomas Chamber wrote in the journal Frontiers of Microbiology.

EHV-1 mostly causes respiratory disease, but viral spread to distant organs enables the development of more severe issues, in particular abortion and neurologic disease.

The virus can also become latent, capable of reactivating to cause infection at any time.

Exposure of horses to either EHV-1 or its close relative EHV-4 occurs very early in life. It has been reported that between 80 to 90% of horses are infected with either pathogen by the time they are two years old.

Vaccines currently available are not completely protective against EHV-1, especially so in the case of equine herpesviral myeloencephalopathy (EHM) — the dangerous neurological form of the disease.

“As a result, EHV-1 still poses a huge threat to the horse industry and efforts geared toward preventing the outbreak of the disease are strongly encouraged.”

They continued: “Several efforts over the decades have been channeled toward understanding and characterizing the protective host immune response against EHV-1 which could be exploited to advance diagnostic approaches and vaccine development,” the review team said.

“However, despite over 80 years since EHV-1 was first recognized, we do not yet fully understand how the virus’s interaction with the host immune response can be harnessed for the development of more effective immunotherapy.”

Equine herpes virus.
The equine herpes virus.

There is no specific drug effective against EHV-1 infection, with strategies aimed at easing symptoms and reducing the risk of spread.

They discussed the trend toward an increasing number of outbreaks of EHM.

The neurological form, which sometimes appears after one week of infection, may lead to deaths of horses, disruption of breeding or training schedules, cancellation of horse events, and extensive movement restrictions, posing difficulties at racetracks, training facilities, and other horse shows.

Even though horses may recover from the disease, their productivity is usually compromised, and the money expended in their care and management may run into several thousands of dollars, depending on the farm size.

“There seems to be no satisfactory scientific explanation for the variable incidence of EHM and different clinical manifestations observed during outbreaks of EHV-1,” they said.

“Several factors including sex, age, immune status of the horse, the reproductive status of the mare (including the stage of gestation), the severity of infection, the type of strain, and perhaps the route of transmission determine the clinical picture of EHV-1 infection.

The clinical presentations are highly variable and widespread, depending on the site of neurologic impact, and usually peak between two and three days from onset.

Generally, there is a loss of appetite, fever, swelling of the lower limbs, abortion, fetal death, or neurologic syndrome. These are variable in different horses, with the extent of effects ranging from temporary unsteadiness of their feet to complete paralysis.

Horses capable of staying on their feet have a good prognosis, unlike recumbent horses that may suffer complications such as pneumonia, colic or bladder rupture. They are generally euthanized.

“Today, a resurgence in the number of EHM cases across the world has necessitated the classification of this syndrome as an emerging disease of the horse,” the review team noted.

According to the Center for Emerging Issues report of 2007, EHM satisfies the requirement for an emerging viral disease based on the more virulent nature of the circulating EHV-1 strains than previously reported and the increased incidence of the disease, with a heightened fatality rate.

Increased outbreaks of EHM have been reported in North America, Europe, Africa, Oceania, and Asia.

“The recent increased incidence of EHM during EHV-1 outbreaks supports the observation that the currently circulating neuropathogenic EHV-1 strain has evolved into a more pathogenic strain, producing a higher rate of morbidity and mortality than previously.

Indeed, EHM has been associated with a particular mutation in the EHV-1 virus. However, other factors may contribute to the virus’s ability to target the brain, given that 14 to 24 percent of EHV-1 strains from horses displaying clinical signs of EHM lack this genetic indicator. This suggests the so-called non-neuropathogenic form of EHV-1 can also cause EHM.

“The associated risk factors for this increased incidence of EHM are still poorly defined.

“However, outbreaks have been reported mostly at places such as racetracks, riding schools, and veterinary hospitals where horses from different origins congregate.

“The high stocking density of stabled horses during events such as horse racing may facilitate the quick spread of EHM by direct contact when outbreaks occur.”

The international movement of horses has also played a role in some recent outbreaks of EHM. Other factors linked to increased incidence of EHM include poor biosecurity measures and the presence of stressors, along with other ill-defined environmental and host factors.

“Importantly, the mutant EHV-1 is now widely distributed within horse populations which implies a tendency toward the increased incidence and severity of recent EHM outbreaks.”

They say detailed information on both the host and environmental factors behind the recent rise in EHM cases remains elusive.

“Further research is required to determine robust epidemiological factors that promote the disease.”

They note that most studies into the equine immune response to EHV-1 infection have been in a laboratory setting, with only a few studies investigating the immunomodulatory effects in live horses.

More studies involving live horses that focus on viral properties important for the evasion of host immunity will help to expose possible therapeutic targets of EHV-1, they suggest.

Future progress in treatment and control of EHV-1 hinges on the combined application of detailed epidemiological data and in-depth knowledge of how the sophisticated viral biology promotes the progression of the disease.

EHV-1: A Constant Threat to the Horse Industry
Fatai S. Oladunni, David W. Horohov and Thomas M. Chambers
Front. Microbiol., 03 December 2019

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

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