Equine Encephalosis Virus has spread beyond southern Africa. What may follow?

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Equine Encephalosis Virus (EEV) is closely related to far more dangerous insect-borne viruses such as African horse sickness and bluetongue.
A working horse in Africa. Image by François Benois

The spread of Equine Encephalosis Virus (EEV) beyond southern Africa serves as a warning of the risks of far more dangerous viruses spreading into new regions, according to the authors of a just-published review.

Equine encephalosis is a fever-inducing disease of horses transmitted by Culicoides biting midges.

The virus was first isolated from a horse in South Africa in 1967 and, until 2008, was believed to have been restricted to southern Africa.

In 2008–2009, the virus was isolated in Israel, after hundreds of horses across 60 premises fell ill with a fever, demonstrating its emergence in new regions.

Further testing of historic samples revealed that EEV had been spreading outside of South Africa since 2001, emerging in central Africa, the Middle East, and India.

“Although EEV normally does not cause severe clinical disease, it should be considered important since it may indicate the possible spread of other related, much more pathogenic viruses, such as African horse sickness virus,” Sharon Tirosh-Levy and Amir Steinman wrote in the journal Animals.

They cited the emergence and global spread of West Nile Virus as an example.

It should, they said, be a reminder not to limit the list of possible causes when facing a possible outbreak or a cluster of undiagnosed clinical cases.

In their review, the pair summarized current knowledge around EEV, including its clinical significance and how it infects horses.

Equine encephalosis was first described over a century ago by Sir Arnold Theiler as a mild form of African horse sickness, under the name “equine ephemeral fever”. The virus was first isolated in 1967 in South Africa from a Thoroughbred mare named Cascara, who died following a fever and neurological problems.

EEV is an orbivirus closely related to several other important pathogenic and emerging viruses affecting livestock, including bluetongue virus, African horse sickness virus, and epizootic hemorrhagic disease virus – all transmitted by Culicoides species.

Tirosh-Levy and Steinman said the clinical significance of EEV infection is difficult to determine but is probably low. Generally, it is associated with mild or subclinical disease in horses, with low death rates.

Characteristic clinical signs in symptomatic horses tend to involve a short period (typically two to five days) of fluctuating fever and poor appetite, sometimes accompanied by a racing heart and rapid breathing. There is no evidence it can spread to other species.

They said the name “equine encephalosis” is misleading, as the disease is not primarily neurologic.

The clinical signs are non-specific, they said, and could be easily confused with that of other viruses.

No specific treatment is available against the virus. Most symptomatic cases recover with no complications. Supportive treatment may be given to decrease fever and inflammation or to relieve other clinical signs. No vaccine is available.

EEV is considered non-contagious, and prevention strategies mainly focus on reducing exposure to Culicoides midges.

“Although EEV is probably not of major clinical importance, its emergence in areas outside southern Africa may precede the spread of other, closely related, Culicoides-borne pathogens, such as African horse sickness virus and bluetongue virus,” they concluded.

“For the time being, the virus has been reported from southern and central Africa, Israel, and India but has not spread to moderate climate countries in Europe or the Americas.

“The lower virulence of this virus, in combination with uncharacteristic clinical signs, makes its diagnosis challenging, especially in areas when it is not known to be endemic.”

They said future research is needed to better understand the spread of the virus and its ability to cause disease, as well as the dynamics of the circulation of various Culicoides-borne arboviruses.

Tirosh-Levy, S.; Steinman, A. Equine Encephalosis Virus. Animals 2022, 12, 337. https://doi.org/10.3390/ani12030337

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

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