Findings of EHV-1 study in horses may explain rise of dangerous neuropathogenic strain

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Most horses by the age of two will have been infected by the EHV-1 virus.
Most horses by the age of two will have been infected by the EHV-1 virus.

The neuropathogenic variant of Equine Herpes Virus type 1 (EHV-1) has been found to produce far more virus shedding that its common counterpart, possibly explaining the dangerous form’s increasing prevalence among domestic horses.

The neurologic form of EHV-1 causes what veterinarians call equine herpes virus myeloencephalopathy, an often fatal condition arising from damage to blood vessels in the brain and spinal cord associated with the infection.

Infections in horses with the milder form − the non-neuropathogenic strain − are common. Indeed, by the age of two, nearly all horses have been infected with EHV-1, usually resulting in little more than mild respiratory problems.

The neuropathogenic variant is more commonly associated with neurological problems and cases are on the rise. Based on two retrospective large-scale studies, it has been estimated that the odds of developing neurological disease with the neuropathogenic form were 490 times greater than with the common variant.

German and American researchers followed up on three recently conducted experiments in domestic horses and ponies that looked for virus shedding differences between the neuropathogenic and non-neuropathogenic variants.

“These results were interpreted as suggesting the absence of a consistent selective advantage of the neuropathogenic variant and therefore appeared to be inconsistent with a systematic increase in the prevalence of neuropathogenic strains,” Mathias Franz and his colleagues reported in the journal Viruses.

In the current study, the researchers integrated the raw data from all three experiments into a single statistical analysis which helped them overcome problems of small group sizes.

The resulting picture was clearer.

“The results of this combined analysis showed that infection with the neuropathogenic EHV-1 variant led to a statistically significant increase in viral shedding,” they reported.

Their modeling indicated that infections with the neuropathogenic variant led, on average, to a four-fold higher amount of nasal EHV-1 shedding compared to infections with the common variant.

“This finding is consistent with the idea that neuropathogenic strains could have a selective advantage and are therefore systematically increasing in prevalence in domestic horse populations.

“However, further studies are required to determine whether a selective advantage indeed exists for neuropathogenic strains.”

The research team comprised Franz and Alex Greenwood, both from the Leibniz Institute for Zoo and Wildlife Research in Berlin; Laura Goodman and Gerlinde Van de Walle, both from Cornell University in Ithaca, New York; and Nikolaus Osterrieder, from the Free University of Berlin.

A Point Mutation in a Herpesvirus Co-Determines Neuropathogenicity and Viral Shedding
Mathias Franz, Laura B. Goodman, Gerlinde R. Van de Walle, Nikolaus Osterrieder and Alex D. Greenwood.
Viruses 2017, 9(1), 6; doi:10.3390/v9010006

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

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