EHV-1 viability has implications for biosecurity

Under some conditions equine herpesvirus type 1 can persist in water for up to three weeks.
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New research suggests that equine herpesvirus type 1 (EHV-1) can persist much longer in water than previously thought, meaning that biosecurity measures taken during an outbreak must include buckets, troughs and natural water sources.

EHV-1 is one of five strains of herpesvirus known to affect horses. EHV-1 infection typically results in respiratory disease (rhinopneumonitis), but it can also cause pregnant mares to abort their foals.

In rare cases, the virus attacks the central nervous system, producing signs ranging from incoordination to paralysis. This potentially fatal neurological form, technically known as equine herpesvirus myeloenceph- alitis, is often referred to as “neuro” EHV-1.

Researchers at the Leibniz Institute for Zoo and Wildlife Research and Freie Universität Berlin decided to investigate the viability of EHV-1 in water after observing interspecies EHV outbreaks at zoos. The animals were in separate enclosures and had no physical contact.

“Water wasn’t generally considered an ‘environmental vector’ for equine herpesviruses,” explains Anisha Dayaram, PhD. “It was generally assumed the virus was not stable in water. However, many other viruses are known to be passed via water, although those viruses that have been investigated to date usually have a direct impact on humans, such as influenza and noroviruses.”

In a laboratory setting, the researchers added a known quantity of the virus to a variety of water samples with different pH levels, sediment content and salt levels. Individual samples were also kept at different tem- peratures: 39, 68 or 86 degrees Fahrenheit. As days passed, the researchers measured the so-called cytopathic effect in each water sample, which indicates whether an organism is able to invade cells.

“Cytopathic effect is a term used in cell culture to describe the infection of cells by virus particles,” says Dayaram. “This can be visualized under a microscope, as the healthy cells will usually form a monolayer; however, when infected with virus, the cells begin to die, creating patches in the monolayer known as cytopathic effect.”

That data showed that EHV-1 remained infectious for the entire length of the study period---21 days---in alkaline water with a pH of 10, at all temperatures and in slightly salty (brackish) water. In water with low- sediment concentrations (7.5 to 14 g/L) and high salinity (similar to seawater), the virus was viable for 14 days. EHV was infectious for three days in water with high sediment concentrations (50 to 100 g/L) and at low salinity levels (similar to fresh water). EHV-1 remained infectious for only one day in water at a pH of 4, which is slightly acidic.

Although this study was limited to a laboratory setting, the potential viability of EHV-1 in water sources has implications for how to manage outbreaks in stables.

“This study suggests that EHV-1 may be transmitted via an indirect route such as water,” says Dayaram. “So communal buckets, troughs and any other standing water sources are potential indirect vectors of the virus. If horse owners suspect an outbreak the best thing they can do to limit transmission is to isolate the suspected individual from other horses, make sure they do not share the same water sources and get the horse tested for EHV to confirm the infection.”

Reference: “Long term stability and infectivity of herpesviruses in water,” Scientific Reports, April 2017

This article first appeared in the July 2017 issue of EQUUS (Volume #378)