Equine herpesvirus type-1 (EHV-1), an important pathogen that affects horses worldwide, can remain stable and infectious under many conditions in water for up to three weeks, German researchers report.
The findings highlight a potentially important transmission route for the troublesome virus during outbreaks of the highly infectious disease.
“Although our experiments do not mimic EHV-1 in the natural environment, they provide valuable insights into factors that may influence their stability and ability to remain infectious in water,” Anisha Dayaram and her colleagues reported in the journal Scientific Reports.
“Further research is necessary to establish whether the laboratory results reflect virus biology in environmental samples,” the study team said.
Horses are natural hosts for equine herpesviruses. The most common, EHV-1, usually causes a fever and respiratory disease in the species. It can also cause fatal neurological problems and trigger abortions.
The researchers from the Leibniz Institute for Zoo and Wildlife Research, working with the Virology Institute at the Free University of Berlin, said there were many factors known to contribute to the ability of viruses to persist in water, including pH, salinity, temperature, and turbidity.
The study team noted that EHV-1 and related forms were able to cause infections in animals other than horses, particularly in captive settings.
Many of these captive mammals did not share enclosures with equids, but water sources overlapped. This was also the case in the wild, where equids encountered many species at waterholes.
The study team hypothesized that EHV-1 was stable in water, thus acting as a vector for the virus.
The study team described a series of controlled experiments conducted over 21 days in which they exposed EHV-1 to various water environments, involving different pH, salinity, temperature, and turbidity.
The results showed that EHV-1 remained stable and infectious for over a week in all the experiments conducted, and for up to three weeks under many of the conditions to which the virus was exposed. Water and pH proved to be the key factors.
The presence and infectivity of the virus was confirmed by both DNA-based testing (using real-time polymerase chain reaction) and cell culture experiments.
“Our results suggest that high pH increases the time that EHV-1 remains infectious, while salinity does not demonstrate a significant effect on the persistence or infectivity of EHV-1,” they reported.
This, they said, indicated that alkaline water sources may be the likeliest sources for EHV-1 infection.
Their results also indicated that EHV-1, when shed into aquatic environments, may physically associate with sediment where the virus may remain stable at a higher concentration than in the water.
The researchers said they were surprised EHV-1 DNA degraded more rapidly in the water than in the sediment, suggesting the soil may interact with the EHV-1 particles, drawing the virus from the water to the sediment, where it may be protected.
They said the data overall suggested that a wide range of salt concentrations will have limited impact on EHV infection, but that high salinity preserves viral DNA at the expense of viral infectivity.
“However, the water sources used by both captive and wild equids are unlikely to have salinity levels equivalent to those that inhibit infectivity and thus EHV is likely environmentally stable across a broad range of salt conditions in nature.”
Temperature had a major effect on its persistence in water, they said, with the evidence suggesting EHV-1 may remain infectious for longer at colder water temperatures (4 degrees Celsius). However, they stressed that EHV-1 was able to remain infectious for extended periods in a temperature range of 4–30 degrees C.
“The stability of the virus across a range of water temperatures increases the opportunity for EHV-1 to be transmitted in many different environments and may suggest transmission is possible across different seasons.”
The findings pointed to the possibility that EHV-1 shed by wild equids at water sources could provide the virus to infect potential hosts sharing the supply, they said.
“This in turn may account for the recent reports of EHV-1-like viruses detected in non-equid species in captivity, as water is a potential transmission conduit for the virus between different species.”
The study team comprised Dayaram, Mathias Franz, Alexander Schattschneider, Armando Damiani, Sebastian Bischofberger, Nikolaus Osterrieder and Alex Greenwood.
Dayaram, A. et al. Long term stability and infectivity of herpesviruses in water. Sci. Rep. 7, 46559; doi: 10.1038/srep46559 (2017).