The little-studied equine herpesvirus 8 (EHV-8) − until now only associated with respiratory disease in donkeys in Australia and horses in China − may well cause abortion in horses, according to researchers.
Nine herpesviruses have been identified that affect the Equidae family, which includes horses, ponies, donkeys and zebras.
It is understood that the horse is the natural host of five of these viruses (EHV-1, EHV-2, EHV-3, EHV-4 and EHV-5), and the donkey the natural host of three (EHV-6, EHV-7 and EHV-8).
It has been suggested that the natural host for the ninth (EHV-9), isolated originally from gazelle, is the zebra, but evidence points to the African rhinoceros as at least an additional potential natural host.
EHV-1 is common and found globally, causing respiratory disease. It is considered the most economically significant EHV because it is also associated with abortion and neurological disease in some cases, including brain swelling.
The disease-causing ability of EHV-8 is not well understood, and to date has been associated only with respiratory disease.
Researchers from Ireland and Scotland, writing in the open-access journal PLOS ONE, described how they sequenced the genome of four EHV-8 strains collected in Ireland between 2003 and 2015.
Two of these strains were collected from aborted foetuses in horses, and were misdiagnosed initially as EHV-1. Two were isolated from nasal swabs taken from donkeys, one with neurological disease.
Marie Garvey and her colleagues reported that the four genome sequences were very similar to each other, which is evidence of minimal diversity. EHV-8 was found to be closer to EHV-9 than it is to EHV-1 in an evolutionary comparison, although all three are closely related.
“The current study suggests that EHV-8 can cause abortion in horses,” they reported.
“The potential threat of EHV-8 to the horse industry and the possibility that donkeys may act as reservoirs of infection warrant further investigation.”
They said that although donkeys have been reported to be the natural hosts for EHV-8, the results show that the virus can cross host species and cause abortion in horses.
Infections caused by cross-species transmission of herpesviruses can result in increased virulence and cause severe or fatal diseases, they noted.
“It is possible,” they wrote, “that EHV-8 is under-diagnosed in horses because of its close relationship to other equid alphaherpesviruses.
“Indeed, the EHV-8 strains isolated from two aborted foetuses of horses in this study were identified incorrectly as EHV-1 during initial testing, due to cross reactivity of the PCR assay [test].
“Viral causes of abortion in horses have only been attributed to EHV-1, and less frequently EHV-4 and equine viral arteritis. In future, EHV- 8 will need to be included in the differential diagnosis.”
They said the availability of the EHV-8 genome sequences generated in the study will allow development of a specific test for EHV-8 in aborted foetuses and newborn foal deaths. It will also allow further investigation into the possible role of EHV-8 in equid neurological disease.
The EHV-8 strains isolated from the two horse abortions originated in two distinct provinces in Ireland about seven years apart. The source of infection in either case was unknown.
Donkeys and horses occasionally share pasture in Ireland, but neither mare had a history of contact with donkeys during the gestation period.
It has been suggested that water can act as a source of herpesvirus infections, and EHV-1 has been shown to remain infectious in water for up to three weeks. “However, there is currently no evidence to support the survival of EHV-8 in water as a basis for cross-species transmission to horses.
“As an alternative, the possibility that EHV-8 circulates continuously in horses, even at a low level, merits investigation.”
The availability of the EHV-8 genome sequences opens the door to development of an EHV-8-specific peptide-based ELISA test similar to that developed for the detection and differentiation of EHV-1- and EHV-9-specific antibodies.
This could be used to discover the prevalence of EHV-8 infections in donkeys and horses.
Alternatively, PCR tests could be used to determine whether these viruses represent sporadic events in which the donkey virus has crossed into each horse, or whether EHV-8 is circulating in some horse populations.
“The co-occurrence of EHV-1 and EHV-8 also needs to be monitored because EHVs have the potential to diversify rapidly by recombination.”
The study team comprised Garvey, Nicolás Suárez, Karen Kerr, Ralph Hector, Laura Moloney-Quinn, Sean Arkins, Andrew Davison and Ann Cullinane. They are variously affiliated with the Virology Unit at the Irish Equine Centre, the University of Glasgow and the University of Limerick.
Garvey M, Suárez NM, Kerr K, Hector R, Moloney-Quinn L, Arkins S, et al. (2018) Equid herpesvirus 8: Complete genome sequence and association with abortion in mares. PLoS ONE 13(2): e0192301. https://doi.org/10.1371/journal.pone.0192301