Analysis by several laboratories in Europe of positive samples from horses with Equine Herpes Virus (EHV-1) has found that the virus does not have the N752D amino acid substitution in the DNA polymerase that has been shown in the past to be associated with neurological disease.
Work on identifying the clade or genetic group to which this virus belongs is ongoing and will assist in tracking the spread of the virus and differentiating it from many other strains of EHV-1 in circulation, the FEI Veterinary Epidemiology Working Group heard at its first meeting on Thursday.
The group was created following the European outbreak of EHV-1 that originated at a competition in Valencia, Spain, in February. The virus is responsible for the deaths of 17 horses in Germany, Belgium and Spain.
Research led by Britain’s Animal Health Trust in 2018 on the diversity of EHV-1 sequenced 78 strains isolated over a 35-year period, and found they included low passage isolates from respiratory, abortigenic and neurological outbreaks. Most neurological isolates had the N752D substitution, whereas most abortion isolates did not, although three of the neurological isolates from linked outbreaks had a different polymorphism. Bioinformatic analysis also suggested that recombination has occurred between EHV-1 clades, between EHV-1 and equine herpes virus 4, and between EHV-1 and equine herpes virus 8.
The Working Group noted the need for improved network tracing, which would require the FEI being granted a greater jurisdiction at FEI events in the event of an EHV outbreak. The group also discussed epidemiological links between events where positive cases have been reported, and further known transmission of the disease at the home stables of affected horses.
EHV is an endemic disease worldwide and is notifiable in only a few countries. Concern was raised that should the disease become notifiable in more countries it could lead to reduced reporting, meaning that outbreaks could become more difficult to manage.
Regarding the evolution of the outbreak, the group agreed that several risk factors could be expected within the next two weeks. They included the transport of horses (potentially causing raised stress levels), which may result in further recurrence of the virus and more confirmed cases. “Efforts must be focused on both preventing the incursion of virus positive horses at events and contingency planning to mitigate the impact of such an incursion, should it occur,” a statement from the group said.
The group stressed that the circulation of active virus is expected to continue because this virus often moves more slowly through groups of horses compared to a virus such as Equine Influenza. Therefore, it may take some time for more recently infected groups of horses to be released from isolation.
Safe resumption of competition involves two key areas: Conditions for entry and the management of horses within the venue. The group’s recommendations included event size restriction, pre-movement testing and pre-travel health certificates issued before the horse travels, routine health monitoring and good separation between horses.