A New Zealand study aims to drill down into the prevalence of equine herpesvirus 1 (EHV-1) in the country, and hopefully provide more answers about the dangerous strain that can cause neurological disease.
EHV-1 is a common virus among horses around the world, including New Zealand.
Data on the prevalence of EHV-1 in New Zealand, which dates back 20 years, suggest up to 70 percent of adult horses carry antibodies to EHV-1.
Horses are usually exposed at a young age. Many infected horses show no clinical signs of disease at the time of EHV-1 infection, while others may develop respiratory disease.
However, on occasion, EHV-1 infection can result in a potentially fatal neurological disease, often referred to as equine herpesvirus myeloencephalopathy or EHM.
Researchers have found that a single amino acid variation in an enzyme that is needed to make copies of viral DNA creates a different type of EHV-1, which is more likely to cause neurologic disorders in horses. This variant is often referred to as “D752” or “mutant EHV-1” to reflect the change in the amino acid sequence at position 752 of the enzyme from a more common form containing asparagine (N) to the “mutant” form containing aspartic acid (D).
Authorities in the United States have noted growing numbers of cases involving the D752 variant of EHV-1, although evidence suggests it has been around since at least the 1950s.
The quarantine restrictions required to contain the regular outbreaks in the US have proved economically damaging, resulting in the closure of racecourses and the cancellation of major events, as the well as the loss of horses.
Scientists have yet to piece together why the small change turns a virus that normally causes little more than a cold into a potential horse killer. They acknowledge that the disease is complex and many questions remain.
For example, not all horses infected with the D752 variant of EHV-1 will develop neurological disease.
Conversely, horses infected with the common type of EHV-1, often referred to as the wild type or N752 variant, may also on rare occasions develop neurological disease.
Both variants of EHV-1 are capable of causing abortions.
New Zealand had not had any confirmed cases of neurological disease in horses resulting from EHV-1 in any form until an outbreak at a Waikato stud farm early in 2014. Reports at the time indicated 14 horses were infected, seven of whom had to be euthanised.
Research undertaken at Massey University showed that the D752 variant of EHV-1 had been present in New Zealand for at least 2 years before the Waikato outbreak of EHM.
Testing of post-mortem samples collected in 2012 from 52 horses at a South Island slaughter plant found that 17 carried EHV-1 identified as the N752 variant – the relatively common wild-type virus. However, one of the horses also tested positive for the D752 variant – the first confirmation of the presence of the potentially more dangerous variant of EHV-1 in the country.
The factors that have made myeloencephalopathy in New Zealand a rarity – at least to date – remain unknown.
Now, funding has been sourced to enable further research into the virus.
Magda Dunowska, senior lecturer in veterinary virology at Massey University’s Institute of Veterinary, Animal and Biomedical Sciences, says much remains unknown about EHV-1.
Scientists have yet to establish what factors are at play in determining whether an infected horse might remain clinically normal, develop respiratory disease, or even the potentially fatal neurologic disease. Similarly, they do not know why only some horses infected with EHV-1 develop neurological disease.
Factors with the potential to influence the outcome of EHV-1 infection might involve the virus itself, the environment in which the horses are kept, or perhaps even something about the EHV-1 infected horses themselves.
Dunowska says one factor that has been identified is age. The ability of the virus to cause neurological problems is age-dependent, with older horses more likely to develop myeloencephalopathy than their younger counterparts.
In fact, foals rarely develop neurological disease following EHV-1 infection, even in an outbreak situation in which their dams are affected.
Dunowska says the new study aims to extend the data obtained from the initial New Zealand-based survey from 2012 by testing further samples from a broader population of New Zealand horses.
The researchers hope to source post-mortem samples from up to 100 horses, to not only determine the prevalence of latent EHV-1 infection among sampled horses, but also the relative frequency of detection of the D752 and N752 variants of EHV-1.
It is hoped the data will provide insights into the geographical distribution of the N752 and D752 variants of EHV-1 within New Zealand.
Dunowska says an effort will be made to obtain background information on each of the horses, such as age, breed, purpose, and recent travel history, in a bid to identify potential risk factors for infection with the D752 variant of EHV-1.
The researchers will test lymph nodes collected from the horses for the presence of EHV-1 DNA by the technique called polymerase chain reaction (PCR).
She says the research will be designed to identify not only horses with an active EHV-1 infection, in which the virus would be in abundance, but also latently infected horses – evidence that the horse had suffered an active EHV-1 infection at some point in the past. Such horses are difficult to recognise, because they are clinically normal and may harbour only very small quantities of the viral DNA in their tissues.
To this end, the actual testing for the virus will be preceded by the enrichment step designed to “pull out” viral DNA sequences from a large pool of unrelated host DNA. Dunowska says that this enrichment step makes the study costly and technically demanding.
The laboratory work will be carried out in the Institute of Veterinary, Animal and Biomedical Sciences’ virology laboratory at the Palmerston North-based university.
Funding for the study will be provided by the Ministry of Primary Industries, the New Zealand Equine Health Association, and the New Zealand Equine Veterinary Association.