New Zealand has been mercifully free of practically all the nasty infections capable of causing neurological disease in horses.
A good number of these are mosquito-borne, such as Eastern Equine Encephalitis, West Nile Virus and the like. Until a few days ago, the nation had never had a reported case of the neurological form of Equine Herpes Virus-1 in horses.
The announcement by the Ministry of Primary Industries that 13 horses have been infected at a Waikato stud farm, seven of which were euthanized, has understandably triggered considerable concern among horse owners.
The ministry has declared its confidence that the outbreak is being managed; that a quarantine is in place.
It has made reassuring noises that the EHV-1 virus is prevalent among horses in this country, with most being exposed to the virus in the first few months of life. Many show no signs or just the mildest of symptoms – perhaps akin to a slight cold – and then get over the infection. However, they remain carriers for life.
Horse owners will be assured by the confidence expressed by the ministry, but it is fair to say this is only part of the story. There are a number of unanswered questions.
Firstly, while EHV-1 is common in this country and infections comparatively harmless, the neurological form is nasty indeed. Once horses go down, they rarely survive and are euthanized.
Unlike West Nile Virus and Eastern Equine Encephalitis, which are spread by the bites of mosquitoes, EHV-1 is spread by direct horse-to-horse contact or by contaminated hands and equipment.
Horses can be carriers, appearing perfectly healthy yet spreading the virus via the secretions from their nostrils.
In short, it is highly infectious and easily spread.
The neurological form of the disease is causing growing concern in the United States, where scientists have noted an apparent increase in the number of cases in recent years. Horses can be inoculated against EHV-1, but current vaccines are not effective against the neurological form.
The disease is concerning not only because of the often dire consequences for infected horses, but because it is economically damaging. Outbreaks must be contained, and training areas, equine centres and even entire racetracks have had to close while the risk of infection passes.
The EHV-1 virus can also trigger abortion in mares, causing distress and economic loss to breeders.
Scientists are well aware that stress can reactivate the virus in older horses, resulting in the shedding of the virus and the infection of other horses. This has proved to be a problem for horse sales and events, where horses may understandably find themselves under some stress.
Why, then, do some horses develop neurological disease and others do not?
Scientists acknowledge that the factors determining whether horses develop neurological disease is poorly understood.
To me, the big question over the New Zealand outbreak is the nature of the strain.
It would appear that the “ordinary” EHV-1 virus is capable on rare occasions of causing neurological disease. Scientists are not quite sure why, but believe that the levels of virus in the bloodstream, the state of the horse’s immune system and environmental factors play a part, but this all remains poorly defined.
However, scientists have identified specific strains of EHV-1 that are known to be more likely to cause neurological disease in horses through what is apparently a quite minor genetic variation.
This variation has been strongly associated with the occurrence of neurological disease in horses. The so-called D752 strains carry this gene variation. Scientists call these the neuropatholgenic strains.
Scientists offer two possible scenarios for the origin of the higher risk D752 strains.
First, the virus is simply reactivated in a horse latently infected or, second, the low-risk variant, known as the N752 strains, spontaneously mutate to the high-risk form.
It is even possible that both events occur at the same time, scientists suggest.
Given the complete absence of outbreaks of EHV-1 causing neurological disease outbreaks in New Zealand until now, one could be drawn to the conclusion that the D752 strains have not been circulating here at all. If it was, why have they not caused neurological disease until now?
If the D752 strain was responsible for the Waikato cases, did it arise as a spontaneous mutation or was it imported by a carrier horse which then shed the virus, potentially even years later?
While the less dangerous N752 strain is known to be capable to causing neurological disease, it does not seem plausible that it would infect 13 horses and cause the euthanasia of seven. The suspicion has to be that this is the D752 strain.
A spokesman for the Ministry for Primary Industries confirmed to Horsetalk that scientists were doing further testing to find out more about the particular strain of EHV-1 behind the outbreak in Waikato.
This is good news, and may provide some answers, but the ministry suggests we may never know the source of the disease.
In the meantime, New Zealand horse owners must remain vigilant, especially so in the Waikato region.
For now, horse owners need to brush up on basic biosecurity measures and manage the contact that other people have with their horses. The stakes are high, and farriers, veterinarians, equine dentists and the like should understand the concern of horse owners in ensuring there is no risk to their animals.
Finally, EHV-1 is not a notifiable disease in New Zealand.
For the time being at least, the ballgame has changed. Neurological cases of EHV-1 must be reported authorities to provide every opportunity to contain any outbreaks within a property legal framework.
This issue needs to be addressed with urgency.
New Zealanders will be hoping this is the last we see of this cruel and dangerous form of the disease. There are no guarantees. For now, we need to stay informed and watch developments closely.