Cases of the neurological form of Equine Herpes Virus-1 (EHV-1) diagnosed on a North Island stud farm may not be New Zealand’s last, but infections will likely be sporadic and rare, a spokeswoman for the Equine Health Association believes.
A total of 14 horses on a Waikato property have now been confirmed with the infection. The Ministry for Primary Industries confirmed a 14th case was diagnosed over the weekend. The death toll remains at seven and the property remains under quarantine.
Marton-based veterinarian Trish Pearce, executive adviser to the Equine Health Association, told Horsetalk that the outbreak started in January, and all of the affected horses have been with in one area of the broodmare operation.
EHV-1 is known in New Zealand, having been isolated from respiratory infections in young horses. It is also thought to be responsible for abortions on studs from time to time, usually in the third trimester, but nothing like the abortion storms that had been reported in other countries, she said.
Most New Zealand studs vaccinated against the virus, she added.
“It’s interesting to us that we’ve never had the neurological form here,” she said.
Pearce said there were certainly cases of neurological disease in New Zealand horses, in which the animals suffered ataxia – a lack of co-ordination – and went down, but veterinarians were usually able to rule out EHV-1, identifying causes such as ryegrass staggers, paddock accidents, wobbler syndrome and the like.
It was possible, she said, New Zealand may even have had cases in the past, but if so, they had never spread to other horses on the scale of the Waikato outbreak.
“This is certainly the first time we have confirmed the neurological form,” she said.
The Ministry for Primary Industries had invested in extra testing capacity, and had been able to confirm that the neurological strain was responsible for the Waikato cases, she said.
Why then, if New Zealand has had EHV-1 circulating for many years, has it not recorded cases of the neurological form until now?
Pearce said it was a question that a lot of people had been trying to answer.
She said there were a couple of theories at this stage, both of which made sense.
Research has shown that a specific sequence of genes in the EHV-1 virus was more closely linked to outbreaks of neurological disease. It was possible that this particular form was not circulating in New Zealand as much as in other countries.
Recent New Zealand studies suggested that the level of EHV-1 virus in the New Zealand population was relatively low in comparison with EHV-2, EHV-4 or EHV-5.
Other theories have suggested that neurological disease is related to repeated exposure to the virus. Horses most commonly contract EHV-1 when young and the virus will often stay in their system. It can reactivate, especially so in circumstances where a horse is under stress. Alternatively, horses can suffer repeat infections.
Pearce said it has been suggested that the more a horse was exposed to the infection, the greater the risk of them developing neurological symptoms via an immune mediated response. It was possible that the virus was not circulating widely enough in New Zealand for neurological cases to develop through this repeated exposure.
“There is a lot known,” Pearce says of the disease, “but there is also an awful lot not known.”
She said the epidemiologists working on the case were still trying to piece together the whole picture, and it was hoped a paper could be presented on the outbreak at an upcoming veterinary conference in a couple of months.
There were also efforts under way to conduct further study on the level of latent infection of EHV-1 and the prevalence of the neurological genotype in the New Zealand horse population.
So should New Zealand expect more neurological cases in the future?
“I would suspect that we might get other cases,” she said.
“If we did, I suspect it would most likely be on a stud that has had the odd abortion in the past.
“As with most disease, three factors all converge to contribute to the final picture. In this case it is EHV-1, the horse’s immune status and environmental factors which can vary and could include any range of stressors.
“We aren’t going to see a huge rush of this. We might see it from time to time, but sporadically,” she said.
“But we can do a whole lot make sure that we don’t.”
Pearce said horse owners had a big part to play. For example, horses with symptoms of a cold or illness should always be left at home. Owners should always be careful around abortions, ensuring that they isolate and properly dispose of all abortant material.
They should employ good quarantine measures, monitoring and keeping new arrivals separated for a time.
Good biosecurity practices should also be encouraged at any horse event as a matter of course, she said.
Pearce said there had been some public discussion about the decision not to name the stud. She stressed it was not about withholding information from the public.
She said the stud owners were under considerable stress, and were being diligent in voluntarily complying with quarantine measures to contain the outbreak.
There was also a concern that if the stud was identified, it might encourage more traffic in its direction.
“The stud owners are trying to do their best. They are being diligent. We felt that, by making their name public, it would perhaps increase their level of stress.”
Naming the stud was not going to decrease the level of risk, she said.
Pearce said the evidence to date indicated the outbreak was contained. If the situation was to change, that decision would be revisited.
Meanwhile, the Ministry for Primary Industries has confirmed that the neurological strain of EHV-1 was detected in samples from one of the affected horses. However, the non-neurological strain had also been detected in one other affected horse on the same property.
“This demonstrates the complex association between EHV-1 variants and clinical disease,” the ministry said.
“There are no clinically affected horses or horses that have been in contact with affected horses leaving the stud farm.
“Any horses that have come from this farm, including yearlings, are healthy.
“All horses that are sick are located in one area of the stud farm, which is under quarantine.”
The area with affected horses is separated from the farm’s yearling operations, the ministry said.
“All routine work, including vet work and farriery, has been suspended on this area of the farm. Quarantine measures will not be lifted on this farm until it is certain that no horses on the property are clinically affected or actively shedding virus.”
Laboratory results from a sample of yearlings from the stud farm have come back as negative on nasal swabs and blood.
This boosted confidence that the yearling operation had not been exposed up to the date of sampling, and provided assurances of the effectiveness of the farm biosecurity practices to date, it said.
“The attending veterinarian and MPI are confident that active infection related to the current outbreak … appears to be contained.
“This outbreak is localised to a single property that does not pose a risk to other farms in the area.
“Veterinarians and horse owners should be reminded that despite the rare presentation of the neurological form of EHV-1, it should be expected to occur sporadically in New Zealand, as it has done in other countries around the world.”