Neurological Herpes: Where Are We Now'

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The year 2003 was shocking for neurological herpes cases, with the heartbreaking stories about deaths at the University of Findlay in Ohio and cases at four locations across the country. The disease isn’t going away.

However, neurological disease has been recognized as a possible consequence of herpes virus — specifically EHV-1 or rhinopheumonitis — infection for a long time. While the cases were horrifying, there was no particular reason to think anything new was going on. It seemed an oddity. That’s no longer the case.

We were back to sporadic cases in 2004, but in 2005 we again noted a number of reports of significant outbreaks of herpes-related neurological disease, involving more horses than was typical of neurological herpes in the past. The progression of the disease also changed, from what used to be a relatively slow progression of symptoms to much more rapid deterioration. It’s a cause for concern.

Equine herpes researchers from international laboratories and universities both here and in Europe (where neurological herpes also occurs) mobilized rapidly to study the problem. They’ve found that these horses may develop unusually high numbers of circulating virus, which may increase the odds that the nervous system is attacked and have also found a specific genetic mutation that may be a marker for the more aggressive neurological cases, although it’s still true that EHV-1 virus strains without this marker can cause neurological disease, too.

What To Do
What all of this means is that, while we aren’t dealing with a new disease, and the old form of neurological herpes is still alive and well, we may be looking at the emergence of a specific strain that is particularly likely to invade the nervous system.

Outbreaks typically occur where horses are congregating in large numbers, such as racetracks, and where there is a lot of contact between horses, movement of people and equipment between horses, horses of varying ages, including young ones without robust immune responses based on prior exposures, and when horses are typically under a lot of physical stress from training, racing and shipping. The appearance of cases across the country, sharing the same genetic mutation, means that the problem is endemic — an established disease not confined to one limited location.

Controlling the risk and spread is ultimately going to come down to a better vaccine. The currently available killed-virus vaccines produce poor immunity, and even the common practice of frequent vaccinations (every two to three months) won’t reliably protect from abortion or respiratory disease. There’s also some suspicion that frequent vaccination may be a risk factor for the neurological disease, possibly because there’s no strong cell-mediated immunity induced by the vaccine, and keeping circulating antibody titers high may decrease the amount of stimulation the immune system gets from natural exposures. Researchers are now considering the advisability of going to modified live-virus vaccines instead. These do provide stronger and more broadly based immunity.

Until then, there are a few things you need to understand to decrease your horse’s risk:

A high percentage of adult horses are carrying inactive herpes viruses in their respiratory tract. Avoiding exposure is impossible, but a robust immune system response usually protects adults from the disease.

Risk of developing herpes-related disease is a complex combination of exposure to the organism, level of exposure (“dose”) and the immunocompetence of the exposed horse. Even within barns on racetracks where the virus is actively circulating, not all horses will come down with the disease.

Horses at highest risk for this, and any other, infectious disease are those that travel frequently and/or are stabled in barns with a lot of traffic. Some tactics for decreasing risk are:

• Strict quarantine of new horses for 10 days to three weeks, with daily temperature taking. Quarantine means quarantine. No direct contact with other horses, preferably by housing in a separate barn, no sharing of equipment between horses, staff dipping boots in disinfectant and washing their hands after working around a new horse.

• Pay careful attention to your horse’s attitude, appetite and energy level. Small changes are often the first clue that he’s not feeling up to par or is under stress.

• Take temperatures daily. Make it part of your routine. It doesn’t take much time to slip in a thermometer and let it “cook” while you brush your horse.

• If you find a mild temperature elevation or suspect the horse isn’t feeling 100% himself that day, don’t work him.

• Make sure the horse is properly conditioned for the level of work you expect from him.

• Allow adequate time for the horse to recover to his usual appetite, brightness and energy level after shipping or heavy exertion. Don’t work the horse until he’s back to normal.

• Give a day or two off after vaccinations, and don’t plan to ship or compete until seven to 10 days after routine boosters, two weeks after the final shot of an initial vaccination series.

• Maintain a high-quality diet, with adequate, balanced levels of minerals, vitamin E and B vitamins.

• Talk to your barn manager about having a meeting with your veterinarian to set up a contingency plan on how an outbreak of infectious disease should be handled. Different diseases may require different choices for disinfection, but the basics of when and how to quarantine sick horses, how to monitor other horses in the barn, how to avoid inadvertently spreading organisms, etc. are basically the same. The most important thing to do is to act quickly to stop the spread. This is easier if a plan is in place.

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