We had a great turnout with excellent questions.
The presenters reviewed the facts about the virus and the disease conditions associated with it in horses. The most important thing to understand is that essentially all horses are infected with some strain of EHV-1 early in life – and it then stays in the horses body in a dormant form – a condition termed latency – and can remerge later during some stress – called recrudescence. This process of latency and intermittent reemergence goes on throughout life. The example of how herpes behaves in humans with the appearance of fever blisters or “shingles” was used as how it behaves in a similar manner in humans. During recrudescence, there may be shedding of virus from the nasal passages and nasal secretions into air and subsequent infection of other horses. This is the cause of the usual pattern of spread o respiratory disease that we have always observed, and similarly, is the manner of spread for the neurologic form of the disease as well as infection of pregnant mares with subsequent abortion or delivery of weak neonatal foals that often die in the first few days of age. We also know that the viral shedding process declines over 14 to 21 days and that after that period the animal generally become non-contagious, but the virus may have returned to its latent form – as with the usual behavior of herpes virus. The issue is that there are some percentage of horses in the world that are latently infected with the so-called “neurotropic” strain. Reactivation of this could produce outbreaks like we saw from the event in Utah. There have always been single cases of EHV-1 associated spinal cord disease scattered around – often as an individual case. These may be from the “wild type” or more common strain of the virus in that there is less of a contagious spread.
What we have learned about EHV-1 is that there are different strains with a certain strain recently called the “neurotropic” strain being more associated with the outbreaks of spinal cord disease, but all strains can cause this condition. The issue with the “neurotropic” strain is that this particular strain divides much more rapidly and producing shedding of much higher levels of virus in the nasal secretions and thus much more contagious within an air space. It is this feature that justifies the restriction of movement of horses.
Clinical signs of disease
We know that the first clinically detectable sign of infection is a fever, thus it is recommended to monitor the temperature of potentially exposed horses twice daily and it is a good practice to monitor the temperature of all horses at events at least once daily. The “incubation period” of the virus – the time from exposure to development of clinical signs can be anywhere from 2 to 10 days. The first clinical signs are usually dribbling of urine, weakness of the tail, dangling of the penis in males, and abnormal rear limb gait that can progress to recumbency. These signs are because the virus seems to affect the spinal cord more toward the back end of the horse, but it can affect all areas of the spinal cord and the brain. A good neurologic examination is important to determine the pattern of involvement of the nervous system as there are many other diseases that can affect the nervous system of the horse such as botulism, other viral encephalitis (Eastern, West Nile, Rabies), or EPM.
Diagnosis of the disease condition Equine Herpes Myeloencephalitis (EHM) requires the following for precise definition by the USDA… exposure to a known case, positive test for the virus (nasal swab for PCR usually), and clinical signs of neurologic disease consistent with the effects of the EHV-1 virus.
Diagnostic testing of affected horses will include a minimum of nasal swabs for PCR testing, and possibly cerebrospinal fluid sampling to determine if changes are consistent with EHV-1.
The panel discussed the issues of diagnostic testing being restricted to select groups of horses – those showing clinical signs or those having been directly exposed to a clinical case. It is not recommended to test clinically normal, unexposed horses in the general population as we know that the virus normally cycles in horses with low levels of detection being possible in a certain percentage of horses at any given time. This is the reason why a so-called “positive” test could be found in a clinically normal horse. It does not mean the horse will develop the disease. This makes life more complicated in that the “positive” test result is officially reportable and could result in restriction of the horse’s movement for 21-28 days. It is so important to understand that this “cycling” of virus in horses is probably occurring intermittently in many horses.
Prevention of the disease
The audience heard the term “biosecurity” several times in the discussion. The concepts of avoiding mixing horses in a confined air space, avoiding sharing water buckets and tack, and hand washing were emphasized in helping prevent spread of not only EHV-1, but also Strangles and influenza at events. The only sure way to avoid contact with horses that might be shedding virus – of any type – is to avoid mixing horses . The issue of the time frame of 21 days was discussed after the Utah event in that this should allow horses to stop shedding. If a new clinical case showed up on a premise, then the clock for that premise would be reset from the last clinical signs.
Common disinfectants were recommended for cleaning tack, stalls, trailers, and equipment. Bleach diluted in a 1 to 20 solution (4 ounces per gallon) is an excellent disinfectant, but requires removal or organic material like fecal matter and thick secretions first. Other disinfectants in the quaternary ammonium category are also very effective against the virus. It can live for several days in the environment, but it is very susceptible to ultraviolet light.
The discussion also covered vaccination. We know that there are no commercially available vaccines that are specific for the neurologic form and that well-vaccinated animals have developed the neurologic disease. What is know that vaccinated animals shed much lower levels of virus and it is that benefit of routine vaccination that could help reduce severity and lessen spread of the virus in groups of horses.
The bottom line
We have always had herpes with us in the horse population. The conditions at the Utah event led to the “perfect storm” situation in that horses shedding the neurotropic form exposed a larger number of horses that subsequently dispersed to many locations throughout the west with potential secondary exposure of horses. Authorities in the different states and groups like the NCHA took appropriately responsible steps in limiting the spread by restricting movement and canceling events for a time to allow potentially infected horses to stop shedding and being contagious. We should be able to get back to business as usual in the next couple of weeks, but the opportunity of this outbreak is that we all should make the effort to become much more vigilant in paying attention to biosecurity when we attend events.
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