Parker County Today March 2018 - Page 84

our expertise: VET COLUMN Equine Viral Rhinopneumonitis By Tom Hutchins, DVM, DABVP North Texas Veterinary Hospital 8283 FM 920 • Weatherford, Tx Serious outbreaks of upper respiratory tract disease can ruin a racing season, show season and foaling season. Administrators of boarding and training facilities have become increasingly au- thoritative about vaccination requirements. Most of these outbreaks are caused by equine viral in- fluenza. But when equine viral rhinopneumonitis is implicated, it can be devastating for the horse- man and the facility itself. The problem is great- est where horses are housed semi-permanently in accommodations provided at the facility. In other circumstances where horses are in respiratory con- tact only on the day of training or competition, the outbreaks are much less serious. 82 The most serious outbreaks in horses occur in the winter and spring with equine viral rhino- pneumonitis being the most common in winter and influenza causing most of the outbreaks in the summer. Two and three-year-old horses are the most frequent sufferers, as most four years of age or older appear to have developed resistance to the infections. In a horse population it is the average age and the mix of ages which largely de- termine its herd resistance, and when 30-40% of that population has not been previously exposed to infection then major outbreaks are likely. All of the equine respiratory viruses cause persistent reduction in the tracheal clearance rate of foreign material for at least 30 days. Clinical signs dis- appear much earlier than this and the recovery period should be extended to ensure that horses are not put back into work too early. There are four common types of equine herpes virus (EHV). • EHV-1 has a few subtypes. Subtype 1 is associ- ated with abortion and subtype 2 is associated Dr. Tom Hutchins with respiratory disease, but also capable of abor- tion. EHV-1 is the most potent equine abortifa- cient. • EHV-2 causes long-term infection in foals, some which develop clinical signs of purulent nasal discharge, fever and swollen lymph nodes in a syndrome that lasts about a week. It is rare for animals to die from this type. • EHV-3 is a venereal disease manifested by le- sions that start out flat, then pustular, then even- tually ulcerative in the vaginal mucosa which is generally reddened. The ulcers may be as large as 2 centimeters in diameter and 0.5 centimeters deep and are surrounded by a zone of redness and inflammation. • EHV-4 causes outbreaks of respiratory disease in any kind of horse population, at any time of the year. The disease is highly infectious, and trans- mission probably occurs by the inhalation of infected droplets or by the ingestion of material contaminated by nasal discharges or aborted fetuses. Immediate infection can occur with the virus surviving for 14-45 days outside the animal. The duration of infectivity of animals is unknown, but it is probably some weeks and possibly longer because carrier animals seem to be necessary for persistence of the disease from year to year. The disease is a mid inflammation of the re-