RESPIRATORY DISEASES IN SHOW HORSES: A Review for Horse Owners Dennis D. French, DVM Diseases of the respiratory system are common in show horses all over the country and are a major contributor to poor performance and dehabilitation in many performance horses. Infectious causes of respiratory disease most of- ten produce serious disease in young horses, especially those being cam- paigned heavily on the show circuit. It is of paramount importance that these diseases be prevented if a suc- cessful show season is to be attained. Covered here are some of the causes of infectious respiratory diseases, the means of prevention that are currently employed, and what may be on the horizon that will help us prevent these diseases in the future. The bacterial causes of primary respiratory disease in the horse are Streptococcus equi, or "strangles", and Rhodococcus equi, or "rattles". Strangles is a highly infectious disease that is transmitted by inhalation or in- gestion of the organism and subse- quently localizes in the mandibular and pharyngeal lymph nodes. The organ- ism is usually spread via nasal dis- charges, by contaminated grooming utensils, or humans. The incubation period is 3 to 10 days and if the organ- ism is introduced into a naive popula- tion, nearly 100% of them will become Author's address: School of Veterinary Medicine, Louisiana StateUniversity,Baton Rouge,LA 70803 infected. With good management, death loss from strangles will be less than 5%. Young horses with this disease usually present with an acute onset of depression, off-feed and nasal dis- charge. Swelling may occur in the sub- mandibular region or in the throat latch area as an initial sign, and this swelling may cause difficult breathing ° hence the name strangles. Clinical signs may persist for days to months and lymph nodes commonly abscess with the dis- ease. Once the node is drained, either following rupture or surgical lancing, recovery is uneventful. Prevention of strangles is improved if strict quarantine measures are em- ployed, which obviously does not ap- ply to show horses. Vaccination against the strangles organism is possible and several vaccines are available. Vacci- nation has been shown to reduce the severity of the disease but it does not prevent the disease. The best immunity comes from a series of three vaccina- tions, followed by annual booster shots. However, injection site swelling, mild depression and inappetence are com- mon following vaccination. Therefore, it is wise to be cautious about the timing of administering strangles vaccine. Rattles is a debilitating disease of foals caused by Rhodococcus. There is a subacute and chronic form of the disease. This disorder is much more prominent in foals kept in large groups and is restricted to foals 1 to 6 months of age. Prevention is only by isolation from others infected. There is no vac- cine. Once a foal is infected, long term antibiotic therapy is indicated. Most foals that get Rhodococcus equi infec- tions will not be capable of being shown for nearly a year following recovery. They become very sick. Viral respiratory diseases are a fre- quent cause of owners seeking atten- tion for their horses. Influenza and her- pesvirus are the post prominent causes of viral respiratory diseases. Outbreaks of viral respiratory disease cause a high degree of wastage and poor perfor- mance in the racing industry. Valuable training time is lost while the horses recover from these viral infections. The state of New York actually had to shut down some of its racetracks during March of 1994 to help control an out- break of herpesvirus (equine rhinopneumonitis). The most common signs of disease include loss of appetite, fever, and de- pression. Nasal discharge is usually present within 24-48 hours after the onset of infection. The respiratory vi- ruses inhabit the upper respiratory epi- thelium and cause ulceration of this tissue. This ulceration may then lead to secondary bacterial infections and pleu- ropneumonia. Once the epithelium has been ulcerated, it takes between 14 and 17 days to restore it to normal structure and function. This is the problem that owners and trainers run into with horses in training. The patience required to wait for the epithelium to heal is very difficult to find. There are no effective treatments for viral diseases. Therefore, preven- tion of disease should be practiced for all show and performance animals be- cause they will be exposed to these viruses. Killed and modified live virus vaccines are available for rhinopneumonitis. The efficacy of these vaccines has been questioned, much as they have for any herpesvirus. It does appear that vaccination will decrease the severity of clinical signs and the duration of viral shedding. Volume 14, Number 5, 1994 279