Ask the Vet: Equine Infectious Disease

Answered by: Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S. Army Reserve (Veterinary Corps) Stillwater, OK.

Question: Can a horse have strangles without the swollen lymph nodes? Answer: While the swollen lymph nodes and nasal discharge are characteristic of clinical strangles, a horse can definitely be harboring the strangles bacteria without showing any external signs whatsoever.

Some horses get strangles and develop enlargement mainly in the retropharyngeal lymph nodes inside the guttural pouches instead of the submandibular lymph node under the jaw. Swelling of the retropharyngeal lymph nodes is often difficult to appreciate externally and is best seen by placing an endoscope in the guttural pouches. Horses will often maintain the strangles organism in their guttural pouches for weeks to months after recovering from clinical illness; the only way to know a horse if a horse is carrying Streptococcus equi equi is by doing several guttural pouch washes several days apart and submitting the fluid for culture or PCR testing.

Your veterinarian can assist you with this. Many horse owners opt to do this after their horse has had strangles so that they know when it is safe to expose their horse to other horses again without potentially transmitting strangles. Some farms also require this testing of new horses before they are allowed to mingle with the general population of the farm.

Question: Do any equine vaccines exist for Lyme Disease?

Answer: There are currently no Lyme vaccines licensed for use in horses. There are anecdotal reports of canine Lyme vaccines being used in horses, but no safety or efficacy data exists for this off-label use.

An equine experiment with a Lyme vaccine using the OspA antigen did show evidence of protection, but there is no commercial vaccine available that is precisely analogous to the experimental vaccine used in that study. For prevention of Lyme disease in horses, tick control and regular grooming are recommended.

Question: My horse is losing weight rapidly for no apparent reason. My horse tested negative for parasites and there has been no change in diet.

Answer: The primary causes of weight loss in horses are parasitism, dental disease and inadequate nutrition. You have already ruled out parasitism and it sounds like his diet was previously adequate; have you had a change in dietary quality (new hay supplier, etc.)? Has your horse had a thorough dental exam?

Once those three potential causes are ruled out, a weight loss workup generally includes an abdominal ultrasound, a belly tap and a rectal examination to start. In these workups, we are often looking for masses, which can be abscesses or neoplasia (cancer). You do not indicate your horse’s age or whether he has had recent illness, so it is impossible to tell which we might be more likely to find.

Some diseases that can cause internal abscesses can be detected via a blood test; these include Corynebacterium pseudotuberculosis (the causative agent of Pigeon Fever) and Streptococcus equi (the causative agent of strangles). These blood tests may help narrow down the cause of weight loss.

Finally, some causes of muscle wasting may appear to be rapid weight loss, such as immune mediated myositis also caused by Streptococcus. There are also infiltrative bowel diseases that can manifest as weight loss, but these are very difficult to diagnose without exploratory surgery and biopsy.

You may need to visit a referral center to obtain some of the above mentioned diagnostics; good luck with your horse!

* Reprinted with permission of the AAEP. To view the entire article please visit  then click Horse owners, Ask the Vet