Dr. Moore Talks Strangles

With strangles outbreaks in both eastern and western Canada, Dr. Alison Moore has provided Trot Insider with some in-depth information about Streptococcus equi.

Dr. Moore explains how strangles operates and offers important steps to aid in quelling its spread.

Dr. Moore's piece appears below.

Strangles Diagnosis and Prevention

With the recent outbreak of strangles at Truro Raceway and Northlands Park, it is important to remind horsepeople of a very important characteristic of this disease.

The bacteria causing Strangles, Streptococcus equi, can be 'carried' within the guttural pouches of horses for months and possibly years following infection. That means once the obvious signs of Strangles are gone, that is the swollen, abscessed area under the jaw, it does not mean the bacteria are gone. Typically, the horse sheds the bacteria through nasal secretions anywhere from three to seven weeks after abscess drainage. In some horses, however, the bacteria gain access to the guttural pouches present on both sides of the pharynx and can remain there indefinitely. Once that horse becomes stressed (ie. from shipping), the organisms may be shed, once again exposing other horses on the trailer and those back in the barn leading to a new outbreak. Therefore, in cleaning up the disease, it is important to identify these carrier horses.

Carrier horses usually show no outward signs of the disease, but occasionally may have intermittent nasal discharge and a sporadic cough which could easily be mistaken for other respiratory diseases. Identification of carrier horses is done most effectively by having your veterinarian perform a nasal lavage and then test the sample using a Streptococcus equi Polymerase Chain Reaction (PCR) that detects very small quantities of the bacterium’s DNA. This test must be coupled with bacterial culture as well. If they are only running cultures on the samples, they will not identify the carrier horses.

In an outbreak situation, sampling of horses should be handled as such: Once the draining abscesses have closed the horse should be tested two to three weeks later for S. equi DNA using the PCR and culture. After three consecutive negative results one week apart, the horse is considered to be clean. In the circumstance whereby the horse remains positive on PCR, the guttural pouches should be examined endoscopically. There may be pus within the pouch or chondroids may be present, which look like stones but represent solidified pus and debris. If these are not removed it is very difficult to clean up the pouch. Usually, when the pouches are found to be infected, they are lavaged and a penicillin gel instilled once a week for three weeks and then the horse is retested.

PCRs should not be run on obvious pus-like discharge as there are inhibitors in this material that can interfere with the test. A culture is best for this material.

There are presently Strangles outbreaks on farms in Canada and the U.S. This is important to remember when buying horses through dealers from both countries. It is always safest to isolate new horses, however, that is not always feasible in training centres. The best idea is then to use what is called 'barrier precaution.' This means minimizing the spread of disease by handling the new horse(s) last (ie. last to muck out, last to feed, last to jog or train), cleaning the water buckets last. Ideally, stalls should have solid sides with no nose to nose contact. A PCR test can then be used to screen the new horses. The turnaround time for the PCR test is only a couple of days, and three to five days for bacterial culture.

Remember, Strangles cannot be controlled or eradicated without identifying the carrier horses. Strangles can not only be a very serious disease to the health of horses but can be economically devastating to the racing industry. So act early, identify the carriers, limit the spread and prevent this disease from gaining access to your stable.