Most of you with horses have probably heard of strangles, a highly contagious upper respiratory tract infection. The disease is caused by a strep bacteria and is transmitted from horse to horse by nasal shedding or discharge from draining abscesses. Dr. John Timoney from the University of Kentucky indicates in Equine Disease Quarterly that nasal shedding of the bacteria begins 4 to 16 days after infection and continues for 2 to 3 weeks in most horses. However, some horses may remain carriers of the organism for months or even years, shedding the organism to other horses.
To determine if the horse is a carrier, three nasal flushes should be performed and the fluid removed is cultured to determine if there are live organisms. There is a test called a PCR test that looks for DNA of the strep bacteria and is very sensitive. However, the PCR test will be positive even if the bacteria are dead so if the PCR test is negative, you can be fairly certain the horse is not a carrier. However, if it is positive, it does not indicate if there are any live bacteria and the horse is a carrier because you can have false positives.
Another method determining if a horse has strangles, particularly if the infection is deep inside, is a blood test called a strep antibody titer. A few rare horses can develop abscesses inside the abdomen or chest, and this blood test is one method of diagnosis. Fortunately, these cases are generally not contagious to other horses as the bacteria is not being shed. Strangles is a highly contagious disease. Talk to your veterinarian about vaccinating for strangles. If one of your horses develops the disease, it is important to follow your veterinarian’s recommendation on preventing spread of the disease.
Strangles is commonly found in horses less than 5 years old. The equine strep organism does not infect people and the human strep does not infect horses. The staff at Kansas State Diagnostic Laboratory reported a case of strangles that came into the lab from a 4-month-old foal that was having difficulty breathing. The foal had severe swelling in lymph nodes around the throat. It was having such a difficult time breathing that a surgical opening was made in the skin of the neck and into the trachea to allow the foal to breathe. Because of the severity of the disease, the foal was euthanized. Afterwards, the foal was found to have severe swelling of all lymph nodes in the back of the throat and the guttural pouches were filled with infection. Because strangles is a highly contagious disease, the rest of the herd was quarantined so no horses entered or left, and the quarantine lasted for 30 days after the last horse had no symptoms of strangles. Multiple horses in the herd developed strangles and after the last case resolved on the farm, thorough disinfection was used.
There are several things to be learned from this case. First, always quarantine any new horses you bring into your herd for three weeks. Second, if you are concerned with strangles, test any new horses before placing them in the herd and vaccinate all horses with intranasal vaccine. And because horses with previous exposure can react to vaccine, blood testing can be used to identify horses that may react to it.