Can We Prevent Strangles?

Strangles is a dreaded disease for horse owners. It is caused by bacterial infection with Streptococcus equi subspecies equi (referred to as S. equi). The bacteria typically infect the upper airway and lymph nodes of the head and neck. Strangles has affected horses for centuries, is highly contagious, can recur on farms with previous outbreaks, and is one of the most commonly diagnosed contagious equine diseases worldwide. S. equi might survive in water sources (buckets and troughs) for more than a month, but primary sources of recurrent infections are asymptomatic carrier horses that shed the bacteria for months to years.

Call your veterinarian right away if you suspect an outbreak; he or she can accurately diagnose illness and recommend control practices. Some state veterinarian offices require practitioners to report cases. Do not transport horses on or off the farm, and take all resident horses' temperatures twice daily (99-101.5°F is normal). Horses can transmit S. equi one to two days after developing a fever, so isolate at the first sign of fever to avoid spreading infection.

In an outbreak divide horses into three groups: 1) infected horses (with fever, nasal discharge, etc); 2) exposed horses ; and 3) unexposed horses. Nose-to-nose contact or bucket sharing should not occur between groups! Unexposed horses should be kept in a "clean" area, and they should have separate caretakers, equipment, water troughs, and pastures. People and shared equipment can transfer bacteria, so diligent hand washing and disinfection of supplies are necessary. If separate personnel aren't possible, then always handle healthy horses first. Use protective clothing (boots, gowns or coveralls, and gloves) when managing infected horses.

Thoroughly clean and disinfect all water troughs daily during an outbreak, following disinfectant label instructions (and ensuring they're labeled as active against S. equi). Disinfect all surfaces after removing manure/organic material, which inactivates some solutions. Compost infected horses' manure and waste feed in an isolated spot--don't put it on pastures. Rest sick horses' pastures for at least four weeks.

Some infected horses don't appear sick and continually reinfect your farm's herd. Ideally, vets should test all horses on the farm (perform nasopharyngeal swabs and washes to sample horses for infection), and they should test sick horses three consecutive times--yielding negative results--before putting them back with healthy horses. Upper airway and guttural pouch endoscopy can be useful for pinpointing carriers. Disinfection, isolation, and diagnosis can be costly, but they are cheaper than additional outbreaks.

Vaccination can prevent infection, but it doesn't guarantee prevention. It reduces the severity of strangles in most infected horses. Improper administration can mean poor protection and/or injection site complications, so your veterinarian should administer the vaccine. Intranasal vaccination confers the best local immunity.

Vaccinating during an outbreak is not recommended, but you can vaccinate healthy horses (without fever/nasal discharge) that have not contacted sick horses. After recovering from active infection, 75% of horses are immune for one to two years. Vaccinating recently exposed horses might result in purpura hemorrhagica (caused by an overactive immune response and resulting in limb and head swelling or small hemorrhages on the gums). Your veterinarian can check a blood antibody level on your horse to help determine if he is a good candidate for vaccination.

Evaluate your horse's exposure risk and your comfort with that risk and discuss vaccination with your vet. Consider vaccinating if your horse travels and is exposed to new horses regularly. Vaccinate broodmares prior to foaling on farms with a history of strangles. The initial vaccination requires a booster dose before it's effective, and it takes about one month from vaccination for immunity to develop. Therefore, be certain to vaccinate your horse before potential exposure to new horses.

About the Author

Amanda Martabano House, DVM, Dipl. ACVIM

Amanda Martabano House, DVM, Dipl. ACVIM, is an assistant professor in the department of large animal clinical sciences at the University of Florida's College of Veterinary Medicine.

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