A new horse arrives at the 20-stall boarding facility where your horse resides. He gets turned out in a field with three other horses, and all seems well. But after about a week, he starts to develop an abscess beneath his jaw, and a couple of the horses he’s pastured with are coughing.
Does the horse have strangles? Is the rest of the barn at risk?
This is a panic-inducing scenario many veterinarians might encounter on clients’ properties. So David Rendle, BVCs, MVM, CertEM(IntMed), Dipl. ECEIM, MRCVS, of Rainbow Equine Hospital, in North Yorkshire, U.K., described how to handle it while at the 2018 British Equine Veterinary Association Congress, held Sept. 12-15, in Birmingham.
Strangles develops when Streptococcus equi bacteria infect horses’ head and neck lymph nodes, sometimes resulting in painful abscesses. It’s highly contagious and can spread rapidly through horse populations and barns.
When evaluating a suspected strangles case, said Rendle, the veterinarian should first gather the horse’s history and a timeline of his clinical signs. This might include asking questions such as, how old is the horse (younger horses are more likely to be affected by S. equi)? What signs has he shown over the past two weeks? Does he have a history of dental or respiratory disease (which can be confused for strangles)?
Typical clinical signs of strangles include lethargy, fever, cough, and that telltale abscess. The first signs you’ll see, usually within three to 14 days of exposure, said Rendle, are lethargy and fever.
“Fever occurs before horses become contagious, providing an opportunity to isolate and halt the spread of infection,” he said.
About two to three days after the horse develops a fever, he typically starts shedding the bacterium via pus-filled nasal discharge. Nasal shedding can last for two to three weeks or longer if infection sets up within the sinuses or guttural pouches (cavities in the back of the head), he said. Not until one to four weeks later will an abscess develop and burst.
Rendle said infected horses might also show less-obvious signs, including mild transient fever, mild clear discharge, and lack of appetite yet never develop abscesses. “Often these horses are the ones that spread the most disease,” he said. “Transmission from outwardly healthy horses is often more of a problem than transmission from horses with clinical signs.”
A veterinarian’s ability to confirm a strangles diagnosis depends the stage of disease, which isn’t always known. Rendle recommended performing a guttural pouch lavage (flush) with culture and PCR testing rather than taking less-reliable nasopharyngeal swabs for chronic cases. For more acute cases, he said, timing is critical.
“S. equi doesn’t reside in the nasal passages, so you’re reliant on timing and luck (with swabs),” he said. “It’s only really effective (as a diagnostic) in early infection with copious discharge.”
At least three weeks after all clinical signs have ceased, he suggests performing blood testing on all horses on the property to determine whether they’re carriers. “Ten percent of horses in an outbreak are expected to develop persistent guttural pouch infection,” he said. These horses show no clinical signs yet continue to spread S. equi.
Preventing Disease Spread
If diagnostics do confirm a strangles case, Rendle said property managers should isolate all horses with fever or other clinical signs immediately. These horses should stay in isolation for at least two weeks after clinical signs cease and should undergo testing to confirm they are not carriers before returning to the herd. Rendle said testing typically comprises a blood test to determine which horses have been exposed to strangles, followed by examination of horses with positive blood tests via endoscopy and a guttural pouch wash to check for any remaining bacteria.
If possible, he suggested separating horses into three groups based on what he calls the traffic light system:
- A green group, which has no known contact with the affected horse;
- A yellow group, comprised of horses that had direct or indirect contact with an infected horse; and
- A red group whose signs and test results confirm they have strangles.
House these groups either in three separate buildings or three separate double-fenced paddocks, if possible. Also dedicate equipment and staff to each individual group. If manpower is an issue, then have caretakers move from low- to high-risk groups when feeding and performing chores, said Rendle. While it might not be a popular decision, he added, stop equine movement on and off the property for about four weeks after all horses are declared negative to prevent spread to other properties.
“Take rectal temperatures in all green and yellow horses twice daily,” said Rendle. “And move any pyrexic (feverish) horses or horses with clinical signs to the red group.”
Always quarantine new arrivals, particularly at barns with large populations of horses, for at least two weeks, Rendle suggested. And if a strangles case does pop up on your property, practice smart biosecurity, and remember that signs can take a while to appear as well as clear.
“Strangles cannot survive outside the horse, so if carriers were all identified, the disease could be eliminated,” he said. “It remains a problem because horse owners fail to implement appropriate disease control measures.”