Potomac Horse Fever: A Happy Outcome
It’s good to be a horse vet in the summer. The spring rush of routine wellness work and breeding and foaling is over, the show horses have gone to work, and it is time to take a breath. Except … summer is the season for diarrhea.

So it was as I was sitting down to dinner one blazing hot evening last summer when my phone alerted me to an emergency. When I arrived at the farm, the mare, Molly, was in her shed with her head down, evening meal untouched. She had a fever of 103.6ºF, an increased heart rate, fluidy gut sounds, and a small amount of diarrhea behind her. I explained to her owner, Joe, that in our area the most likely diagnosis that would fit her signs was Potomac horse fever (PHF).

He was puzzled at my statement, as his horse was current on her vaccines, which included PHF. And, besides, his horses never left the farm or had contact with other horses. He was also worried about her herdmate and asked what he could do to protect the gelding.

I explained that the biggest risk factors for his horses were that they lived in an endemic region (where PHF commonly occurs) within 5 miles of a body of natural water and that we were having a hot, rainy summer. Potomac horse fever is not contagious; rather, snails and aquatic insects carry the causative agent (­Neorickettsia risticii). The bacteria live within parasites that infect the snails and insects. This Russian-doll quality has made this organism challenging for researchers to describe. Horses get sick when they ingest parasites (in natural bodies of water) or infected insects. Measures to protect the horses’ feed, hay, and water include turning off barn lights that attract insects at night, keeping stored hay covered with a tarp, and limiting grazing next to streams and creeks.

Joe asked me how his horse could be sick if she was current on her vaccines. I assured him it was good she was vaccinated. The vaccine doesn’t always prevent disease completely—in part because there are multiple strains—but it helps reduce disease severity and complications such as laminitis. Horses with PHF, just as those with other conditions that cause colitis (lower intestine inflammation), can become gravely ill and require hospitalization for intensive care to manage potentially fatal complications.

I suggested we collect blood to evaluate Molly’s blood cell counts, proteins, and organ function, to help determine if we could treat her on the farm. While I was at it, I grabbed a tube for PHF blood testing (the bacteria can be detected by PCR in blood and manure) and some manure to test for PHF and other potential diseases. Unfortunately, I wouldn’t have the results until the following week. Molly would need to be treated promptly, so I would have to treat her based on her history, physical exam, and bloodwork. Before leaving the farm, I gave Molly a dose of flunixin meglumine (Banamine) to help lower her fever.

Molly’s bloodwork showed that her body was mounting a major inflammatory response, with decreased white blood cells; her white cells were on high alert and not floating peacefully through the lazy river of her bloodstream as they do in healthy horses. Low concentrations of certain electrolytes and proteins revealed that her intestinal lining was leaky and the site of all that inflammation. Fortunately, these changes weren’t severe, and her kidney and liver function markers were fine.

When I called Joe to let him know the bloodwork results, he said Molly was feeling better and was grazing and drinking water. We weighed the pros and cons of taking Molly to a hospital for treatment, and Joe elected to try treatment on the farm. For the next five days, a veterinarian would need to visit Molly and administer intravenous oxytetracycline (vets can also administer oral tetracyclines). I returned to the farm to give her the first dose, and we devised a plan to support Molly and monitor her progress.

As it was, Molly responded well, feeling much better after her first dose of antibiotic, normal after the second, and on the last day of medication she was hard to catch. The tests we sent out confirmed PHF. I was grateful that Joe’s prompt attention to Molly and her vaccine protection helped us give her a good outcome. For more information on PHF, visit ­TheHorse.com/topics/potomac-horse-fever.