What is Potomac Horse Fever?

Published: November 01, 2013
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As I write this, we are having unseasonably warm weather here in Maryland this autumn. It’s also been dry enough lately that I haven’t seen the typical fall bouts of laminitis in horses that comes with some fall rain and a boost in grass growth after the scorching heat of summer. These Indian Summer days remind me of a case of Potomac Horse Fever I saw a few years ago that occurred in similar weather. Allow me to share the case with you.

Potomac Horse Fever (PHF) is an infectious disease caused by a microbe called Neorickettsia risticii. First noted in the Potomac region of the country (where I practice, mind you), this organism causes diarrhea and other complications in horses, including fever, depression, edema, mild colic, and laminitis.

N. risticii has a complicated life cycle. It infects a flatworm parasite that infects aquatic snails. In warm weather, these immature flatworms are released from the snails into the water. Horses become infected by either drinking infected water or by ingesting small insects such as mayflies, since the microbe can also infect these aquatic insects. 

Once ingested, N. risticii invades the horse’s bloodstream, taking up residence inside intestinal cells and a specific type of white blood cell called a monocyte. From there, they induce fever and diarrhea in about 80 percent of infected horses. This diarrhea can be severe, depleting the horse of fluid very quickly, potentially leading to shock and septicemia. 

My first experience with Potomac Horse Fever occurred during a bout of warm autumn weather a few years ago. An Appaloosa mare by the name of Gracie was reported by her owner as being lethargic, having severe diarrhea, and losing weight. Gracie was also nursing a young, feisty colt.

When I arrived at the farm, I found a very thin mare with a fever, swollen legs from edema, and a butt stained with watery diarrhea. A nasogastric tube was passed and I administered Pepto and electrolytes into her stomach and gave IV fluids. The treatment for PHF is the antibiotic oxytetracycline and although we didn’t yet have a definitive diagnosis, I started her on the antibiotics right away. Blood results later showed Gracie positive for PHF (clinical signs of PHF can mimic other infectious diseases, notably salmonellosis). 

My two main concerns for Gracie were the drains on her body from the diarrhea and nursing and the threat of laminitis. I made multiple visits over the next two days to administer more fluids, Pepto, and IV fluids and antibiotics, nervously monitoring her diarrhea, appetite, hooves, and the body weight of her colt. Slowly, after about five days, we seemed to have a breakthrough.

Gracie went 24 hours without diarrhea and her leg swelling decreased. Her appetite increased and she looked perkier. As another week passed, she continued to gain strength, put a little weight back on, and the diarrhea completely resolved.  Somehow, we managed to avoid laminitis, which is often the killer with PHF. Gracie made it.

One of the things I remember most about Gracie’s case was her colt. Always getting in the way of the nasogastric tube or chewing on her IV line when no one was looking, he was a precocious thing that I could just tell was going to be a handful when he got bigger. Sure enough, six months later, his owners called to schedule a castration appointment for the now named Chrome. Much bigger now, Chrome had really muscled out and although still cheeky, was innocent in his intentions. Gelding him also allowed me to check in on Gracie, whom I was pleased to see fatter and much happier.

As an afterword, I would like to mention that there is a vaccine available for PHF. Many horses in my area are vaccinated against the disease and Gracie’s owners were informed of the vaccine for their other horses. Interestingly, Chrome never showed signs of PHF during his mother’s illness. My guess is that he was still nursing and didn’t expose himself to N. risticii from ground water or pasture grass.

Dr. Anna O'Brien

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