Potomac Horse Fever & Mayflies

Dr Lisa Nesson

Written By: Lisa Nesson, DVM

It’s July, so Mayflies are hatching in Wisconsin. Next comes Potomac Horse Fever (PHF) cases. PHF causes clinical signs such as lethargy, fever, loss of appetite, diarrhea and laminitis in late summer to early fall. Here’s what you need to know about PHF.

PHF Background

Veterinarians first identified Potomac Horse Fever (PHF) in 1979 in the Potomac River Valley in Maryland. Now, PHF is spreading throughout the United States. 43 states, three Canadian provinces, South America, Europe and India all reported cases of PHF. It generally occurs in geographical pockets. Southern Wisconsin is seeing an increase in the number of cases  each year over the past several years.

PHF isn’t a reportable illness, largely because it’s not contagious from horse-to-horse, or horse-to-human-to-horse in the sense of more traditionally contagious illnesses. PHF does impact horses living in the same space and cluster outbreaks do occur. While the disease is still surrounded by some mystery, it’s believed that mayflies, caddis flies and other aquatic insects carry the Neorickettsia risticii (NR) organism which causes PHF. Mayflies and other insects transfer NR to horses when the insects are inadvertently ingested by the horse when grazing near or drinking from ponds and streams where the insects live. Horses that live in areas with standing fresh water (ponds, lakes, streams, etc.) or in irrigated pastures are at a higher risk for PHF.

PHF Clinical Signs


Once exposed to NR, the incubation period (or time until clinical signs appear) is 1-3 weeks.  Common clinical signs of PHF include depression or lethargy, followed by a biphasic fever ranging from 102-107 degrees F (a fever with two peaks and a period of normalcy in between), colic, anorexia (off feed), dehydration, and diarrhea (in 60% of cases) within 24-48 hours of the second fever spike.  Laminitis (founder) is seen in about 40% of PHF cases, sometimes before they develop diarrhea, but more frequently seen in conjunction with the diarrhea.  Unfortunately, the laminitis can be severe and progress despite the improvement of the other clinical signs. Of all of these clinical signs, the diarrhea and laminitis are the life threatening issues which need to be carefully monitored.  The fatality rate for this condition varies from 5 to 30% and depends primarily on the strain involved.  Chronic infections do not appear to occur but the laminitis can result in long term problems for your horse.

PHF Diagnosis and Treatment

Veterinarians diagnose PHF based on typical clinical signs, season of the year and access to a water source that could serve as a source of infective insects.  Currently the diagnostic test of choice for the NR organism is a polymerase chain reaction (PCR), which labs run on a blood sample or a fecal sample.  Labs can also run an antibody test, but it is of less use in reaching a timely diagnosis. This antibody test will be affected by the vaccination status of your horse, while the PCR test will not.  I know you’re thinking to yourself, “If I vaccinated my horse, isn’t an antibody test a non-issue, because he won’t contract PHF?”. Not always the case, unfortunately. But we’ll get to that.

So, your horse has the clinical signs and your veterinarian drew blood to run a PCR test, which you’re waiting to get back. However, if your horse has PHF, you will want to have him treated immediately in order to avoid the more life threatening clinical signs listed above. Because of that, veterinarians will often begin treatment right away, and use treatment as a diagnostic test in and of itself.

We will treat a horse with PHF will with flunixin meglumine (ie. Banamine) to control fever, and an antibiotic, tetracycline, to treat the actual infection. We will dilute the tetracycline and give it intravenously to avoid potentially fatal complications. We complete these treatments one to two times per day, for five to ten days, or until clinical signs resolve. Your horse’s fever should break and his manure should firm up within 48 hours. He may also require supportive care to rehydrate him, replace lost protein and treat any laminitis that may develop. While many early cases can be treated successfully on the farm, some need to be hospitalized in order to get the intensive care they need.

To vaccinate, or not to vaccinate? That is the question.

The vaccines currently on the market for PHF were manufactured against a single strain of the organism. That means vaccinations are not providing strong protection against naturally occurring infections. If the vaccine is used, it should be given twice a year, in the spring and again in mid summer to early fall to offer it’s best protection. The vaccine may lessen the severity of the illness and may augment your horse’s natural immunity.

Beyond vaccination, plan to take other preventative measures to protect your horse. Wisconsin has many lakes, so controlling those environmental factors can help protect your horse from the infection. Limit access to freshwater streams, ponds, lakes and irrigated pastures during the late summer and early fall. Leave trees, bushes and grasses that grow up around these water sources in place – these natural barriers will encourage insects to stay near the water and not travel out into the pastures or barn.  Finally, keep the barn lights turned off at night to discourage drawing insects. These aquatic insects hatch in the evening, and will be attracted to the bright lights, even if they’re away from water.  Keep the insects from coming to your property, and you’ll lower the incidence of your horses ingesting them.

Questions, Comments, Concerns?

We’re here to help! Contact us to discuss if vaccinating your horses for PHF is right for you.