Potomac Horse Fever: The Newest Threat in the Midwest

Written By: Lisa Nesson, DVM

Written By: Lisa Nesson, DVM

As late summer approaches, concerns arise for a particular disease horse owners and veterinarians hadn’t seen much of in this area of Wisconsin until several years ago. While Irongate Equine Clinic saw our first case of Potomac Horse Fever this season in late July, the UW School of Veterinary Medicine has seen several throughout the month of July so this disease continues to be of increasing concern to horse owners, horses and veterinarians again this year. It appears to be here to stay and should be considered as the cause of typical clinical signs such as lethargy, fever, loss of appetite, diarrhea and laminitis seen in late summer to early fall. The following is a review of information regarding where Potomac Horse Fever comes from, how it’s treated and how it can be prevented in your horses and/or herd.

PHF Background

Potomac Horse Fever (PHF) was first identified in the 1979 in the Potomac River Valley in Maryland, primarily showing up in the middle to late summer. Since then, it has spread further inland in the United States while increasing in prevalence on the east coast as well. It has been reported in 43 states, three Canadian provinces, South America, Europe and India. Generally it occurs in pockets within the state, with southern Wisconsin seeing an increase in the number of cases seen each season over the past several years.

Mayfly, the Potomac Horse Fever Carrier

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 often 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 organism which is transferred 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, Tests and Treatments

Once exposed to Neorickettsia risticii, the incubation period (or time until clinical signs appear) is 1-3 weeks.  Common clinical signs of this condition 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 cases with PHF, 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.

The diagnosis of PHF is frequently 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 Neorickettsia ristiici organism is a polymerase chain reaction (PCR), which can be run on a blood sample or a fecal sample, preferably both.  There is also an antibody test that can be run 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.

A horse with PHF will typically be treated with flunixin meglumine (ie. Banamine) to control fever, and an antibiotic, tetracycline, to treat the actual infection. The tetracycline should be diluted and given intravenously to avoid potentially fatal complications. These treatments are given one to two times per day, generally 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.

PHF: To vaccinate, or not to vaccinate?

The vaccines currently on the market for PHF have been made against a single strain of the organism, so strong protection against naturally occurring infections is not likely following vaccination.  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. Limiting access to freshwater streams, ponds, lakes and irrigated pastures during the peak season is one method of controlling your horse’s chance of exposure.  Also, leaving the natural barrier of trees, bushes and grasses that grow up around these water sources in place will encourage these insects to stay near the water and not travel out into the pastures or up to the barn where horses may come into contact with them.  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?

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