Potomac horse fever reemerges with increase in areas where disease has not been seen before

by Kenneth L. Marcella, D.V.M.

POTOMAC HORSE FEVER was first described in 1979 when horses at a racetrack along the Potomac River in Maryland became ill. Shortly thereafter, many horses in nearby Virginia, Kentucky, and Ohio began experiencing similar clinical signs, and an alarming number of fatalities were recorded.

Affected horses showed depression and lethargy initially coupled with a decrease in appetite and the presence of a high fever of more than 104 degrees. Most clinicians initially felt these were influenza-like signs, especially in a racehorse population that is under stress from transportation and exercise and is constantly exposed to horses coming from many locations.

Within seven to ten days of the initial signs, however, horses with the condition progressed to showing symptoms of colic and having loose manure to profuse watery diarrhea; edema or fluid swelling of the head, legs, and abdomen; and severe, potentially life-threatening laminitis. Pregnant mares that were affected by the disease experienced late-term abortions.

Because the initial cases seemed to occur along streams, rivers, or other freshwater sources and because the initial temperature was so severe, the newly emerging disease was dubbed Potomac horse fever.

In the months and years that followed, scientists uncovered plenty of information about the disease, including evidence that it probably had existed for many years prior to the 1979 outbreak that gave it its name and that it exists in many other states and in Europe. Researchers began a search for the causative organism or agent almost immediately. They first investigated flies, insects, rats, and mice--the usual suspects in a disease. Eventually the culprit responsible for Potomac horse fever was proven to be a rickettsial parasite that was given the name Ehrlichia risticii.

Rickettsial organisms cause other diseases that are commonly flea- or tick-borne, such as Rocky Mountain spotted fever and typhus. Further research showed that Potomac horse fever attacked certain blood cells called monocytes, and the name of the disease was changed to equine monocytic ehrlichiosis, though Potomac horse fever has stuck with the average horse owner and is the much more common term in use today. Information gathered later allowed the parasite to be more correctly renamed Neorickettsia risticii, and additional information about the life cycle of the parasite continued to be discovered.

Multiple carriers

John Madigan, D.V.M., M.S., and other scientists at the University of California at Davis College of Veterinary Medicine recently uncovered the various stages of the very complex system that supports Potomac horse fever. They confirmed that N. risticii is carried by freshwater snails. This fact answers the question about why Potomac horse fever almost always seems to occur in proximity to streams and rivers.

Madigan and his colleagues then discovered that an immature life stage of the rickettsial parasite, called a metacercariae, is next passed to immature or adult forms of any one of 17 species of insects such as dragonflies, caddis flies, mayflies, damselflies, or stoneflies. These insect species spend part of their life cycle in water and then lay eggs that hatch and fly onto pastures and grasslands. Horses grazing these areas then can be accidentally infected.

Veterinarians in California have recorded significant increases in cases of Potomac horse fever within seven to ten days following excessively large hatches of caddis flies in their practice areas. Investigation is currently under way to determine if these insects can remain infective in hay and stored feedstuffs. The answer could explain the small percentage of cases that have occurred in horses far removed from any water source because potentially infected hay can be shipped long distances before it is fed to horses.

Not a regional disease

The discovery of the role that caddis flies, dragonflies, and other insects play in the transmission of Potomac horse fever has presented researchers with one plausible explanation for the recent increase in the number of cases. Last year, there was an increase in the number of cases of Potomac horse fever and--almost more significantly--an increase in cases in areas where the disease had not been seen before.

Many veterinarians feel the scientific community did a disservice to horses and their owners when they came up with the name Potomac horse fever. Until last year, most owners believed if they did not have their horses near fresh water along the Potomac River or Ohio River valleys, they need not worry about the disease. Surely horse owners in New York, Kentucky, Arizona, or California would not have to fear a disease found mostly in Maryland. A regional name for a nonregional disease fostered a casual attitude among some owners and even among some veterinarians.

Mare reproductive loss syndrome (MRLS) that struck Central Kentucky taught us that one cannot overlook the power of nature and the environment to change situations we have become accustomed to. This same principle of nature seems to be at work in the resurgence of Potomac horse fever. Last year, there was a dramatic increase in severe hurricanes throughout the Southeast, and heavy rains and flooding occurred in many areas of the country. These conditions resulted in standing fresh water in many areas that typically would have been dry. The fresh water provided an ideal environment for the growth of snails.

The intense and expansive high winds that accompanied the major hurricanes as they made landfall in Florida and continued on throughout the country were easily capable of blowing entire hatches of insects to areas several hundreds to thousands of miles away. As these environmental conditions developed last year, veterinarians in New York, Tennessee, Kentucky, and other states previously devoid of significant cases of Potomac horse fever began to see an increase of the disease.

Another factor contributing to the increase in cases of Potomac horse fever is probably the fact that a vaccine was rapidly developed when the initial outbreak occurred. Many horses were vaccinated in the Potomac region, and the disease slowly began to decrease in appearance. Because Potomac horse fever has been relatively quiet over the past few years, many owners and veterinarians have not been as diligent in their management practices regarding the disease. New research also shows the disease itself may be changing.

There are currently six known strains of Potomac horse fever--all causing similar signs of the disease. The initial Potomac horse fever vaccine is unlikely to be protective against all six strains, though some cross-protection does occur. A new vaccine has been developed that will offer wider protection and better efficiency, but there is currently not enough research to know if present vaccines are completely protective against all strains of Potomac horse fever.

This means that recognition and early treatment of Potomac horse fever are still very important, but vaccination for the disease actually contributes to problems in this area. Vaccination for Potomac horse fever, while not thought to be protective for all strains, might cause the horse to exhibit much milder clinical signs if it acquires the disease. These horses could show depression and an elevated temperature and then have soft manure for a few days. These are such nonspecific signs that they may be missed or instead attributed to a mild virus or cold.

These horses might be under the weather for a few days and never show any further signs. Owners and veterinarians, however, could be totally unaware that what they are seeing is a case of Potomac horse fever, and they may not be alerted to the potential for more severe cases in the area among unvaccinated horses.

Diagnosis and treatment

Treatment for Potomac horse fever should be initiated as early as possible and involves oxytetracycline or doxicycline, which are antibiotics with a special affinity for rickettsial organisms. Horses are given fluids and electrolytes to replace the losses due to diarrhea, and preventive care is done in anticipation of laminitis. If vaccinated horses show mild signs that could be easily attributed to a low-level bacterial cold or virus, early treatment is sometimes not started, and these horses are at risk for possible serious complications later.

Diagnosis of Potomac horse fever can be done through several different tests, but there still remains no correlation between a titer (level of antibodies in the blood) and the likelihood of disease. Most tests cannot differentiate between active infection and disease versus a protective antibody titer from vaccination.

Most tests are also slow enough that treatment should really be started before the results are returned. Withholding or postponing treatment until results are available can lead to laminitis, toxemia, and possible death in severe cases. If the disease is suspected, the best approach is to treat the horse for Potomac horse fever.

Whether you live in Wisconsin, Texas, California, or Georgia, Potomac horse fever is back. The signs might be a bit more subtle. And because the disease is now occurring in areas where owners and veterinarians do not routinely think about it, Potomac horse fever is more likely to be misdiagnosed or missed altogether. Potomac horse fever should be suspected in any horse with an elevated temperature and soft manure, and diagnostic testing and early treatment should be done.

Until they are officially eradicated, no diseases ever really go away. Be on the lookout for Potomac horse fever; discuss your horse's actual level of risk with your veterinarian; and adjust your management and vaccine program accordingly.