Conditions collide to propel PHF/Potomac horse fever must be treated rapidly to dodge fatalities

In these days of alphabet diseases, there are plenty of things to worry about: EPM, EIA, PSSM, HYPP, MRLS and the list goes on. Recently, it seems as though a somewhat forgotten problem is staging a comeback, and Potomac horse fever (PHF) now should be added to dastardly list of diseases to watch.

Freshwater snails carry Neorichettsia risticii, which explains why pastures with proximity to water can carry a greater risk to horses.

Many veterinarians say environmental conditions are playing an important role in the re-emergence of PHF. An extremely active hurricane season that resulted in heavy rains on most of the eastern half of the country this year. The resultant flooding and standing water has provided optimum conditions for the spread of PHF, and the recent increase in clinical cases in the Southeast, Midwest and the Northeast confirms this theory.

In a situation very reminiscent to the climactic and environmental coincidences that helped cause Mare Reproductive Loss Syndrome (MRLS) throughout Kentucky and neighboring states, a number of factors have collided to propel a sharp rise in the number of PHF cases this past year.

Fresh water exposure

To really understand the return of PHF, it is necessary to take a look at where the disease came from and what is currently known about the condition. Potomac horse fever was first definitively described in 1985, though it is postulated to have existed for a number of years before the outbreak that gave it its official name. Horses at a racetrack along the Potomac River in Maryland became ill with very high fevers and diarrhea. Many horses in Maryland, Virginia, Kentucky and Ohio soon were affected with an alarmingly high number of fatalities, and researchers hurried to identify the causative agent and the method of transmission.

The disease was discovered to be caused by a Rickettsial parasite that was given the name Ehrlichia risticii. It was shown that this parasite invaded monocytes in the bloodstream, and the disease was then referred to as Equine monocyte ehrlichiosis. The name of the causative organism has been corrected to Neorichettsia risticii since then, but the name Potomac horse fever has stuck. The search for the method of transmission proved to be much more difficult than isolating the causative organism. Originally researchers implicated the usual suspects—flies, mosquitoes, mice and such. Knowledge of a Rickettsial parasite then leads researchers to consider other possible vectors. Finally Dr. John Madigan and scientists at the University of California-Davis College of Veterinary Medicine uncovered the various stages of a very complex system that supports this disease.

Freshwater snails carry Neorichettsia risticii. This part of the cycle partially addresses the question of proximity to water. Most cases of PHF have occurred in areas near fresh-water streams or rivers. Some cases have been reported that lack this detail, but these are much less common. Dr. Madigan and his research group then discovered that a metacercariae—an intermediary form of a trematode parasite—was passed from the snail to immature or adult forms of any of 17 species of insects, such as dragonflies, caddisflies, mayflies, damselflies and stoneflies. These flies also spend part of their lifecycles in water, and then they hatch and fly onto pastures and grasslands. It is now believed that horses become infected by accidental ingestion of these insects. In fact, field veterinarians in California have reported large increases in PHF cases within 7-10 days following excessively large hatches of caddisflies in their practice areas. This explanation of transmission of PHF helps explain this year's increase in clinical cases. Flooding and standing water provide habitat for both the fresh-water snails and the insect vectors of PHF. It remains to be seen if other environmental conditions have produced an exceptionally large hatch of insects this year, and such research is currently underway.

Scientists also are looking into the effects of flooding on grazing management to see if this might have affected equine exposure. Other investigations are looking into the question of whether insects carrying N. risticii can remain infective in hay and stored feedstuffs. The answer to this question might help explain those cases that occur away from obvious water sources because possibly infected hay sometimes can be shipped long distances before it is fed to horses.

Symptoms and treatment

Currently, six strains of PHF have been recognized, and all strains cause similar disease. Affected horses typically show initial signs of depression and a transient high fever. These early signs contribute to the difficulty in diagnosing this disease because depression and variable high fever can be seen with any number of bacterial and/or viral diseases. Typical management of such cases might be supportive care with pain relief and a fever-reducing agent, such as flunixin. A complete blood count done initially might show a mild leucopenia or a normal hemogram. In 7 to 10 days from the initial signs, effected horses progress to show loss of appetite, colic, loose manure to profuse watery diarrhea, edema of the head, legs and abdomen and potentially life-threatening laminitis. Pregnant mares effected with PHF might experience late-term abortions.

Treatment is most effective when initiated early in the course of disease. Practitioners that suspect a case of PHF should begin aggressive fluid therapy to counteract the dehydration that is seen with this disease and to correct the electrolyte imbalance created by the diarrhea. Anti-rickettsial therapy with oxytetracycline also is crucial, and current recommendations are to treat with a 6.6 mg/kg IV daily for up to five days. Response to treatment is often quick and marked. Doxycycline also has been used recently by many clinicians, too.

Diagnosis of PHF still remains problematic. Testing can be done by IFA, Elisa and, more recently, by a PCR test. There still remains no correlation between titer and the likelihood of disease, and many of the testing methods cannot differentiate between a vaccine titer and actual clinical disease. Paired titer samples can be helpful, but they take too much time, and isolation of the causative agent, which is definitive, also will be too slow for proper and timely treatment decisions. Veterinarians are left with the reality that if they suspect PHF, then treatment must begin immediately because toxemia, laminitis and death can occur rapidly.

The need for rapid treatment is one of the factors that seems to be contributing to the recent re-emergence of PHF. The trend toward fewer vaccinated horses also appears to be contributing to the disease's rapid rise. Because this disease has been relatively quiet during the last few years, many owners and veterinarians have not been as diligent in their management practices or as aggressive with treatment in cases that present with PHF-like signs.

There are many areas of the country that have not had a case of PHF in a long time. Owners in these locations are not as concerned with this disease as they had once been. Veterinarians do not think of PHF perhaps as often as they should when presented with a depressed horse with a high fever. The recent increase in PHF cases points out the need for more vaccination and for heightened awareness on the parts of both owners and veterinarians.

There are a number of vaccines available for PHF. Initially a vaccine was available that protected against only one PHF strain. Newer vaccines—one released and approved as recently as this summer—offer wider protection and better efficacy. There have been reported cases of horses contracting PHF however, despite the use of routine and repeated vaccinations. It should be noted that the vast majority of horses that are vaccinated and still contract PHF actually develop a much milder form of the disease and usually are spared the more-serious consequences of the condition.

The fact that the disease presents in a slightly different way in vaccinated horses makes clinical diagnosis even harder for veterinarians. Vaccinated horses can show mild depression, a less-severe temperature elevation and slightly soft manure for a few days and never develop any other signs. Practitioners attending such a horse might be unaware that what they are seeing is an active PHF case, and therefore, they might not be alerted to the potential for other more severe cases in their area among unvaccinated horses.

Robert Holland, DVM, PhD and a senior equine technical services veterinarian at Pfizer Animal Health, is understandably in favor of more-frequent vaccination for PHF.

"Horse owners and veterinarians are concerned about potentially fatal diseases like West Nile Virus, but the truth is that as many, or even more horses in certain regions, may contract, or even be killed, by Potomac horse fever this year," he says.

Other veterinarians not affiliated with vaccine companies, such as those practitioners in northwestern New York state and in other areas experiencing PHF outbreaks this year, tend to agree. This year has seen conditions that favor the development and spread of PHF, and this disease is back. Clients in potentially susceptible areas or owners with horses traveling through such areas should be advised to vaccinate with a product that will provide as much protection as possible. Veterinarians need to be alert for potential cases of PHF and should be aware of the altered presentation possible in vaccinated horses.

Until it is officially eradicated, no disease ever really goes away. Conditions such as PHF, West Nile Virus and MRLS, which depend on insect or parasite vectors, climactic conditions, and the number of vaccinated animals in the population, will increase and decrease continually. It is important that veterinarians remain vigilant in the field and continually remind owners of these diseases so that the proper management and treatment protocols can be taken.

Dr. Marcella, a 1983 graduate of Cornell University's veterinary college, was a professor of comparative medicine at the University of Virginia. His interests include muscle problems in sport horses, rehabilitation and other performance issues.