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Potomac horse fever more dangerous than West Nile Virus?
Potomac horse fever (PHF) is an often fatal disease of horses, caused by different strains of Rickettsia. Unfortunately, the signs are pleomorph and differ also between vaccinated and non-vaccinated animals. Early and aggressive therapy is mandatory, and this excellent summary gives important informations for every horse practitioner.

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

Source: Kenneth L. Marcella (2005): Conditions collide to propel PHF/Potomac horse fever must be treated rapidly to dodge fatalities. In: DVM Newsmagazine Jan 15, 2005. www.dvmnewsmagazine.com/dvm/









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