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.