OCTOBER 1995 BACK ISSUE

Part of Horse Previews Magazine website. Posted on 10/1/95; 10:00:00 AM.

Morris Animal Foundation

News Release

Identifying a better test for Potomac Horse Fever

Englewood, CO - Despite the availability of vaccines, cases of Potomac horse fever (PHF) continue to plague horses & cause alarm for owners. Jonathan Palmer, VMD, of the University of Pennsylvania indicates: "There has been an increase in reports of PHF in many areas of the country during last summer and so far this summer. But the true prevalence of PHF is difficult to estimate because the diagnostic test is so inaccurate. With false positive tests, PHF may be diagnosed in cases where there is another cause. With false negative tests, it may be discounted when it is the real culprit."

PHF outbreaks are random and unpredictable and tend to break out in the late spring to early fall. This disease can attack any breed of horse at any age, and it's seen in many areas of the United States and other areas of the world.

Scientists know that a parasite called Ehrilichia risticii causes colitis which leads to the PHF syndrome. Recent studies have also shown that E. risticii causes spontaneous abortion. According to Dr. Palmer, there have been many studies attempting to show that PHF is transmitted by insects, but they have been negative. Although scientists aren't sure how it is transmitted, they believe it is associated with hot weather. Potomac horse fever is named after the Potomac river area of Maryland, the region where this disorder was first identified in 1979.

Signs of PHF usually appear in several stages with fever, loss of appetite and leg swelling appearing first. Then, more severe symptoms surface that may include depression, colic, diarrhea and laminitis. Most treated horses survive PHF. Dr. Palmer says: "The estimates of fatality rates for PHF are almost always too high. Even in bad outbreaks, they are usually not greater than 10 percent. When treated, they are lower."

The current diagnostic test is inaccurate because of false positive test results - a side effect of frequent vaccination. Dr. Palmer is studying a more accurate test to diagnose PHF. The revolution in molecular microbiology has paved the way for new techniques in diagnosing infectious diseases. Dr. Palmer is using one such technique - polymerase chain reaction (PCR), a DNA test. This Morris Animal Foundation sponsored pilot study is titled "Detection of Ehrilichia risticii by PCR in Equine Tissues." Development of a PCR test could result in an accurate, rapid diagnostic test.

This study compares PCR detection of E. risticii with culture isolation of E. risticii obtained from equine fetal tissues. An initial small study has indicated a correlation between PCR detection and cultures. To prove the value of PCR detection, this larger blind study is being performed. Dr. Palmer will have more information about the test results when the blind study is completed.

Contributions to support this PHF study can be sent to Maratez+/ Memorial Fund established through Morris Animal Foundation. This fund was set up by Sandie Cafritz in honor of Maratex+/, as Arabian stallion, thought to be the world's first horse to receive a PHF vaccination for USDA testing.

Morris Animal Foundation is a nonprofit organization that funds humane health studies for companion animals and wildlife. All unrestricted, annual contributions support animal programs, no administration. For more information call toll-free 1-800-243-2345.