Vigilance Key in Managing for PHF

Tennessee Cases Increase

Potomac horse fever (PHF) probably has been endemic in the Western Hemisphere as long as horses have inhabited the region—perhaps even since the prehistoric era--and it's here to stay. As awareness of the disease increases, so does the need to remain vigilant in managing horses to reduce the incidence, and severity, of cases.

PHF is also quite likely under diagnosed. In summer and fall of 2006, the University of Tennessee (UT) College of Veterinary Medicine tested 55 horses with clinical signs of PHF and found the incidence of cases to be 20% for the area, which includes most of East Tennessee. "It's likely that many more horses had PHF in our area, but had mild signs or were not tested and just treated by our local veterinarians," said UT's Frank M. Andrews, DVM, MS, Dipl. ACVIM, professor and section chief of Large Animal Medicine.

Understanding the nature of Neorickettsia risticii--the organism that causes PHF--is key to managing the disease said Yasuko Rikihisa, PhD, a professor in the Department of Veterinary Biosciences at The Ohio State University (OSU).

Potomac horse fever was first identified in the horse industry in the mid-1980s with an outbreak in the Potomac River area of Maryland. Researchers were able to link the causative agent (a bacteria named Ehrlichia risticii, which has been re-named Neoriketssia risticii) to parasites of freshwater snails. Scientists at the University of California, Davis, were able to complete the natural transmission cycle of PHF in 2000 when they found that the bacteria infects the larvae of mayflies and caddis flies in fresh water. The larvae then mature into adult flies and are ingested by horses when they inadvertently consume the insects while grazing or in feed. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease.

Dead-End Host
The horse is considered a dead-end host of the organism when it becomes infected by the flies. "The flies that bear the organism, including mayflies, caddis flies, dragonflies, and damsel flies, can fly or be swept by the wind long distances," Rikihisa said. The common stable fly is not considered a vector.

N. risticii lives symbiotically in the trematode, a fluke-like organism that infests aquatic snails. A study performed at OSU and published in Environmental Microbiology in 2005 showed the trematode Acanthatrium oregonense as a natural reservoir and probable vector of N. risticii. The study was conducted on big and little brown bats, another dead-end host of the bacteria, collected in Pennsylvania. These are insectivorous (eats insects) bats, not blood-sucking bats, and direct transmission between dead-end hosts is not possible, says Rikihisa.

"When the trematode eggs are released into the water, they hatch and enter into aquatic snails. Bacteria get a free ride to the next stage, aquatic insects," she said. As the flies mature into adults, "thousands of them come out from the water (fly hatch), and they carry the trematode in their tissues. When the flies die, they drop on the grass and the water, all over the place."

Management Challenges
Multiple strains of the bacteria also make protection and treatment a challenge. Rikihisa and colleagues have isolated different strains of N. risticii in the blood of horses and in flies from different geographical areas in Kentucky, Pennsylvania, Ohio, and California. Not all strains are protected against by the available vaccine or respond to the tetracycline class of antibiotics used to treat PHF.

"Isolating the strains and studying the genetic differences and immuno-protection mechanism of the bacterial infection is the first step toward finding a more effective vaccine," she said. "We don't know if the organism is changing, or if it has always been diverse."

Early diagnosis is critical in managing the disease. "The mortality rate is fairly low with this disease, but the laminitis and diarrhea we see in about 40% of cases can be fatal," said Andrews. Lethargy, depression, and high fever that do not respond to the usual analgesics should raise suspicion. "The first thing to do is take the horse's temperature," he said. In fact, it's important to take the horse's temperature on a regular basis, "so you get to know what is normal in your horse at different environmental temperatures."

Vaccination, fencing horses out of streams and ponds, cleaning water buckets and troughs frequently, and eliminating standing water are important, but the organism is so prevalent and the clinical signs so variable that vigilance in noticing early clinical signs (such as a higher temperature) is essential.

Early Diagnosis Crucial
If a horse has even vague symptoms, and the temperature is between 101 and 104 degrees Fahrenheit for more than one day, it's time to call the veterinarian, who can draw blood and send it to any of a number of labs that perform a PCR (polymerase chain reaction) test for the presence of Neorickettsia risticii DNA and for the presence of antibodies.

Cindy Backus, MS, DVM, PhD, with Countryside Veterinary Service in the metropolitan Knoxville region, treated five horses all within a month, some on a farm near a major river. "If we get to them early, they don't get the diarrhea and the founder," she said. If you live by a creek, a pond, or a river, you should vaccinate with two doses initially and a booster every six months. On one farm with multiple cases, she gives the booster four times a year. "If anything has diarrhea, it's PHF until proven otherwise," she said.

About the Author

Elise LeQuire

Elise LeQuire is a freelance writer specializing in science technology and the environment.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More