Detecting PHF in Oklahoma; Behind the Headlines

The recent confirmation of Potomac horse fever (PHF) in an Oklahoma horse was possible because of relatively new--and inexpensive--testing techniques that help speed diagnose of the disease. This allows horse owners to be alerted to the active presence of the disease's causative agent in their area. Recognition of the disease in areas unused to seeing PHF is important. This burden falls to treating veterinarians, horse owners, and diagnostic laboratory officials who take or request blood or tissue samples from horses with clinical signs of PHF.

The Lucy Whitier Core Molecular Laboratory at the University of California, Davis (UC Davis) School of Veterinary Medicine, reported the Oklahoma equine case on Aug. 22 after tests were completed that day on blood and feces.

Potomac horse fever's causative agent, the bacterium Neorickettsia risticii, has been linked to parasites of freshwater snails. The parasites are called cercariae, and they also infect the larvae of mayflies and caddis flies in fresh water. When the infected fly larvae mature into infected adult flies, they can be ingested by horses inadvertently consuming the insects while grazing or eating feedstuffs.

John Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in PHF research for nearly 10 years. He was part of the research group that linked caddis flies and mayflies to the life cycle of PHF in 2000, demonstrating that they carry the infectious agent. That group also did the first experimental reproduction of the disease by feeding horses infected caddis flies. Researchers at The Ohio State University have since reproduced the UC Davis study and confirmed the initial finding of transmission by ingestion of insects.

The laboratory at UC Davis uses a "Real-time TaqMan PCR (polymerase chain reaction) test" developed in the late 1990s. This test detects the infectious agent by amplifying a specific gene in the agent. It is a "closed-tube" test, meaning that there is a reduced risk of contamination compared to other PCR tests. The test only costs about $30 per sample; relatively inexpensive for its sophistication and speed.

Madigan explained that the infective agent of PHF does not just circulate around in the blood and feces of a horse. "If it's there, it's causing disease," he said, and therefore the horse will begin building antibodies against it, which tests prior to the PCR were designed to detect. However, these antibody tests often yield false positives, and sometimes they are difficult to interpret because of the wide variance of antibody reaction to PHF at different times during the disease process. Also, these antibody tests won't work on a dead animal." The new PCR test can work on some tissues from dead horses saved from prior necropsies.

Bodies of Water Implicated, But Not Necessary

The PHF agent is present in water worldwide—a colleague recently reported to Madigan that the agent was found in freshwater stream snails in Korea. Horses kept in close proximity to water are at a higher risk for getting PHF. But not all PHF-affected horses are kept adjacent to bodies of water such as rivers, ponds, or creeks. This fact has horse owners concerned.

"When we get to a site where we have PHF, we can usually find water (sometimes just in a ditch), and there will be snails, and the snails will have the infectious agent; hence, PHF is also called "ditch fever," said Madigan.

"What we've done is perform a pretty careful investigation in the areas in northern California, and historically, the cases that start in the spring are closer to the river," he said. Later in the year, the cases were found farther inland, suggesting a migratory drift of the insects (containing the agent). "We don't know enough about the wind and mayflies and caddis flies and how far they go," he said. "I think that it's going to be several miles."

He continued, posing the question: "Could the insects be baled in hay, then be fed to horses? The fact that there's a seasonal occurrence suggests that's not the case." If the horses were eating hay year-around, and insects in the bales were the cause, then PHF cases would be seen year-around.

In the case of PHF in Oklahoma, scientists will be mapping a source in the environment for the infectious agent, whether yards or miles away. "We've put out a snail request (to scientists in the area)," said Madigan.

Also, colon samples from horses affected by colitis in Oklahoma are being shipped to UC Davis so lab personnel can try to diagnose PHF cases retrospectively.

As for PHF nationwide in 2003, Madigan said that the number of samples for PHF testing submitted this year has been comparable to past years, but this could be due to lack of testing and vaccine protection reducing awareness and detection. Samples that are positive have arrived from places such as Northern California, Oregon, Tennessee, Montana, Texas, and New York.

"There's not a great awareness that there's a good, specific, reliable PHF test for around $30," stated Madigan. "It is a yes or no test" that could alert horse owners and veterinarians of the risk of PHF in their area.


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