Conditions collide to propel PHF/Potomac horse fever must be treated rapidly to dodge fatalities

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Jan 15, 2005


Freshwater snails carry Neorichettsia risticii, which explains why pastures with proximity to water can carry a greater risk to horses.
In these days of alphabet diseases, there are plenty of things to worry about: EPM, EIA, PSSM, HYPP, MRLS and the list goes on. Recently, it seems as though a somewhat forgotten problem is staging a comeback, and Potomac horse fever (PHF) now should be added to dastardly list of diseases to watch.

Many veterinarians say environmental conditions are playing an important role in the re-emergence of PHF. An extremely active hurricane season that resulted in heavy rains on most of the eastern half of the country this year. The resultant flooding and standing water has provided optimum conditions for the spread of PHF, and the recent increase in clinical cases in the Southeast, Midwest and the Northeast confirms this theory.

In a situation very reminiscent to the climactic and environmental coincidences that helped cause Mare Reproductive Loss Syndrome (MRLS) throughout Kentucky and neighboring states, a number of factors have collided to propel a sharp rise in the number of PHF cases this past year.

Fresh water exposure To really understand the return of PHF, it is necessary to take a look at where the disease came from and what is currently known about the condition. Potomac horse fever was first definitively described in 1985, though it is postulated to have existed for a number of years before the outbreak that gave it its official name. Horses at a racetrack along the Potomac River in Maryland became ill with very high fevers and diarrhea. Many horses in Maryland, Virginia, Kentucky and Ohio soon were affected with an alarmingly high number of fatalities, and researchers hurried to identify the causative agent and the method of transmission.

The disease was discovered to be caused by a Rickettsial parasite that was given the name Ehrlichia risticii. It was shown that this parasite invaded monocytes in the bloodstream, and the disease was then referred to as Equine monocyte ehrlichiosis. The name of the causative organism has been corrected to Neorichettsia risticii since then, but the name Potomac horse fever has stuck. The search for the method of transmission proved to be much more difficult than isolating the causative organism. Originally researchers implicated the usual suspects—flies, mosquitoes, mice and such. Knowledge of a Rickettsial parasite then leads researchers to consider other possible vectors. Finally Dr. John Madigan and scientists at the University of California-Davis College of Veterinary Medicine uncovered the various stages of a very complex system that supports this disease.

Freshwater snails carry Neorichettsia risticii. This part of the cycle partially addresses the question of proximity to water. Most cases of PHF have occurred in areas near fresh-water streams or rivers. Some cases have been reported that lack this detail, but these are much less common. Dr. Madigan and his research group then discovered that a metacercariae—an intermediary form of a trematode parasite—was passed from the snail to immature or adult forms of any of 17 species of insects, such as dragonflies, caddisflies, mayflies, damselflies and stoneflies. These flies also spend part of their lifecycles in water, and then they hatch and fly onto pastures and grasslands. It is now believed that horses become infected by accidental ingestion of these insects. In fact, field veterinarians in California have reported large increases in PHF cases within 7-10 days following excessively large hatches of caddisflies in their practice areas. This explanation of transmission of PHF helps explain this year's increase in clinical cases. Flooding and standing water provide habitat for both the fresh-water snails and the insect vectors of PHF. It remains to be seen if other environmental conditions have produced an exceptionally large hatch of insects this year, and such research is currently underway.

Scientists also are looking into the effects of flooding on grazing management to see if this might have affected equine exposure. Other investigations are looking into the question of whether insects carrying N. risticii can remain infective in hay and stored feedstuffs. The answer to this question might help explain those cases that occur away from obvious water sources because possibly infected hay sometimes can be shipped long distances before it is fed to horses.