LEXINGTON, KY--- Four additional cases of Potomac horse fever (PHF) have been confirmed at two veterinary hospitals in Lexington, Ky. According to Nathan Slovis, DVM, Dipl. ACVIM, of Hagyard-Davidson-McGee (HDM) Veterinary Hospital, two of the horses at HDM are recovering, and one was euthanized. Tests are pending on four other horses at HDM, with their symptoms resembling those found in the clinic's three confirmed cases. One additional case was seen at Rood and Riddle Equine Hospital.

The first recorded Kentucky fatality from PHF in 2002 was a Thoroughbred filly, which was euthanized June 18 at Rood and Riddle Equine Hospital in Lexington (see http://www.TheHorse.com/news.asp?fid=3702). The cause of the filly's symptoms was a mystery before the Kentucky Livestock Disease Diagnostic Lab (LDDC) concluded on June 27 that PHF caused the illness.

Bill Bernard, DVM, Dipl. ACVIM, of Rood and Riddle, reported the additional case of PHF diagnosed at that clinic, although the outcome of that case is unknown. Potomac horse fever is normally detected in only one or two horses per year in Kentucky.

Symptoms of Potomac Horse Fever

Commonly recognized PHF symptoms include diarrhea, depression, colic, anorexia, dehydration, and signs suggestive of laminitis or founder. However, the cases at Hagyard-Davidson-McGee have not exhibited the characteristic diarrhea. The horses had low white blood cell (WBC) counts, a little heat in their feet, and were depressed.

If symptoms are caught early, the horse can be treated with intravenous tetracycline, said Slovis, who suggests administration twice daily for five to seven days, although treatment protocols can vary. Many horses respond to treatment in about 24 hours and have a dramatic turnaround if the symptoms are caught early.

"The key is early treatment," said Slovis. "You have to treat within several days or you're in trouble. The broodmare died after four days of fevers, a low white blood cell count, and eventually had laminitis. If your horse gets a fever, don't just blow it off. When they're off their feed, dull, and depressed, pay attention."

One of the two confirmed cases that are recovering is still on tetracycline. The horses underwent considerable damage to their bowel, but they are starting to eat more, according to Slovis. One horse from Cincinnati developed laminitis, but is slowly getting better. The suspect cases which are being treated with tetracycline are of various ages and breeds--proof that the disease is non-discriminatory.

Neoriketssia ristici (the causative agent) does its damage when it gets into the horse's blood and migrates toward the bowel, where it enters the enterocytes or colonocytes lining the mucosa of the bowel. The reason affected horses have low WBC counts is because these animals are exhibiting endotoxemia, causing the white blood cells to stick to the blood vessel walls, and some rush to the bowel to fight the bacteria.

"They get toxic, and the toxins leak into the body," explained Slovis. "The integrity of the bowel is compromised."

Potomac horse fever is not as highly contagious as some other diseases caused by bacteria like salmonella. While PHF can be spread from horse to horse, it can only be done so with difficulty, as a horse would have to eat the feces of another affected horse.

About the Illness

Potomac horse fever hit the headlines in the mid-1980s, when an outbreak in the Potomac River area of Maryland drew attention to the disease. The causative agent, a bacteria named Ehrlichia risticii (recently renamed Neoriketssia ristici, 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 fly larvae mature into adult flies, they sometimes are ingested by horses which inadvertently consume the insects while grazing or eating feedstuffs. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease because of the close proximity of the aquatic insects.

Slovis formerly worked with PHF researcher John Madigan, DVM, MS, Dipl. ACVIM, while a resident at the University of California, Davis. Therefore, Slovis was familiar with PHF research being performed there and sent whole blood samples of the first suspect horse to Madigan for polymerase chain reaction (PCR) testing. This type of testing looks at the DNA of the bacteria for identification, and can be completed in about 24 hours. Tests for PHF antibodies also can be performed locally at the Livestock Disease Diagnostic Center in Lexington, with results returned in about a week.

When Madigan reported positive results on the PCR tests, Slovis realized that there were two other horses at HDM's medicine clinic with similar symptoms. "In a matter of a week, we've had three positives," said Slovis. "So there are probably more out there people don't know about."

The first positive horse was a mixed-breed horse from Cincinnati, which is recovering. The other two confirmed cases from Central Kentucky include a young Thoroughbred, which is recovering, and a Thoroughbred broodmare, which was euthanized after she developed laminitis, a common side effect of PHF.

It was notable that the horse from Cincinnati did not live anywhere near a stream or pond.

"It's out there, even though with PHF, people claim you need an aquatic environment," said Slovis. "We think that with the drought being the way it is, horses might be foraging where they haven't foraged before and ingesting those insects. There might be lusher grass closer to where there is runoff of water, and in that water might be dead insects.

"The cercariae is in the snail, and when it gets hot out, the cercariae will leave the snail and go into the water," Slovis explained. "The larvae of different insects ingest cercariae in an aquatic area, be it in a stream or be it runoff from your wash stall. Underneath the mats of your wash stall, for example, there may be moisture. So (the bacteria) doesn't need to have running water or a pond-like environment. Also, these insects can travel. The (affected) animal could be a ways from a water source and still be infected."

While researchers have only positively linked PHF to mayflies and caddis flies, Slovis says that there are up to 17 water-loving insects that are suspected of carrying the cercariae bacteria.

"The insects carrying the bacteria fly around and when they die, they fall into the grass or into the grain, and the animals accidentally ingest them and get the disease," he explained.

On the Lookout for PHF

Neil M. Williams, DVM, PhD, of the Lexington Disease Diagnostic Center, did his doctoral studies on PHF. He said, "This year is different in that we've had very few cases in (recent years), but there have been years when we've had many more than what we've had to this point. We may get more cases as we get into the fall, so it's still a possibility that there might be a pretty good problem with this disease this year, relatively speaking, compared to other years."

Williams explained that like many diseases, PHF might appear more in some years than others. This could be for numerous reasons, such as the susceptibility of the population of horses, vaccination practices, or weather conditions that might result in higher numbers of vectors that transmit the disease. While a vaccine is available for the disease, it has been noted in literature that it is not very effective.

Slovis said, "We're working with Drs. Madigan and Nicola Pusterla (at UC Davis) to try to see what percentage of the Central Kentucky population might be affected. We're trying to collect a bunch of samples. It's not an outbreak-we're just looking to see what the prevalence is in our population just this year. We've never looked. We're sending in some controls, and seeing what we get."

According to Slovis, once unusually hot weather hits on the East Coast, 10-18 days later horses can show symptoms of the disease, and that might be what has happened in Kentucky.

"It's something we have to keep on our minds. This should not be considered an outbreak, but something to warn clients about, just like West Nile virus," said Slovis. "We haven't had any Potomac horse fever cases (at HDM) in the past two years. Now we have three in a week."

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