Four Additional Cases of Potomac Horse Fever Confirmed
at Lexington, Ky Veterinary Hospitals
by: Stephanie L. Church, News Editor, The Horse Magazine; TheHorse.com
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.
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.
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.
of Potomac Horse Fever
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.
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.
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."
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.
get toxic, and the toxins leak into the body," explained Slovis.
"The integrity of the bowel is compromised."
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.
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.
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.
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
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.
was notable that the horse from Cincinnati did not live anywhere
near a stream or pond.
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.
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."
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.
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.
the Lookout for PHF
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."
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.
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."
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.
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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