Potomac Horse Fever Fatality in Kentucky
Reported by: Stephanie L. Church, News Editor, The Horse Magazine;
LEXINGTON, Ky.-- A Thoroughbred filly in Central Kentucky recently
succumbed to Potomac horse fever (PHF), a disease that is detected
only once or twice per year in the Commonwealth. The cause of death
was a mystery until test results were received from the University
of Kentucky Livestock Disease Diagnostic Center (LDDC) on June 27.
horse fever first hit 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. The
Kentucky filly was kept on a farm with several small streams and
two-year-old filly went off her feed on June 5, displaying the first
of an array of symptoms that puzzled the owner, farm veterinarian
(who treated the horse for the seven days prior to referral), and
referring veterinarian Bonnie Barr, VMD, of Rood and Riddle Equine
Hospital in Lexington, which admitted the filly to its isolation
facility on June 11. Barr explained that the filly, "had the
history of having diarrhea on the farm, she was depressed, showed
signs of colic, anorexia, and dehydration. Colic isn't something
you see all of the time (with PHF), but you do see it in about a
third of the cases. She showed signs suggestive of laminitis or
kind of go through your head the different diseases that would cause
the horse to have these symptoms-Salmonella and Clostridium--and
although Potomac horse fever is in the back of your mind, you wouldn't
think of it in this area."
owner described the filly's demise after several days in the clinic.
"She had vasculitis, or extreme swelling. She was a pretty
diminutive filly, and her face, belly, and legs blew up the size
of a draft horse. At first they thought it was the result of self-inflicted
injuries (when the horse had a violent episode of abdominal discomfort
prior to referral). They did every test under the sun--spinal taps,
X rays, you name it-but could not figure out what it was,"
the owner said. "She was there eight days before the decision
was finally made to put her down (June 18)."
explained that what usually debilitates the horse in a PHF case
is founder, which is what happened to this filly. "You can
treat the PHF, but if the horse founders, you have a whole lot of
things to deal with. If you're suspicious, you treat them with the
oxytetracycline." She said that one can test the blood of the
horse for PHF, but often by the time the results are returned the
horse is either on the road to improvement or beyond treatment.
was surprised when Dr. Williams (Neil M. Williams, DVM, PhD, of
LDDC) called and told me the results," said Barr. "Maybe
this case will make us a little more aware. Sometimes things come
in a cycle, and maybe we'll see a few more cases."
who did his doctoral studies on PHF, said in Kentucky, "We
only see one or two cases per year. The number of cases have really
fallen off dramatically since the 1980s. The fact that it is not
that prevalent, and that the signs can vary greatly from case to
case, makes it a diagnostic challenge. This disease is very sporadic;
usually there's only one case on a farm."
first signs that are typically seen with PHF would be lethargy,
fever, loss of appetite, and possibly diarrhea, said Williams. Any
time those symptoms are encountered, he added, you should seek veterinary
literature describes it as being only 5-20% fatal," said Williams.
"Obviously, we would only deal with the fatal cases (here at
the LDDC). Blood samples could also be sent in from live horses
for testing purposes (no vet clinics in the Lexington area test
for PHF in-house). That is the best way to practice surveillance,
since fortunately most of the horses don't die.
there is a vaccine, we're not necessarily recommending vaccinating
all horses with it. It's available and you may want to consider
it, but that would be best discussed with your veterinarian,"
added Williams. "The vaccine has been reported to be not completely
Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in
PHF research for nearly 10 years and 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 and did
the first experimental reproduction of the disease by feeding horses
caddisflies. Researchers at The Ohio State University have since
reproduced the UC Davis study and confirmed the initial finding
of transmission by ingestion of insects.
time to recall this very important disease," said Madigan.
"There are some new and effective DNA diagnostics available,
and the disease is not a mystery any more--if the disease is found
on a premise, a person can go in and do some control measures to
potentially reduce the risk of getting the disease. At River Downs
in Southern Ohio on the Kentucky border (where a large outbreak
occurred in 1980s), they filled in all the ditches (and areas of
fresh water) and the outbreak went away. Madigan suspects the disease
is much more common and lack of testing and vaccine protection have
reduced the awareness and detection. It is also a cause of abortion
in mares and not easy to detect in aborted fetuses, Madigan explained.
a horse comes in with a fever, colic, and/or diarrhea, and they
live close to bodies of water, it is prudent to get them tested,
or be thinking of prompt treatment in areas where the disease has
been confirmed," Madigan said. "If you think about it,
it can often be treated, otherwise it's a killer."
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