How is it spread?
How can you protect your horse?
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Fever is caused by an organism known as Neorickettsia
risticii (formerly called Erhlicia risticii). This
organism is similar in many ways to the organisms that
cause Lyme Disease and Rocky Mountain Fever. In lower
Michigan, we see many cases of Potomac Horse Fever each
summer. It is a serious disease that results in fever
and severe diarrhea. Some horses will also develop
laminitis( serious inflammation of the hooves, also
known as founder) as a result of the disease. Although
it is treatable in the early stages with the antibiotic
oxytetracycline, it often goes unrecognized until it is
difficult to treat. Some affected horses will die, and
often the ones that live have difficult recoveries. In
vaccinated horses, PHF can still cause disease but it is
usually more mild and responds well to treatment.
swimming cercaria leave the snails
caddis flies carry PHF
flies carry PHF
swarming around Stable light bulb
In the past few years, researchers at the University
of California-Davis Veterinary school have discovered a
lot about how this disease is transmitted. As a horse
owner, understanding the life cycle of the disease will
help you protect your horse from it.
The PHF organism lives inside tiny trematodes (fluke
parasites) that infect snails. When these trematodes
reproduce, they hatch cercaria, tiny swimming larvae,
that leave the snails and go into the water environment.
These larvae carry the PHF organism inside them.
These PHF-carrying cercaria are eaten by other
larvae— The water-dwelling larvae of the mayfly and
caddis fly. When the larvae hatch into flying insects,
they take the Cercaria, and the Potomac Horse Fever,
These flies do not generally live very long. Their
life span is only a few days. As they die, they may
contaminate pasture, feed, hay , bedding and other
surfaces that your horse has contact with.
When your horse accidently ingests one of these
flies, the Potomac Horse Fever organism comes with it.
In the horses intestines, the PHF organisms are released
from the cercaria and begin to multiply, causing
inflammation of the gut. This inflammation results in
the fever, diarrhea, and toxemia( release of toxins from
infection into the bloodstream) that we recognize as
Potomac Horse Fever .
How does knowing this help you as a horse owner?
Well, first of all, we know that the PHF organism is
almost always going to be found near water. Creating
boundaries of trees, hedges, and other vegetation
between your property and water sources can help insure
that the insects do not travel to your property.
Also, we know that certain flies can carry the
disease. These flies are attracted to light during the
night hours. So, avoiding leaving on lights in the barn
that will attract the flies can help lower your horse’s
risk of coming in contact with the disease. Whenever
possible, keep feed and hay stored away from insects.
Finally, since the disease carrying organisms are
dependent on water, avoid allowing standing water on
your horse properties. Keep water buckets and troughs
cleaned out regularly. Establish grading or drainage so
that standing water does not develop around pasture
areas. This will have the additional benefit of reducing
the mosquito population. Mosquitoes carry other diseases
such as West Nile, so you don’t want them around any
There is a vaccine for Potomac Horse Fever, and we
recommend it in the spring for all horses because the
disease is so prevalent in our area. However, the
vaccine produces limited immunity(resistance to
infection). Usually the immunity is short-lived, about 3
months. For this reason we routinely recommend a booster
in July to keep the horse fully protected. While
vaccinated horses do occasionally get PHF, they usually
get only a mild illness and recover.
Watch your horse closely during the summer months,
especially if there is a noticeable rise in the number
of insects around. Potomac Horse Fever is what we call
biphasic, meaning it has two separate phases. The horse
will have a fever for 1-2 days, then his temperature
will return to normal. As the fever fades, the diarrhea
begins. Usually there is a second high fever when the
diarrhea develops. The antibiotic is most effective if
given before the diarrhea begins. Unfortunately, a lot
of horse owners do not realize the horse is sick until
the diarrhea starts. At that point antibiotic treatment
is less effective. By paying close attention, you will
identify it when it is still very treatable. Every barn
should have a digital thermometer. If your horse does
not seem “right” during the summer months, check his
temperature and call your vet if it is above 102.5 F.
Treatment of PHF involves intra-venous(IV)
antibiotics, Banamine to manage the fever and toxemia,
and often large volumes of IV fluids. Keeping the horse
from dehydrating often requires a lot of fluids because
the diarrhea is profuse. Some horses will develop either
severe anemia(low red blood cells) or very low white
blood cell counts. In very severe causes, plasma
transfusions may be necessary. If laminitis develops,
more intensive care is required to help the horse
recover. Horses often lose several hundred pounds during
the course of treatment. The exception is vaccinated
horses, who usually get only a mild fever and mild
|Foot of a
horse who developed laminitis from PHF
||Horse On IV fluids
Denise Bickel DVM
Whole Horse Veterinary Services
Phone # 517-474-4050
Fax # 517-764-7710
3906 Seymour Rd
Jackson, MI 49201