Page 24 - New Mexico Horse Breeder, Fall

Bacteria resides in a certain type of fluke (trematode, or parasitic flatworm) that in its different life stages, inhabits a number of different species including aquatic snails and some aquatic insects like Mayflies and caddis flies. These bacteria have also been found in birds and bats.
Potomac Horse Fever
Potomac Horse Fever is sporadic and seasonal and while most cases occur during summer and fall, cases are occasionally seen
at other times of the year. This disease is technically known as equine monocytic ehrlichiosis, but because it was first documented in the Potomac River Valley of Maryland in l979, most horsemen know it as Potomac Horse Fever (PHF).
This disease probably existed before 1979 and in other areas; “river valley fever” and “Shasta River crud” have been a problem in northern California for a long time. Affected horses developed diarrhea, fever and usually died unless treated with tetracycline--the same drug currently used to treat horses with PHF. It just happened that a virulent strain emerged in 1979 in Maryland and gained attention and a name.
Dr. Katherine Wilson (Clinical Assistant Professor, Large Animal Medicine, Virginia- Maryland Regional College of Veterinary Medicine, Blacksburg, Virginia) says the
disease is caused by a bacterium formerly called Ehrlichia risticci, but its name has been changed to Neorickettsia risticci.
“There are several things we now know about this disease and the life cycle of the organism, but we still don’t totally understand the true life cycle in terms of what other organisms it utilizes as hosts and how it is transmitted to horses through all of that. One thing that we do know is that the bacteria reside
by Heather Smith Thomas
in a certain type of fluke (trematode, or parasitic flatworm) that in its different life stages, inhabits a number of different species including aquatic snails and some aquatic insects like Mayflies and caddis flies. These bacteria have also been found in birds and bats. Whether the birds and bats are natural hosts and whether they play any role in transmission of the bacteria to horses is still unknown,” she says.
“It’s a complicated cycle. How the bacteria gets into the horse is still a little bit of a mystery. We don’t have much evidence yet to show how horses are getting this disease,” she says. At first, people thought the disease was transmitted to horses via biting insects, since this particular pathogen belongs to the same family of pathogens that cause tick fever. Then scientists discovered that snails were involved in the process.
“For many years we thought horses had to actually eat the snails to get the bacteria and that only the horses with access to running water would be at risk. But PHF often occurs in horses that are not near water where snails live. So now there’s a question about whether horses are being infected by ingesting the insects,” says Wilson.
“One theory is that these aquatic insects are attracted to lights around the barn and fall into water troughs or into feed. PHF is definitely not transmitted by mosquitoes or biting insects. We may learn more in the next 10 years,” she says.
THE DISEASE
PHF is seasonal and usually appears in the summer months and into the fall and even early winter. “Once the horses are infected with the bacteria, it may take anywhere from one to three weeks for them to come down with the disease. PHF has variable clinical signs. The classic situation is a horse going off feed, with diarrhea and fever. This
is usually a biphasic fever, which means they will have a fever and then their temperature comes back down to normal, and then they develop another fever. Then most horses develop profuse diarrhea and become severely endotoxic and dehydrated, losing protein through diarrhea,” says Wilson.
“The classic scenario is that these horses are very sick and about 30 to 40% of them develop laminitis. If they end up dying from PHF, most of those fatalities result from either not being treated for the dehydration and shock, or development of laminitis,” she says.
“There are wide ranges of clinical signs, however. Some horses are just off feed, with fever, and may have decreased fecal output, acting a little colicky—and never develop diarrhea. When we run bloodwork on those horses, they have a low white blood cell count, and if we test them for PHF they test positive,” she says.
If a pregnant mare gets sick with PHF she may abort. “In my experience, these mares
are pretty sick, and abort secondary to this disease.” Any horse that is dull and off feed, and certainly any horse that has profuse diarrhea should be checked by a veterinarian.
22 New Mexico Horse Breeder