Horse illness raises questions of exact bacterial cause

Potomac fever in horses can cause severe diarrhea, high temperatures, abortion of fetuses, lethargy and anorexia

Scientists are investigating whether a species of bacteria shown to cause symptoms similar to Potomac fever in horses is present in Alberta.

It raises the possibility of extra biosecurity and isolation requirements, which could impact the owners of affected horses, said a researcher.

“We have a lot of horses that come in that we think have Potomac fever,” said Ashley Whitehead, an associate dean of clinical programs at the University of Calgary’s Faculty of Veterinary Medicine.

Although the horses respond appropriately to treatment for the fever, they come up negative in tests for the disease, she said.

Such tests are designed to identify Neorickettsia risticii, a bacteria known to cause the illness, which is fatal in about one-third of infected horses, she said. They cannot detect Neorickettsia findlayensis, an “extremely similar” species of bacteria of the same genus found in horses with symptoms of the fever in eastern Ontario and the United States, said Whitehead.

She cautioned there is much to learn about how the new bacteria affects horses and that any conclusions must be proven through research. It has not yet been confirmed if it is even present in horses in Alberta, although researchers suspect it is, she said.

“This is kind of like we’re on the breaking edge of learning something pretty new here,” she said. “It’s a really big unknown here in Alberta.”

Besides severe diarrhea, Potomac fever can lead to temperatures of up to 41.6 C in horses, along with abortion of fetuses, lethargy and anorexia. Another common effect is laminitis, a painful inflammation within the hoofs that can lead to their loss — “and obviously, no foot, no horse,” she said.

The province has experienced a Potomac fever outbreak this year that has lasted far into the fall, said Whitehead. She cited the wet weather earlier this year as a likely cause, which created pools of standing water. That was followed by a fall with prolonged warm temperatures.

These conditions mean things such as aquatic insects that help transmit the disease, which are then inadvertently ingested by horses, have been active for longer than usual, sparking cases well into late September, she said.

“The most common times that we see Potomac horse fever is in June, July and August in Alberta,” said Whitehead.

It is likely that Neorickettsia findlayensis has a similar life cycle as Neorickettsia risticii, she said. As part of initial research involving the University of Calgary that included the Moore Equine Veterinary Centre, the University of Guelph in Ontario and Ohio State University in the U.S., fecal and blood samples were collected from horses in Alberta, she said.

Researchers were waiting as of Oct. 8 for the results of cultures to determine if the new bacteria is in the province, said Whitehead, who is also a senior instructor of equine clinical sciences. She hopes to receive funding to expand the research next summer.

Scientists in the U.S. and Ontario found that Neorickettsia findlayensis can cause symptoms similar to Potomac fever, she said, adding the link was determined using both naturally occurring and induced cases.

“This will mean we will have false negative tests, which can delay treatment,” she said in an e-mail Oct. 8.

Although Potomac fever caused by Neorickettsia risticii can lead to severe diarrhea, the disease isn’t contagious between horses, she said. But other causes of diarrhea “can be very contagious or even zoonotic,” which means they can also infect people, similar to salmonella, she said.

“If we get false negative tests, then we have to assume that the condition is contagious, which can mean additional biosecurity and isolation requirements which are not only costly, but time-consuming for (horse) owners,” she said.

It isn’t clear if Neorickettsia findlayensis can cause symptoms in horses as severe as those caused by Neorickettsia risticii, she said. However, the existing vaccine for Potomac fever likely won’t be effective against the new bacteria, a possibility that must be confirmed by research.

“It’s such an unknown… you know, is it out there? Is it here?” she said.

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