A woman died earlier this year after visiting her daughter’s riding stable in King County. The 71-year-old victim was killed by a type of bacteria common in horses.

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In a rare case of a fatal infection associated with exposure to horses, a woman died earlier this year after visiting her daughter’s riding stable in King County.

The 71-year-old victim was killed by a type of bacteria common in horses. Called Streptococcus equi zooepidemicus, the pathogen usually causes no health problems in horses and only rarely sickens people, said Dr. Vance Kawakami, an epidemiologist for the Centers for Disease Control and Prevention stationed in Seattle.

“This is an unfortunate, tragic occurrence,” he said. “There are not many cases worldwide of this bacteria causing human illness.”

Because the incident is so unusual, Kawakami and his colleagues reported it in this week’s CDC Morbidity and Mortality Weekly Report.

The woman was visiting her 37-year-old daughter on March 2 when she was stricken by diarrhea and vomiting. On March 3, she was found unconscious and taken to the hospital, where she died that day.

The mother had been spending time at her daughter’s stable, where she petted, walked and rode a horse that later tested positive for the strain of bacteria that caused her death.

The daughter also tested positive for the bacteria but did not get sick.

It’s possible that the mother was vulnerable because of her age, and because of an apparently unrelated respiratory infection, Kawakami said.

In the handful of cases documented around the world, the bacteria seem to cause more serious illness in older individuals and people with compromised immune systems, he explained.

The horse that probably infected the mother had also recently suffered from a respiratory virus, which might have weakened its immune system and allowed the strep bacteria to proliferate.

The other five horses at the stable were also tested. One was found to carry the identical strain of bacteria, and one carried a different strain.

Officials from Public Health – Seattle & King County notified about 25 people who had recently ridden horses or boarded their animals at the stable. About 15 of them opted for screening. Three of the clients tested positive for the bacteria, but none fell ill.

The incident doesn’t mean that people should avoid horses, Kawakami said.

“It’s important for people to be aware that animals can definitely pass disease onto us, but good hygiene can reduce the risk,” he said.

He advises those who work and play with horses — and all animals — to wash their hands regularly and minimize contact with sick animals.