A woman from Seattle, Washington, died from a bacterial infection she probably caught from a horse, researchers report.
The woman, 71, was found to have been infected with Streptococcus equi subspecies zooepidemicus (S. zooepidemicus).
S. zooepidemicus is a zoonotic pathogen that rarely causes human illness and is usually associated with consuming unpasteurized dairy products or with direct horse contact.
It is part of the usual bacterial flora found in horses, but can occasionally cause respiratory, wound, and womb infections.
Vance Kawakami and his colleagues have reported on the case in the journal, Morbidity and Mortality Weekly, published by the Centers for Disease Control and Prevention.
Public health services in Seattle and King County in Washington were notified on March 17 this year of a mother and daughter who had been diagnosed with S. zooepidemicus infections.
The daughter, 37, ran a horse boarding and riding facility in King County. She fed, groomed and exercised the facility’s six horses and cleaned the stalls daily.
During the week of February 21, she developed mild pharyngitis – a sore throat – and cough. That same week, a horse developed an eye and nose pus discharge, and was lethargic. On February 29, the daughter started the horse on a 10-day antibiotic course, and the horse recovered without incident.
The mother, a previously healthy woman aged 71, also developed symptoms consistent with an upper respiratory infection during the week of February 21 while visiting her daughter and living in the same household.
On March 2, she developed vomiting and diarrhea. On March 3, she was found unconscious and taken to hospital, where she died that day. She is known to have had close contact with the horse in question – that is, riding, petting, and walking the animal – on at least February 25 and February 29.
Nasal swabs collected on March 10 from the horse in question, and two others that appeared well, were positive for S. zooepidemicus.
The daughter did not report consumption of unpasteurized dairy products or exposure to other animals, apart from one healthy cat, during the preceding two months.
A throat culture from the daughter taken on March 10 and blood cultures from her mother grew S. zooepidemicus isolates that were indistinguishable in analysis from isolates cultured from the original infected horse and a second horse at the facility.
S. zooepidemicus cultured from a third horse did not match other isolates, the researchers reported.
“The epidemiologic and laboratory evidence from this investigation linked a fatal S. zooepidemicus infection to close contact with an ill horse,” they wrote.
The mother might have been at increased risk for invasive disease by S. zooepidemicus because of her age and her possible preceding upper respiratory infection, the study team added.
Because the daughter specifically sought health care and a throat culture as a result of her mother’s death, determining whether the S. zooepidemicus infection preceded or followed her mild illness about two weeks earlier was not possible, they said.
“Although S. zooepidemicus is a rare zoonotic pathogen in humans, older persons might be at increased risk for a fatal outcome from this infection,” the authors noted.
In 32 reported cases, the average age was 61 years, with a range from less than 1 to 83. There were 7 deaths, for a case-fatality rate of 22%.
“Consistently practicing thorough hand washing with soap and water after contact with horses and other animals or areas where animals are housed is recommended,” they said.
“This outbreak highlights the need for more research regarding risk factors for zoonotic transmission and spectrum of human illness associated with S. zooepidemicus.”
The team who reported on the case are affiliated with a range of Washington institutions and the Centers for Disease Control and Prevention.
Kawakami V, Rietberg K, Lipton B, et al. Notes from the Field. Fatal Infection Associated with Equine Exposure — King County, Washington, 2016. MMWR Morb Mortal Wkly Rep 2016;65:788. DOI: http://dx.doi.org/10.15585/mmwr.mm6530a5
The report can be read here.