Streptococcus Equi: What the CDC Wants Horse Owners to Know About Bacterial Infection Transmission from Horses

When was the last time you washed your hands?

The Internet lit up today with news of a woman in Washington who died of bacterial meningitis related to contact with a horse. It is Old News–the woman died in March–but it is also Big News, since most people do not understand that there is a risk of cross-transmission between species, and this tragic death is a stirring reminder that it can happen.

The United States Government’s Centers for Disease Control (CDC) has shared a documentation of this usual case in Washington State. A woman there apparently contracted a bacterial infection from a horse she was treating. Her visiting mother also became sick and died quite quickly from bacterial meningitis. 

The report from the CDC is written because the woman’s death resulted from a cross-species infection. While streptococcus equi is known to be present in horses suffering from the disease we know as “strangles”, it is unusual for humans to be infected. The subspecies bacteria involved, S. zooepidemicus, is known as a zoonotic pathogen, meaning that it can infect both horses and humans, or be passed from a horse to a human.

Rather than re-write the CDC’s important documentation of this case, the entire report is posted here. It was originally posted as Notes from the Field: Fatal Infection Associated with Equine Exposure — King County, Washington, 2016 in the Morbidity and Mortality Weekly Report (MMWR) on the CDC website.

• • • • •

On March 17, 2016, Public Health—Seattle & King County in Washington was notified of two persons who received a diagnosis of Streptococcus equisubspecies zooepidemicus (S. zooepidemicus) infections. 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 (1). In horses, S. zooepidemicus is a commensal bacterium that can cause respiratory, wound, and uterine infections (2). The health department investigated to determine the magnitude of the outbreak, identify risk factors, and offer recommendations.

Patient A, a previously healthy woman aged 37 years, operated a horse boarding and riding facility in King County, Washington. Patient A fed, groomed, and exercised the facility’s six horses and cleaned the stalls daily. During the week of February 21, 2016, patient A developed mild pharyngitis and cough. During the week of February 21, horse A developed mucopurulent ocular and nasal discharge and lethargy. On February 29, patient A began administering 10 days of sulfa-based antibiotics to horse A, which recovered without incident.

Patient B, a previously healthy woman aged 71 years and the mother of patient A, developed symptoms consistent with an upper respiratory infection during the week of February 21 while visiting patient A and living in the same household. On March 2, she developed vomiting and diarrhea. On March 3, she was found unconscious and transported to a hospital, where she died that day. Patient B had close contact (i.e., riding, petting, and walking) with horse A on at least February 25 and February 29.

Culture results of nasal swabs collected on March 10 from horse A and two other horses that appeared well were positive for S. zooepidemicus. Patient A did not report consumption of unpasteurized dairy products or exposure to other animals, apart from one healthy cat, during the preceding 2 months. A throat culture from patient A obtained March 10 and blood cultures from patient B grew S. zooepidemicus isolates indistinguishable by pulsed-field gel electrophoresis from isolates cultured from horse A and a second horse at the facility. S. zooepidemicus cultured from a third horse did not match other isolates.

The epidemiologic and laboratory evidence from this investigation linked a fatal S. zooepidemicus infection to close contact with an ill horse. Patient B might have been at increased risk for invasive disease by S. zooepidemicus because of her age and her possible antecedent upper respiratory infection. Because patient A specifically sought health care and a throat culture as a result of patient B’s death, determining whether the S. zooepidemicus infection preceded or followed her mild illness approximately 2 weeks earlier was not possible.

Although S. zooepidemicus is a rare zoonotic pathogen in humans, older persons might be at increased risk for a fatal outcome from this infection; in 32 reported cases, the median age was 61 years (range = <1 to 83 years) with 7 deaths (case-fatality rate = 22%) (1). Consistently practicing thorough hand washing with soap and water after contact with horses and other animals or areas where animals are housed is recommended (3). This outbreak highlights the need for more research regarding risk factors for zoonotic transmission and spectrum of human illness associated with S. zooepidemicus.


Supervisors and staff members of the Reference and PulseNet laboratories, Washington State Department of Health; Shelly McKeirnan, MPH, Public Health–Seattle & King County, Washington; Washington State Department of Health; Washington Animal Disease Diagnostic Laboratory; Northwest Equine Veterinary Associates, Washington; Bernard Beall, PhD, Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, CDC.


  1. van Samkar A, Brouwer MC, van der Ende A, van de Beek D. Streptococcus equi meningitis. Clin Microbiol Infect 2016;22:e3–4. 
  2. Clark C, Greenwood S, Boison JO, Chirino-Trejo M, Dowling PM. Bacterial isolates from equine infections in western Canada (1998-2003). Can Vet J 2008;49:153–60. 
  3. CDC. Wash your hands. Atlanta, GA: US Department of Health and Human Services, CDC; 2016.


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:

(Note: this CDC document is in the public domain.)





Related Posts

Gray horse head in profile on EQ Extra 89 cover
What we’ve learned about PPID
Do right by your retired horse
Tame your horse’s anxiety
COVER EQ_EXTRA-VOL86 Winter Care_fnl_Page_1
Get ready for winter!


"*" indicates required fields


Additional Offers

Additional Offers
This field is for validation purposes and should be left unchanged.