A recent report in MMWR describes a pair of Streptococcus equi subsp. zooepidemicus infections in people. The cases are noteworthy because this bacterium doesn’t often cause human infections, and because one of the infected people died.
More commonly referred to simply as Strep zoo, this bacterium is very commonly found in perfectly healthy horses, although it can cause a variety of infections in this species as well. It can also be found in some other animal species. In dogs, we see it periodically as a cause of pneumonia, which tends to be very severe and often fatal. The same applies in cats. However, it can also be found in healthy dogs and cats, so it’s usually thought of as an opportunist, i.e. a bug that causes disease in unusual situations and secondary to some other inciting cause such as a viral infection or trauma. Human infections also occur, but they are uncommon and usually aren’t severe.
The two cases in the MMWR report were from Washington state. The first individual was an otherwise healthy woman who operated a riding stable. She developed mild cough and pharyngitis (sore throat), and at the same time, a horse she was taking care of developed eye and nasal discharge, and lethargy. The horse received antibiotics, and both horse and caretaker recovered uneventfully.
The second individual was the mother of the first patient. She visited her daughter and stayed at her house during the time that the daughter and her horse were sick, and she had direct contact with the sick horse on at least two days. The mother soon developed signs of an upper respiratory tract infection. About a week later, she started vomiting and had diarrhea, and was found unconscious the next day. She died the following day.
Strep zoo was not surprisingly cultured from a nasal swab of the sick horse. Also unsurprisingly, it was isolated from two healthy horses on the farm. Strep zoo was also isolated from a throat swab of the first woman and blood samples from her mother. When these bacteria were tested, the isolates from the first horse, one healthy horse and the two people were all an identical strain of Strep zoo. (The isolate from the other healthy horse was a different strain).
- Finding Strep zoo in the horses, sick and healthy, is unsurprising.
- Finding Strep zoo as a cause of mild disease in someone caring for an infected horses is not particularly surprising, although we don’t recognize it very often.
- Fatal infection is surprising, although the age of the woman probably contributed to the severity. Whether she was infected from the horse or her daughter is unknown, but ultimately this was an equine-associated infection.
What came up with this case, and with similar mild cases in the past, was the question of “what to do” with the farm and with sick horses in general. There are no clear recommendations. Rarely do people take any precautions when handling horses that have (or might have) Strep zoo infections. You can see why, because human infections are so rare despite the fact that exposure of horse personnel to the bacterium is presumably very common.
But, what level of precaution is indicated?
Do we need to rethink what we usually do?
Does that change with people that are at increased risk because of age or immunocompromise?
- It probably should.
How far to go in order to prevent what is probably a very rare outcome is hard to define.
The article takes a pretty balanced and practical approach, saying “Consistently practicing thorough hand washing with soap and water after contact with horses and other animals or areas where animals are housed is recommended. This outbreak highlights the need for more research regarding risk factors for zoonotic transmission and spectrum of human illness associated with S. zooepidemicus.”
That’s probably all we can say with much confidence.