A common limitation of studies or case reports of zoonotic pathogens or infections is that they are one sided – they often just discuss the human case(s), or they just report carriage of a pathogen in animals. Case reports of human infections often only go as far as saying something along the lines of “this bug is most commonly associated with dogs, so the person must have been infected by the pet dog.” Those are usually reasonable assumptions in most cases, but they’re still assumptions. Sometimes, it’s not possible to properly investigate the animal link. Most often, no one bothers.

A recent paper in Emerging Infectious Diseases (Sleutjens et al. 2019) shows what can/should be done when investigating suspected zoonotic infections. It’s a report describing a human case of endocarditis (infection of the inner lining the heart, typically involving heart valves) caused by the bacterium Streptococcus equi zooepidemicus (Strep zoo for short).

The patient was admitted to hospital with malaise and a fever, and infection involving a prosthetic aortic valve that was placed 9 months earlier was noted. Strep zoo was isolated from the patient’s blood, which facilitated appropriate and effective treatment. The patient was discharged from hospital 6 weeks later.

As you can probably guess from the “equi” part of the name, this bacterium is commonly found in horses. (It’s less commonly found in other animal species like dogs and cats, and rare human infections have been associated with those species too.) An investigation of possible sources was performed, and since the patient had frequent contact with 7 horses that were stabled at his yard, those horses were tested. Two of the horses had had upper respiratory tract infections around the time the patient got sick. Whether that’s relevant or co-incidental isn’t clear, since Strep zoo can cause respiratory tract disease in horses, but most often is found in healthy horses. Regardless, the bacterium was isolated from 4 of the 7 horses.

What sets this investigation apart from most others is the additional testing that was done. They performed whole genome sequencing (WGS) of the bacteria isolated from the horses and the patient. Not surprisingly, they were  essentially the same, pretty much confirming that the horses were the source of infection. The figure below shows how the human isolate, in grey, is closely related to (and presumably originated from) the horse strain at the centre of the collection.

Strep zoo infections in people are rare, but it’s important to understand how, when and why bacteria move between species.

As the authors concluded “Systematic reporting of suspected or confirmed transmission of pathogens between horses and humans is lacking. Such reporting would support the estimation of the burden of equine-origin zoonotic infections in humans, which is needed as the equine industry continues to grow. Collaboration among disciplines to develop such a reporting system is fundamental for enabling reliable assessment of the potential risk for humans to become ill after contact with horses and the usefulness of implementing precautionary measures for patients with specific conditions.”

This report doesn’t mean horses pose any more risk now than they did before. It’s just a reminder that uncommonly, infections like this can occur.

In addition to the reporting need that the authors of the paper identify, a key step is even considering zoonotic infections. “Do you have pets or have contact with animals?” isn’t asked enough when people go to the doctor. Most of the time it’s not relevant, but occasionally it’s very relevant, and if the question isn’t asked, zoonotic infections can be missed (sometimes with fatal consequences).