As someone who works with zoonotic diseases, I often find myself fighting battles on both sides of the issue. One side is trying to increase awareness about zoonotic diseases (i.e. those caused by microorganisms that are transmitted between animals and humans) and getting people to think about the potential role of animals in human infection. However, I often also have to deal with trying to keep things in perspective, and prevent people from over-reacting to disease risks. Part of this is helping people understand that disease transmission is typically a two-way street. While animal-to-human transmission is usually the greatest concern, human-to-animal transmission of a variety of bugs also occurs, and this can cause problems for the animals, and for people who subsequently have contact with those animals.
A recent paper in the journal Mycoses (Van Rooij et al 2013) highlights one such scenario. The paper describes ringworm in a dog that was associated with the fungus Trichophyton rubrum, which is not the typical ringworm species (Microsporum canis) that we find in dogs. Trichophyton rubrum is a common cause of infection in people, particularly tinea pedis (athlete’s foot) and onychomycosis (fungal infection of finger and toe nails). The authors did something that’s often lacking in reports of animal-human transmission: they actually tested both the person and the pet. Here, they found that the owner was a carrier of this fungus. They were able to isolate the fungus from his skin and determined that he likely had an asymptomatic infection that was subsequently transmitted to his dog. (He’d previously had untreated and self-resolving athlete’s foot, and presumably remained a carrier after that). Since this ringworm species is predominantly found in humans, it’s a reasonable assumption that it started with the person and the problem was only identified when the dog developed disease. In this case, the dog was old and had been treated with corticosteroids, both of which probably affected it’s immune system and made it more susceptible to this uncommon cause of canine disease.
It’s important to remember that while zoonoses are important, pathogens go both ways.
In the end, we’re all animals.