Candida auris is an emerging infectious disease threat. This fungus causes disease almost exclusively in immunocompromised people, with infections most often acquired in hospital. While infections are rare, it’s a bit problem because mortality rates tend to be very high (20-60%), it can cause outbreaks in healthcare facilities, it can live on the skin of healthy people, antifungal resistance is common and routine disinfectants don’t always work very well.

That’s a perfect combination for a nasty hospital-associated infection, and when something is a problem in hospitals, we have to consider whether that will spill into the community. Since infections are most often in highly compromised patients the community risk is probably low, but since it’s an emerging disease, we need to pay attention.

Below is a map of human Candida auris cases in 2022 from the US Centers for Disease Control (CDC).

As I often say, when we see a new disease threat in people, we need to look at the risk to (and from) animals as well. Not to blame them, and not to freak people out, but to understand the risks. More often than not, the animal side of the equation is completely ignored, or there’s overreach and unnecessary blame on an animal source. A logical middle ground is needed, whereby we look and try to understand, addressing any potential problems proactively, and without panic or sensationalism.

What do we know about C. auris in animals?

Very little so far. A study of 87 dogs in India (Yadav et al. 2023) reported detection of C. auris in the ear of 3 dogs and the skin of 1 dog. It’s a bit hard to follow the results of that paper, and not all the infections are well described, but at least one of those dogs had chronic skin and ear infections, and had been previously treated with antibiotics and antifungals. Candida auris was identified on two samples from that dog taken a month apart; that’s important information as we need to understand whether animals can be long-term carriers, short-term carriers or are largely resistant to infection (in which case a positive test result is simply the result of transient contamination, not carriage or infection).

Another study of 251 shelter dogs in Kansas (White et al. 2024) found C. auris in an oral swab from 1 dog, a two-year-old Labrador cross that had recently been surrendered. When they looked at the genetic makeup of the C. auris, it was the same clade (group) as C. auris isolates that have been found in people in a couple of places in the US, including Chicago.

• The isolate was resistant to fluconazole, amphotericin B and terbinafine, but susceptible to voriconazole, itraconazole and caspofungin. So, it was resistant to some common antifungals but not others, but that degree of resistance is still a concern, since our antifungal treatment options are even more limited than our antibacterial treatment options.
• It’s hard to say where the dog got infected. It could have been acquired in the shelter or before the dog was surrendered. No one at its previous home was immunocompromised, and the dog didn’t visit human hospitals (those are the first two risk factors for which I’d look). No shelter workers were known to have been infected, but the staff weren’t tested. The dog hadn’t traveled, had no previous medical issues and had not been treated with antifungals. No other dogs in the shelter tested positive for C. auris, including the littermate with which the affected dog was surrendered. The dog was subsequently adopted, and 2 years later,neither she nor the other dog in her new household were positive for C. auris. (Why was the dog only re-tested 2 years later? Probably because the C. auris was a secondary finding from the original study, and it took that time for the side-work to be done, plus the confirmatory testing).

When we’re thinking about emerging diseases like C. auris in animals, there are two main aspects to consider:

1. Can animals be a source of infection for people? i.e. is the disease zoonotic?
2. Can the pathogen cause disease in animals? Sometimes this animal health component is overlooked.

Actually, we should probably add one additional consideration: can animals be infected by people?  While it’s usually overlooked, we have ample precedent of infectious diseases emerging in human healthcare and spilling over into animals, particularly companion animals (e.g. methicillin-resistant Staphylococcus aureus (MRSA) and various other multidrug-resistant bacteria).

While it’s rare, it’s clear that dogs can be infected with C. auris, at least temporarily. It’s fair to expect that that applies to other species too.

So, that leads to the big question… can animals infected with C. auris infect people?

• Who knows, but probably.

It’s logical to assume that an infected dog could infect a susceptible person, just like an infected person could infect another susceptible person. Fortunately, this organism tends to infect highly immunocompromised people, and most dogs don’t have much contact with those individuals. The risk would be greatest from a dog that had direct contact with an infected person and then contact with another high-risk person (or a dog that lived with an infected person who was successfully treated, but could then potentially be a source of re-infection for that same high-risk person).

What is the risk of C. auris to and from hospital visitation dogs?

Hospital visitation dogs are a unique group with which I’ve worked on and off. We know that these dogs are at increased risk of picking up a variety of infectious agents during hospital visits (e.g. high rates of (albeit transient) MRSA acquisition). A similar situation could potentially occur with C. auris. Fortunately, the risk of exposure of dogs is going to be much lower than with many other hospital-associated pathogens, since C. auris is still very rare even in people. However, this should be yet another reminder of the need to follow appropriate guidelines for canine hospital visitation programs, and to take efforts to reduce the risk of exposure of dogs during such visits.

When MRSA first emerged in dogs, we tended to see it most in pets of healthcare workers and dogs that visited hospitals. Visitation dogs are at the forefront of exposure to a variety of things that are concentrated in human healthcare facilities. If we’re going to see an animal spreading C. auris to people, this is a prime way for that to occur.

We need to think about potential human-animal and animal-human aspects of C. auris, and use basic infection prevention and control measures to reduce the risk of transmission, in either direction. We shouldn’t over react since we have no evidence of a problem, but we also shouldn’t wait for definitive proof before taking reasonable precautions.