People, dogs and bacteria

When penicillin was first introduced in the early 1940s, it was effective against nearly all bacteria. But nearly 70 years later, penicillin has lost its punch: while bacteria such as Streptococci are still frequently susceptible to the antibiotic, Staphylococcus aureus are often resistant.

“Antibiotic resistance has emerged very quickly because bacteria have a very short generation time and can evolve and adapt rapidly,” says Dr. Joseph Rubin, a postdoctoral fellow at the Western College of Veterinary Medicine (WCVM). “When you double every 20 minutes, you have a lot of evolutionary power.”

After graduating from the WCVM in 2007, Rubin began graduate studies in the field of veterinary microbiology. His PhD thesis work focused on antimicrobial susceptibility of canine and human Staphylococcus aureus in Saskatoon, Sask. With the guidance of his supervisor, WCVM professor Dr. Manuel Chirino-Trejo, Rubin looked at methicillin resistant S. aureus (MRSA) and the risk of transmission between dogs and people associated with it.

“There isn’t very much information available about MRSA in dogs or people in Saskatchewan,” says Rubin, whose research work was supported by the Companion Animal Health Fund (CAHF). “My study’s goal was to provide a baseline for future S. aureus surveillance and resistance monitoring.”

Q. What is Staphylococcus aureus?

S. aureus is a gram-positive bacterium. It’s a very common organism that’s found in the nose of about 30 per cent of people at any given point in time. In colonized dogs, it can be found in the intestines or on mucous membranes (back of the throat and anterior nares). It’s an opportunistic pathogen – it doesn’t usually cause a problem unless there’s a breach in the normal host defenses. For example, a skin wound provides an entry point for S. aureus to set up an infection.

Q. What is methicillin resistant S. aureus (MRSA)?

This name is misleading because MRSA is resistant to more than just methicillin. It’s actually resistant to a class of antibiotics known as beta-lactams that includes penicillins, cephalosporins and carbapenems. While MRSA is always resistant to all beta-lactams, it’s often resistant to other classes of antimicrobials such as macrolides, tetracyclines and fluoroquinolones.

Common infections associated with MRSA in dogs include skin, ear and surgical incision infections. In people, MRSA is frequently involved in skin and soft tissue infections as well as in hospital-acquired infections.

Q. Why is antimicrobial resistance a concern?

We care about antimicrobial resistance because it means we have less effective treatment options available for patients whether they’re animals or people.

These organisms are continually evolving and we’re not developing new drugs as quickly as we were during the 1950s and 1960s. We don’t have anything new being developed so we’re in a situation where there’s nothing left to treat some of these really nasty bacteria.

 Q. How is MRSA transmitted from dogs to people?

We have very close contact with dogs. They’re part of the family in many households. We’re petting them, giving them baths, sleeping with them and they’re licking our faces. A lot of places where MRSA tends to live in or on a dog are places that we may have inadvertent contact with.

An important point is that canine MRSA infections are still rarely identified in Western Canada. S. aureus is more of a human pathogen. We believe that people pass it on to their dogs and the dogs end up giving it back. So the route of transmission is primarily from humans to dogs, but it can go both ways.

Q. Can you describe your study?

From May to November 2008, I collected swab samples from the nasal passages, the throat and the rectum of dogs visiting the WCVM’s Veterinary Medical Centre (VMC) for routine health checks and annual vaccinations.

The swabs were plated out on special growth media every day and any colonies that looked like potential S. aureus were biochemically tested to determine the species.

Saskatoon’s Royal University Hospital supplied a number of S. aureus isolates from people. These isolates, along with the S. aureus derived from the dogs, were exposed to 33 different antimicrobial drugs. Some of these drugs are used exclusively in veterinary medicine or human medicine; some are used in animals and people.

Over a two-year period, I performed several genetic tests to determine if the S. aureus isolates from dogs and people were either MRSA or methicillin susceptible S. aureus (MSSA).

Q. What were your findings?

I found that MRSA was in 0.7 per cent of healthy dogs, but S. aureus in general (MRSA and MSSA) was in 10 per cent of healthy dogs, which is slightly higher than previously reported.

The genetic tests revealed that canine and human MRSA are closely related in Saskatoon, providing evidence that transmission occurs between people and dogs in this area. This data also confirms previous findings of studies done in other parts of the world that canine and human MRSA from the same region tend to be related.

One interesting finding was that much of the S. aureus isolates colonizing healthy dogs were found to be fairly susceptible to many of the drugs. Unfortunately, this is changing so the study should be repeated in a couple of years to see what’s going on there.

 Q. How are your results significant for pet owners and veterinarians?

My investigation will help raise awareness among veterinarians about these organisms. MRSA in dogs is a relatively new phenomenon and it’s only recently that it’s becoming more widely acknowledged by the veterinary profession. I hope that awareness will allow vets to better treat their patients and to help protect their clients’ health.

 Q. Any suggestions on how to prevent an MRSA infection?

Good hand washing is key! While it’s important that veterinary and human medical facilities ensure basic biosecurity measures are in place, hand washing is the most important method of prevention. And this is not only critical for avoiding MRSA infections, but really any type of bacterial or viral infection.

Visit WCVM Today to read more WCVM news.