The prevalence of methicillin-resistant Staphylococcal spp infections in animals is increasing, and this is no longer an “uncommon” finding in clinical practice.
Excellent information for clinicians is available in the recent edition of Infectious Diseases of the Dog and Cat, and University of Guelph's Worms and Germs Blog is a great resource for clients. Until you have time to read these sources, here are answers to some of the most commonly asked client questions:
1. This is the “doggie” MRSA, right?
It cannot be stressed enough that a distinction needs to be made between methicillin-resistant infections in dogs and cats and in humans. The primary pathogen of concern for this type of infection in humans is Staphylococcus aureus, and resistant infections are referred to as methicillin-resistant Staphylococcus aureus or MRSA. These infections can be hospital or community acquired. Hospital-acquired or nosocomial infections are contracted while the patient is in a hospital; in contrast, community-acquired infections refer to those that develop in patients with no known exposure to a healthcare setting. The primary pathogen of dogs is Staphylococcus pseudintermedius, and this is referred to as methicillin-resistant Staphylococcus pseudintermedius or MRSP. MRSA and MRSP are two different organisms with different biological behaviors. The similarity shared between these two organisms is their resistance pattern to antibiotics (ie methicillin resistance). These organisms share the mecA gene that confers resistance to all beta-lactam antibiotics (all classes and all generations of penicillins and cephalosporins). Methicillin resistance does not mean the organism is more virulent, and most infections can be treated successfully.1
Related Articles: Methicillin-Resistant Staphylococcal Infections
2. Can this infection be transmitted to humans?
The short answer is that transmission to humans is highly unlikely and very rare. This organism is of low zoonotic risk. When discussing this aspect, it is often helpful to first define the difference between “colonization” and “infection” for clients. Briefly, colonization refers to the presence of bacteria that are causing no harm (tissue invasion or damage), whereas infection refers to bacterial invasion and infliction of clinical signs. Unlike in humans, in which S aureus colonization is common, colonization with S pseudintermedius is unlikely even among humans that have frequent contact with animals. In one study in which 144 healthy veterinary staff members were cultured, only one showed colonization.2 When hospital records of 3397 cultures were reviewed, only 2 S pseudintermedius isolates were identified.2
However, the following needs to be noted: dog bite wounds are a risk factor for infection, with one report from the UK identifying S (pseud)intermedius in 6/34 bite wounds.2 Transient colonization can occur in humans that are treating dogs with deep pyoderma due to MRSP. Two groups of owners were compared: owners with contact with a dog with deep pyoderma and owners without contact. S pseudintermedius was isolated from 6/13 owners of dogs with deep pyoderma compared to 1/13 in the non-deep pyoderma group. Typing revealed that the strains were identical between human and canine isolates. When owners were re-cultured once the deep pyoderma was resolved, they were found to no longer be carriers.3 Contact with purulent material was the most likely source and exposure to this and/or contact during bathing may be risk factors for colonization. Extra attention to good hygiene is always recommended when in contact with patients with MRSP infections, especially in anyone that is at increased risk for infection in general.
Related Article: A Brief History of Staphylococcus