Good news when disease not detected

Staphylococcus pseudintermedius (S. pseudintermedius) is a really common bacteria carried by dogs, most commonly contributing to skin and ear infections, but also able to spread to humans. Studies from around the world have shown carriage by normal healthy dogs of anywhere from 16-87%.


Most of these infections are by methicillin-susceptible S. pseudintermedius which can usually be treated by antibiotics. There is a strain of this bacteria that carries the mecA gene making it resistant to treatment with the normal arsenal of antibiotics available however. This strain has been called methicillin-resistant or MRSP.


There is no data on rates of carriage of S. pseudintermedius in New Zealand. However, a previous study on S. pseudintermedius isolated from diagnostic samples from dogs with active infections in New Zealand showed that only 40% of isolates were susceptible to all the tested antibiotics, and that 38% were methicillin-resistant due to carriage of the mecA gene. 


Methicillin-resistant and multi-drug resistant (defined as an isolate with resistance to antibiotics of three or more different classes) S. pseudintermedius have been reported globally, including from clinically normal dogs, at rates of approximately 3%


MRSP has previously been detected in clinically significant levels the Canterbury region, so what this study set out to look for was what frequency both the methicillin-susceptible and methicillin-resistant strains were in clinically normal dogs within Christchurch and the wider Canterbury region. It was hypothesized that infection rates would be similar to those seen globally i.e. 3% of normal dogs.


Swabs were collected from mouths and bums of 126 healthy dogs presented for either neutering or vaccinations from 9 different practices across Christchurch and the Canterbury region. 130 isolates of S. pseudintermedius were detected across these samples.


The study detected S. pseudintermedius in 73% of the dogs tested, with the mouth being the most common area it was detected from. No MRSP was isolated from these clinically normally dogs, however resistance was found to penicillin (85.1%), tetracycline (25.4%), erythromycin (2.3%), clindamycin (2.3%), trimethoprim (1.5%), chloramphenicol (0.8%), and gentamycin (0.8%).


No resistance to enrofloxacin, ciprofloxacin, cefpodoxime or rifampicin was detected. Two isolates were resistant to antibiotics from three different classes, thus were classified as multi-drug resistant. 


This means that the first-line antimicrobials amoxicillin-clavulanate and cephalosporins, which are often used empirically, should be clinically efficacious in clearing skin infections caused by these wild-type S. pseudintermedius. However, the pervasiveness of resistance to penicillin (85%) means that use of this antibiotic is unlikely to be clinically successful, while the frequency of resistance to tetracycline (25%) suggests antimicrobial susceptibility testing should be performed before use. 


The conclusion was that while most clinically normal dogs in the studied region are likely to be carriers of S. pseudintermedius, only a small proportion, if any, are likely to be carriers of MRSP. While no MRSP was detected in this study, the detection of multi-drug resistant bacteria in a healthy dog is of concern. If we want to continue the low risk of MRSP and multi-drug resistant bacteria from our Canterbury dog population, then appropriate use of antibiotics only where truly required is very important to maintain low-level circulation of drug-resistant bacterial lineages.