Dogs, cats and methicillin-resistant Staphylococcus aureus
STAPHYLOCOCCUS aureus is commonly carried by asymptomatic humans but has serious pathogenic potential in susceptible individuals. The organism is a particular problem because of its ability to develop and transfer antibacterial resistance through multiple mechanisms. This allows rapid adaptation to antibacterial exposure and the potential for multiple drug resistance.
Resistance to penicillin was identified 60 years ago (Kirby 1944) and resistance to the next generation of anti-staphylococcal drugs (the semi-synthetic penicillins) was reported soon after their introduction to clinical practice. Methicillin was first used in 1959 and the first resistant strain reported in 1961 (Jevons and others 1961). Methicillin-resistant S aureus (MRSA) is not a new entity but the recent exponential rise in its prevalence in hospitalised humans and those working in hospitals, in combination with the development of multiple antibacterial resistance, has resulted in increased mortality, morbidity and the cost of healthcare.
In addition, community- acquired rather than nosocomial MRSA infection is being increasingly reported in humans, irrespective of age and in the absence of any of the risk factors previously identified (Frank and others 1999). In either situation, humans are the main reservoir of infection and the major transmission method is direct human-to-human contact.
New focus of attention
Recent press headlines such as “‘Hospital superbug” MRSA spreads to animals’(Revill 2003) and ‘MRSA on the rise in UK veterinary clinics’ (Veterinary Times, October 2004) have focused attention on the potential risk of MRSA to companion animal health and also on the role of dogs and cats in the epidemiology of human MRSA infection.
During the first half of 2004, the BSAVA Scientific Committee asked one of its members, Dr Tim Nuttall, of the University of Liverpool, to spearhead the preparation of an evidence-based review of current literature on MRSA in dogs and cats. The review, ‘Methicillin-resistant Staphylococcus aureus in dogs and cats: an emerging problem?’, by Dr Robert Duquette and Dr Nuttall, is published on pp 591-597 of this issue of the Journal of Small Animal Practice and provides both historical and current information on the epidemiology of MRSA in dogs and cats and the risk of zoonotic spread.
It also provides a scientific basis on which to identify those animals that may be at higher risk of contracting infection, and to underpin any future recommendations on control of MRSA in companion animals and in veterinary hospitals and clinics. The review by Duquette and Nuttall (2004) reinforces the fact that dogs and cats may become infected with MRSA but at a very low rate and with infected humans being the most common source.
What is not yet clear is the degree to which dogs and cats, once infected from a human source, may provide a ‘reservoir’ for susceptible people. There is also evidence that MRSA in companion animals may be acquired both in community and hospital/clinic environments. The same molecular characteristics and antibacterial resistance patterns that distinguish human community-derived MRSA infections from nosocomial infections occur in strains isolated from dogs.
Documented MRSA infections in companion animals
Duquette and Nuttall (2004) have reviewed published reports of MRSA infections in dogs and cats, and they are associated with wound infections (including postoperative infections), prolonged hospitalisation and/or immunosuppressive therapy.
A case of multiple-resistant (including methicillin) S aureus infection associated with septic arthritis in a dog following elective joint surgery for cranial cruciate ligament rupture is also published in this issue of the Journal of SmallAnimal Practice (pp 609-612). The authors, Dr Martin Owen and colleagues, point out that joint sepsis (usually due to Staphylococcus intermedius) is an uncommon, although serious, complication of surgical intervention and that large-breed dogs undergoing stifle joint surgery are more predisposed.
The implications of poor response to broad-spectrum antibacterial drugs was recognised quickly in this case and culture and sensitivity from joint fluid revealed a multiple-resistant S aureus. This was forwarded to a Health Protection Agency laboratory for further evaluation and methicillin resistance identified. The multiple-resistance pattern of the isolate in this case suggests that the MRSA strain was derived from a human nosocomial infection and the implications for both the hospital staff and the owners in this particular case are discussed.
In particular, references are given for barrier nursing methods. Using a treatment strategy relatively novel for dogs, an absorbable gentamicin-impregnated sponge was surgically implanted into the affected joint. This had the benefit of achieving prolonged high intra-articular concentrations of gentamicin while limiting the requirement for systemic administration to that appropriate for elimination of S aureus in other sites. This also minimised both the risk of nephrotoxicity and the potential risks of MRSA contact by humans. The treatment was successful. In retrospect, it would have been interesting to know if this dog was indeed carrying MRSA in its nasopharynx and if the owners had previous hospital contact.
Implications for veterinary surgeons.
The published information on MRSA in dogs and cats currently reflects a biased population made up of individuals that are highly predisposed to bacterial infection and are likely to have had prior antibiotic therapy. In all cases, it is unknown if the affected animals were carrying MRSA prior to lesion development or prior to hospitalisation.
The call by Duquette and Nuttall for prospective, properly controlled epidemiological studies to establish the prevalence of MRSA and other resistant staphylococcal species in the healthy dog and cat population is timely. However, in the immediate future, veterinary surgeons need to be aware of the risk (although low) of MRSA in their patients, and its implications for the animal, its owners and hospital/clinic staff. Any isolation of antibacterial-resistant S aureus should be fuily investigated and appropriate hygiene measures instigated.
Susan E. Shaw
Chairman, BSAVA Scientific Committee
Susan Shaw graduated from the University of Sydney, Australia, in 1974. She is based at the University of Bristol, where she is senior lecturer in the department of clinical veterinary science and is director of the Acarus Unit which investigates arthropod-borne diseases in companion animals. She Is a diplomate of the American College of Veterinary Internal Medicine and a fellow of the Australian College of Veterinary Scientists.
DUQUETTE, R. A. & NUTTALL, T. J. (2004) Methicillin-resistant Staphylococcus aureus in dogs and cats: an emerging problem? Journal of Small Animal Practice 45, 591-597
FRANK, A. L., MARCINAK, J. F., MANGAT, P. D. & SCHRECKENBERGER, P. C. (1999)
Community-acquired and clindamycin-susceptible methicillin-resistant Staphyloccocus aureus in children. Pediatric Infectious Disease Journal 18, 993-1000
JEVONS, M. P., ROLINSON, G. N. & KNOX, R. (1961) Celbenin-resistant staphylocci. British Medical Journal 1,124
KIRBY, W M. M. (1944) Extraction of a highly potent penicillin inactivator from penicillin resistant staphylococci. Science 99, 452 -453
OWEN, M. R., MOORES, A. P. & C0E, R. J. (2004) Management of MRSA septic arthritis in a dog using a gentamicin-impregnated collagen sponge. Journal of Small Animal Practice 45, 609-612
REVILL, J. (2003) ‘Hospital superbug’ MRSA spreads to animals. Observer, December 14