Finola Leonard, MVB, PhD, MRCVS; Y. Abbott; B.K. Markey
There are 36 species of the genus Staphylococcus, the majority of which are rarely associated with disease. Staphylococcus aureus is the most important pathogen of humans while Staphylococcus aureus and Staphylococcus intermedius are the species most frequently associated with disease in small animals. S. intermedius is the more common pathogen of the two in dogs and cats and is frequently isolated from skin infections in dogs and cats. However, both species are also associated with suppurative conditions such as otitis externa, endometritis and cystitis. As in humans, S. aureus can cause life-threatening bacteraemia and septicaemia.
Barber (1961) first recorded resistance to meticillin in human medicine. By the 1990s, nosocomial infection with MRSA had become a worldwide problem. Strains of S. aureus which are resistant to meticillin possess the mecA gene. This gene encodes a penicillin-binding protein (PBP2a) which has a low affinity for β-lactam antibiotics, and thus mecA-positive strains are resistant to all β-lactams. In addition, most MRSA isolates are resistant to several antimicrobial classes although the extent of resistance differs for different clones of the organism.
Emergence of MRSA Infection in Animals
Devriese et al. (1972) reported the first isolation of MRSA in animals, from mastitis in cows. Small animal colonisation with MRSA was reported by Scott et al. (1988) who described a cat as the suspected source of a MRSA outbreak in a human geriatric ward. MRSA infection in dogs suffering from post-surgical infections was recorded by Smith et al. (1989). MRSA infections have now been recorded in many animal species including dogs, cats, horses, cattle, sheep, pigs, rabbits, chickens and a seal and there is evidence that MRSA can act as a nosocomial pathogen in veterinary medicine as occurs in human medicine.
The most frequently described clinical conditions associated with MRSA infections in dogs and cats are wound infections, in particular post surgical wound infections. As in human medicine, infections associated with orthopaedic implants may persist for months or years. Morris et al. (2006) found that MRSA was significantly more frequently associated with deep infections in dogs than were susceptible S. aureus strains, the latter being more common in superficial infections such as skin infections and otitis. MRSA have also been isolated from a variety of other types of infections in dogs and cats including pyoderma, respiratory and urinary conditions.
Diagnosis and Strain Typing
Detection of MRSA in clinical specimens is usually performed using conventional microbiological methods. Sensitivity of detection can be improved by the use of an enrichment step in which the specimen is incubated in salt broth before plating on solid media. Classically, MRSA strains are isolated on standard media such as Colombia blood agar and identification is confirmed using biochemical tests and antimicrobial resistance typing. A number of commercial chromogenic agars have been developed for the isolation of MRSA in clinical specimens.
Rapid methods for the detection of MRSA have been developed and are used in human medicine. These methods are based on the detection of the mecA gene using polymerase chain reaction techniques. Reports suggest that the sensitivity and specificity of a commercially available real-time PCR test for MRSA in human nasal swabs are 97.8% and 100% respectively (Gregson, et al., 2003). Another study reported sensitivity and specificity of the same test to be 96% for both parameters (Desjardins, et al., 2006).
Several molecular methods are employed for typing of MRSA strains, the most important of which include pulsed field gel electrophoresis (PFGE), multilocus sequence typing (MLST), typing of the SCC mecA element (staphylococcal chromosomal cassette mec element) and typing of spa (the gene encoding staphylococcal protein A). PFGE typing provides great discrimination between strains and is useful for the investigation of local outbreaks of infection. MLST is used for the comparison of the genetic relatedness of MRSA isolates in international epidemiological studies. It provides less discrimination than PFGE but a system devised by Enright et al. (2002) permits MRSA clones to be described using their MLST profile in combination with their SCCmec type and antimicrobial resistance phenotype. This system is now used internationally to characterise human isolates of MRSA and these typing methods are being used increasingly to describe MRSA isolates from animals.
Epidemiology of MRSA Infection in Pets
A limited number of studies are published on MRSA prevalence in animals. Prevalence figures of 14% for MRSA infection in pets suffering from clinical infections were reported in a study of veterinary teaching hospitals in the USA (Middleton, et al., 2005). MRSA was isolated from 8% of dogs with clinical infections and 0.6% of healthy dogs in a survey of Irish veterinary practices (Abbott, et al., 2006). Four of 45 dogs (9%) sampled in the Royal Veterinary College, Queen Mother Hospital for Animals were positive for MRSA (Loeffler, et al.,2005). In contrast, only 0.5% of dogs entering the Ontario Veterinary College Veterinary Teaching Hospital were positive for MRSA (Hanselman, et al., 2008).
Risk factors for the acquisition of MRSA in pets are currently under investigation. Molecular typing studies of MRSA strains isolated from pets usually indicate that strains isolated from dogs and cats match the prevalent hospital-associated human strains in the local geographical area. This finding suggests that the original source of MRSA infection or colonisation in pets is likely to be through contact with infected or colonised people. However, the same animals may subsequently serve as a source of infection for people as documented by Manian (2003). Recent studies suggest that colonisation rates with MRSA in veterinary personnel may be higher than in the general human population and in some cases may be higher than in healthcare workers. Hanselman et al. (2006) reported colonisation rates of 7% in veterinarians and 15.6% in veterinary technicians attending an international veterinary conference. Thus, veterinary personnel may act as a source of infection for their patients and clusters of infection in animals associated with colonised veterinary staff have been reported (Leonard, et al., 2006). Few data are available on the role of environmental contamination in transmission of MRSA between humans and pets. A study in an equine veterinary hospital found widespread environmental contamination with MRSA suggesting that indirect transmission of MRSA may be significant in horses (Weese, et al., 2004). Comparable data for small animal hospitals are not available but it is likely that surfaces frequently touched by hands such as tables, taps, keyboards and stethoscopes may become contaminated.
Control of MRSA in Animals
MRSA strains are usually resistant to a number of antimicrobial classes in addition to the β-lactam antibiotics and thus antimicrobial susceptibility testing is essential for selection of the correct antimicrobial agent for treatment. The strain currently prevalent in human hospitals in the UK and Ireland, and which is usually isolated from MRSA infections in pets, is susceptible to tetracycline and the potentiated sulphonamides. However, treatment with the appropriate antibiotic is not always successful and may have to be coupled with the removal of orthopaedic implants if present (Leonard et al., 2006).
Control measures for MRSA in animals have been adapted from guidelines developed in human medicine and few studies investigating the effectiveness of control measures in animals have been published. Weese et al. (2007) reported the successful control of MRSA transmission in an intensive care unit using measures such as active screening of all animals in the unit, barrier nursing and hand hygiene. However, scientifically published data supporting currently available guidelines such as those published by the British Small Animal Veterinary Association on its website (http://www.bsava.com/), are not available.
Control of MRSA in veterinary practice is based on prevention of introduction of the organism where possible and the curtailment of direct and indirect transmission between animals and between animals and practice personnel:
Screening of animals for carriage or infection before admission to a veterinary practice is not practical in many situations. Some secondary and tertiary referral practices may carry out such screening, with isolation of the patient until MRSA-free status is established.
Screening of suspect cases should be carried out on animals with:
Infections which are non-responsive to antimicrobials.
A history of previous mrsa infection.
Wound infections where the owner has healthcare connections.
Control measures such as isolation and barrier nursing should be instituted when dealing with these cases.
Infection control procedures should be in place to prevent transmission of MRSA (and other infectious agents). Measures should include the following:
Suspect MRSA cases should not enter the waiting rooms.
Correct hand washing.
Alcohol-based hand sanitizers.
Cover wounds and skin lesions (both in animals and staff).
Use gloves, masks, disposable aprons and eye protection when in contact with wounds, body fluids or other contaminated materials.
Observance of strict hygiene measures including regular cleaning and disinfection of consulting rooms and kennels, in particular following occupation by suspect or known MRSA-positive animals. Hand touch surfaces including keyboards, stethoscopes and pens must be frequently cleaned and disinfected also. It is good practice to use dedicated equipment for known positive animals.
Strict aseptic procedures during surgery.
Screening of staff for carriage of MRSA is controversial and should be carried out in consultation with the medical profession. It is most useful as part of infection control procedures following an outbreak or cluster of animal MRSA-positive cases within a practice, in order to determine possible sources of infection. Screening of staff needs to be carried out with sensitivity and in confidence in order that no stigmatisation of individuals occurs.
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