Introduction

Methicillin-resistant Staphylococcus aureus (MRSA) infection is now recognised as a worldwide problem in human medicine. Epidemic hospital strains (EMRSA) are a common in human medical institutions and strains that are distributed amongst people in the community (community-acquired MRSA) are being recognised increasingly. The very broad antimicrobial resistance profile of MRSA makes it a major hazard in human hospitals and to vulnerable individuals in the community; it is significant cause of human mortality.¹

In 1988 colonisation of a cat with MRSA was first recognised when in-contact patients in a geriatric ward developed recurrent MRSA infection² and this case demonstrated that transfer from animals to man could occur. There are now many reports documenting transfer of MRSA, most commonly hospital EMRSA, from humans to animals,³ and colonisation and infection of dogs and cats with MRSA is increasingly recognised in veterinary practice⁴, particularly in the USA and UK.

The consequence of the occurrence of MRSA in domestic pets is that practitioners are now obliged to consider more carefully: 1) the possibility that animals they are treating may be carriers or infected with MRSA, 2) the consequences this may have for treatment of affected animals, and 3) the risks of transfer to other animals and to veterinary staff.

Recognising MRSA Infection and Colonisation in Dogs and Cats

Staphylococcal infection is well-recognised in small animal veterinary practice. Normally S. intermedius is the cause and isolates seldom have very broad antimicrobial resistance. The risks to associated humans are very low. S. aureus causes similar clinical presentations but infection in pets is much less common. In the past, S. aureus strains associated with pet animal infections have often shown a broader range of antimicrobial resistance than S. intermedius but with the advent of highly resistant MRSA, S. aureus presents a much greater challenge.⁴

In the British Isles, two reports in 2004, provided warning that MRSA infection was becoming a problem in small animal practice. Rich and Roberts⁵ reported in 2004 isolation of 95 MRSA from specimens submitted to a veterinary diagnostic laboratory during 2003. In March 2004, Boag et al.⁶ reported an increase in cases of MRSA infection seen at a small animal referral hospital; 12 cases had been confirmed in dogs and cats over the previous 5-months.

There is now increasing evidence that veterinary staff can become colonised by MRSA at relatively high frequencies and that transfer amongst staff and animals in veterinary practice can readily occur.³ Furthermore, owners of MRSA-infected animals may be the original source of infection, particularly if they have had contact with human healthcare facilities, or may become colonised by MRSA from their infected or colonised pets. Thus animals that are susceptible to bacterial infection, especially those being treated with antimicrobials, are at risk of acquiring MRSA from owners or veterinary staff and from other MRSA-infected or colonised animals, and may then be much more difficult or impossible to treat effectively.

Recognition of MRSA infection in pet animals should occur when diagnostic microbiology is carried out on appropriate samples but this is not always the case. Laboratories that are expecting to isolate S. intermedius may misidentify S. aureus particularly those strains which have very low levels of golden pigmentation. Suspicion should be raised if an isolate reported as S. intermedius has a very broad resistance profile, especially if it is resistant to cefalexin. Any case of bacterial infection that does not respond to properly administered antimicrobial drugs or suffers from recurrent infections should also be suspected. If there is doubt, the laboratory should be asked to recheck the identity of isolates or new specimens should be submitted with a request that checks be made for the presence of S. aureus.

Treatment of MRSA Infection in Pets

MRSA can be very highly resistant. In some cases there may be no antimicrobials that are effective against them. Fortunately, isolates from animals have generally proved to be susceptible to potentiated sulphonamides and oxytetracycline, and also topical products including fusidic acid and mupirocin.³ Clindamycin sensitivity is quite common but inducible resistance to this antibiotic has been reported in 71% of 285 MRSA animal isolates and screening for such inducible resistance is recommended if clindamycin is to be used.⁷

There is evidence indicating that when small animals become infected with MRSA, the nasal mucosae commonly become colonised. This colonisation may persist for a substantial time. Thus when animals are found to be infected or colonised with MRSA, the need for decolonisation must be considered. Otherwise the animals may continue to pose a risk to themselves, to other animals and to people who are in contact. No well-established methods for decolonisation have been described but combination of systemic therapy with treatment of mucosal sites with topical antimicrobials to which the MRSA is sensitive may be effective. Fusidic acid has been shown to be effective, at least in the short-term, with S. intermedius.⁸ Treatment for about three weeks with topical mucosal application twice or three times daily may be effective.

Controlling Transfer of MRSA Infection in Practice

Preliminary data indicate that owners and veterinary staff in contact with MRSA-infected dogs and cats may often be colonised by MRSA. Although in healthy individuals the risk posed by MRSA appears to be no greater than that of methicillin-sensitive strains, the risk of transfer to susceptible animals or people must be considered. There is now abundant evidence that this occurs and infection of animals under veterinary treatment, particularly those with wounds or subjected to surgery, has been documented. In addition, transfer of MRSA to the environment can readily occur and survival of such organisms in the environment for many months is possible.⁹

Thus veterinary surgeons need to monitor possible MRSA colonisation amongst their staff, and MRSA infection and colonisation amongst the animals they treat. Hospital hygiene methods need to be rigorously maintained at a high level of efficiency and when MRSA infection is recognised or suspected, comprehensive disinfection must be carried out. Members of staff need to be given appropriate training so as to understand the risks posed by MRSA and enable them to adopt appropriate disinfection and aseptic techniques. Isolation facilities should be reserved for infected animals or if these cannot be supplied, strict barrier nursing must be maintained.

In the UK, the British Small Animal Veterinary Association has published very useful guidelines for dealing with MRSA in small animal practice at its website (http://www.bsava.com/resources/mrsa/mrsaguidelines/).

References

1.  Appelbaum PC. MRSA--the tip of the iceberg. Clinical Microbiology and Infection 2006; 12 Suppl 2: 3-10.

2.  Scott GM, Thomson R, Maloney-Lee J, Ridgway GL. Cross-infection between animals and man: possible feline transmission of Staphylococcus aureus infection in humans? Journal of Hospital Infection 1988; 12: 29-34.

3.  Loeffler A, Boag AK, Sung J, et al. Prevalence of methicillin-resistant Staphylococcus aureus among staff and pets in a small animal referral hospital in the UK. Journal of Antimicrobial Chemotherapy 2005; 56: 692-7.

4.  Duquette RA, Nuttall TJ. Methicillin-resistant Staphylococcus aureus in dogs and cats: an emerging problem? Journal of Small Animal Practice 2004; 45: 591-7.

5.  Rich M, Roberts L. Methicillin-resistant Staphylococcus aureus isolates from companion animals. Veterinary Record 2004; 154: 310.

6.  Boag AK, Loeffler A, Lloyd DH. Methicillin-resistant Staphylococcus aureus in small animal practice. Veterinary Record 2004; 154: 411.

7.  Rich M, Deighton L, Roberts L. Clindamycin-resistance in methicillin-resistant Staphylococcus aureus isolated from animals. Veterinary Microbiology 2005; 111(3-4): 237-40.

8.  Saijonmaa-Koulumies L., Parsons E., Lloyd, DH. Elimination of Staphylococcus intermedius in healthy dogs by topical treatment with fusidic acid. Journal of Small Animal Practice 1998; 39: 341-7.

9.  Wagenvoort JH, Sluijsmans W, Penders RJ. Better environmental survival of outbreak vs. sporadic MRSA isolates. Journal of Hospital Infection 2000; 45: 231-4.