|
Methicillin-resistant Staphylococcus aureus
in horses and horse personnel, 2000-2002
Why MRSA has emerged in the equine population is unclear. It may reflect increased exposure of horses to MRSA-infected persons, a unique ability of CMRSA-5 to colonize horses, the increasing use of certain antimicrobial drugs in veterinary medicine, or a combination of these factors. Several recent investigations in humans suggest that the fluoroquinolones themselves may actually predispose patients to infection with or carriage of MRSA (27,28). Two case-control studies examining risk factors for MRSA found a significant association between fluoroquinolone exposure and MRSA isolation or infection (4,29).
Continued from page 2.  Whether fluoroquinolone use in horses has facilitated emergence of MRSA is not known, since no current data exist on fluoroquinolone use in veterinary medicine. Enrofloxacin is widely used in some sectors of the Ontario horse population, particularly racing horses; however, it is rarely prescribed at OVC-VTH and uncommonly used on breeding farms (J.S. Weese, unpub. data). While MRSA has been considered primarily a hospital-associated pathogen in humans (10), the increasing incidence of community-acquired infection is concerning (8,9,30,31). Similar to the equine and equine-associated cases reported here, community-acquired infection with SCCmecIV strains has been reported in humans (32-34). Most reports of community-acquired-MRSA in humans involve skin and soft tissue infections, and production of PVL has been implicated as the possible virulence factor in community-associated MRSA infection in humans (31,35). None of the isolates from clinical equine or equine-associated infections in this study contained PVL genes; however, definitive conclusions regarding the role of PVL in equine-associated infection cannot be made with the reasonably small number of clinical isolates evaluated here. Isolates in this study were also multidrug resistant, in contrast to results of many reports of community-associated MRSA in humans (36,37). Therefore, determining the origin of these isolates (community versus hospital) is not straightforward and requires further study. The lack of proven, safe, and acceptable options of eradication of nasal colonization in horses creates potential management problems. To date, isolation of infected horses and use of barrier precautions have been employed (J.S. Weese, unpub data). However, such methods may be difficult on farms, particularly if colonization is prolonged. Our study has shown that MRSA infection may be an emerging disease in horses, which agrees with earlier reports (13,14). MRSA infection also may become an important nosocomial problem in the veterinary hospital setting and become endemic on horse farms, particularly in foals. Because of the extensive movement of horses, especially thoroughbreds and standardbreds, between and within Canada and the United States, MRSA colonization and infection may be more widespread than recognized. Emergence of MRSA as an equine pathogen is of additional concern because horses may be a community reservoir of MRSA and source of infection or reinfection for persons. In view of the size of the North American horse population and the frequent close contact between many persons and horses, this concern must not be dismissed. Further study is required to clarify the role of this pathogen in equine disease and transmission between horses and humans. Table. In vitro antimicrobial susceptibilities of 101
methicillin-resistant Staphylococcus aureus isolates from humans and
horses *
MIC in [micro]g/mL
Antimicrobial agent 50% 90% Range
Tetracycline >16 >16 [less than or
equal to] 4-
[greater than
or equal to] 16
Doxycycline 8 8 [less than or
equal to] 4-8
Minocycline [less than
or equal [less than
to] 4 or equal
to] 4 [less than or
equal to] 4-8
Erythromycin >8 >8 [less than or
equal to] 0.5->
8
Gentamicin >16
([dagger]) >16
([dagger]) [less than or
equal to] 2->
16([dagger])
Rifampicin >4 >4 [less than or
equal to] 1->4
TMP/SMX 8/152 >8/152 [less than or
equal to] 2/38-
8/152
% of isolates
Antimicrobial agent Intermediate Resistant
Tetracycline 0 96
Doxycycline 78.2 0
Minocycline 14.8 0
Erythromycin 0 86.1
Gentamicin 9.9 88.1
Rifampicin 10.9 71.2
TMP/SMX NA 79.2
* MIC, minimum inhibitory concentration; TMP/SMX,
trimethoprim-sulfamethoxazole; NA, no intermediate MIC category in
NCCLS guidelines.
([dagger]) All strains were susceptible to gentamicin at a
concentration of 500 [micro]g/mL.
References (1.) Barber M. Methicillin-resistant staphylococci. J Clin Pathol. 1961;14:385-93. (2.) Ito T, Katayama Y, Asada K, Mori N, Tsutsumimoto K, Tiansasitorn C, et al. Structural composition of three types of staphylococcal chromosomal cassette chromosome mec integrated in the chromosome of methicillin-resistant Staphylococcus aureus. Antimicrob Agents Chemother. 2001;45:1323-36. (3.) Daum RS, Ito T, Hiramatsu K, Hussain F, Mongkolrattanothai K, Jamklang M, et al. A novel methicillin-resistance cassette in community-acquired methicillin-resistant Staphylococcus aureus isolates of diverse genetic backgrounds. J Infect Dis. 2002; 186:1344-7. (4.) Graffunder EM, Venezia RA. Risk factors associated with nosocomial methicillin-resistant Staphylococcus aureus (MRSA) infection including previous use of antimicrobials. J Antimicrob Chemother. 2002;49:999-1005. (5.) Hartstein AI, Mulligan ME. Methicillin-resistant Staphylococcus aureus In: Mayhall CG, editor. Hospital epidemiology and infection control. Baltimore: Williams and Wilkins, 1996. p. 290-306. (6.) Casewell MW. New threats to the control of methicillin resistant S taphylococcus aureus. J Hosp Infect. 1995;30:465-71. (7.) Baggett HC, Hennessy TW, Leman R, Hamlin C, Bruden D, Reasonover A, et al. An outbreak of community-onset methicillin-resistant Staphylococcus aureus skin infections in southwestern Alaska. Infect Control Hosp Epidemiol. 2003;24:397-402. (8.) Centers for Disease Control and Prevention. Four pediatric deaths from community-acquired methicillin-resistant Staphylococcus aureus--Minnesota and North Dakota. MMWR Morb Mortal Wkly Rep. 1999;48:707-10. (9.) Centers for Disease Control and Prevention. Public Health Dispatch: outbreaks of community-associated methicillin-resistant Staphylococcus aureus skin infections--Los Angeles County, California, 2002-2003. MMWR Morb Mortal Wkly Rep. 2003;52:88. (10.) Simor AE, Ofner-Agostini M, Bryce E, Green K, McGeer A, Mulvey MR, et al. The evolution of methicillin-resistant Staphylococcus aureus in Canadian hospitals. CMAJ. 2001; 165:21-26. (11.) Simor AE, Ofner-Agostini M, Bryce E, McGeer A, Paton S, Mulvey MR. Laboratory characterization of methicillin-resistant Staphylococcus aureus in Canadian hospitals: results of 5 years of national surveillance, 1995-1999. J Infect Dis. 2002;186:652-60. (12.) Shopsin B, Gomez M, Montgomery SO, Smith DH, Waddington M, Dodge DE, et al. Evaluation of protein A gene polymorphic region DNA sequencing for typing of Staphylococcus aureus strains. J Clin Microbiol. 1999;37:3556-63. (13.) Seguin JC, Walker RD, Caron JP, Kloos WE, George CG, Hollis RJ, et al. Methicillin-resistant Staphylococcus aureus outbreak in a veterinary teaching hospital: potential human-to-animal transmission. J Clin Microbiol. 1999;37:1459-63. (14.) Hartmann FA, Trostle SS, Klohnen AAO. Isolation of methicillin-resistant Staphylococcus aureus from a post-operative wound infection in a horse. J Am Vet Med Assoc. 1997;211:590-2. (15.) Cefai C, Ashurst S, Owens C. Human carriage of methicillin-resistant Staphylococcus aureus linked with pet dog. Lancet. 1994;344:539-40. |