Methicillin-resistant Staphylococcus aureus in horses and horse personnel, 2000-2002

Continued from page 2.
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).

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->
Gentamicin                >16
                       ([dagger])       >16
                                     ([dagger])     [less than or
                                                    equal to] 2->
Rifampicin                 >4            >4         [less than or
                                                   equal to] 1->4
TMP/SMX                  8/152         >8/152       [less than or
                                                   equal to] 2/38-

                             % 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.


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