Treatment of MRSA and MRSP: more than meets the eye
Methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant S. pseudintermedius (MRSP) are important causes of infection in pets, and are resistant to many different antibiotics. It's therefore very important that these bacteria are cultured and tested in a lab to determine what antibiotics may be effective. Even then, choosing an effective medication can be difficult due to the limited number of potentially effective drugs.
Treatment selection is further complicated by problems that can occur when methicillin-resistant staphylococci are tested against certain drugs in the lab - problems that are not readily apparent unless extra testing is done. One "problem" drug is the antibiotic clindamycin. Clindamycin can be an effective treatment for MRSA or MRSP infections, but some MRSA strains may be resistant to it despite appearing to be susceptible based on normal laboratory testing. This is because the bacteria can have "inducible resistance". Inducible resistance can be detected by a special test called a D-test (see picture), but this test is not widely performed in veterinary laboratories.
In recent study presented by Dr. Meredith Faires (University of Guelph) at a conference last week, 55% of MRSA from dogs and cats that were reported to be resistant to erythromycin and susceptible to clindamycin were actually inducibly resistant to clindamycin. But no MRSA that were susceptible to erythromycin were inducibly resistant to clindamycin. In contrast, inducible resistance to clindamycin was not identified in any MRSP. Therefore, in the absence of specific testing for inducible resistance (i.e. a D-test), it is wise to consider all erythromycin resistant MRSA to be clindamycin resistant as well, regardless of the results of traditional antibiotic susceptibilty testing.
How did dogs get MRSA?
A common question from owners of pets infected with methicillin-resistant Staphylococcus aureus (MRSA) is "Where did it come from?" The answer is not completely clear, but evidence strongly suggests it probably came from humans. The MRSA strains found in pets are almost always teh same as those found in people (including people that don't have pets) in the same geographic area. This strongly suggests that the MRSA came from the same source. Considering the fact that S. aureus in general is much more common in humans than in pets, and how long MRSA has been around in people, and that MRSA can sometimes be found in both people and pets in the same household, it is very likely that MRSA can move between species and originally went from humans to animal.Even though MRSA in pets probably originated from people, now that it is in the pet population, pets can spread it to other animals and back to people. It is unclear how frequently this occurs - most people are still more likely to encounter MRSA from another person than from an animal.
The emergence and spread of MRSA in humans was largely driven by extensive antibiotic use in people. Once MRSA crossed over to pets, antibiotic use in these animals almost certainly played a role in helping MRSA spread in the pet population. Antibiotic treatment has been identified as a risk factor for development of MRSA infection in dogs, and for MRSA carriage in horses.
Prudent antibiotic use is likely the most important factor in reducing the risk of MRSA in individual pets. Using antibiotics prudently means doing things like:
- Only using antibiotics when they're really necessary
- Using the most basic antibiotic possible (instead of the fanciest or most powerful antibiotic)
- Finishing every prescription completely and as directed)
EU Antibiotic Awareness Day
Antibiotics are incredibly important drugs and save countless lives (human and animal) on a daily basis. Resistance to antibiotics is a major threat to human and veterinary medicine. While antibiotic resistance is a complex issue, overuse and improper use (in animals and people) undoubtedly contribute to the emergence and spread of resistant bacteria like MRSA. Awareness of these concerns is important for healthcare professionals (both human physicians and vets) and the general public. Here are some general points to remember with respect to pets and antibiotics:- Do not encourage your vet to prescribe antibiotics when they are not necessary. Vets (and physicians) often feel pressure to prescribe 'something', even though they have no clear evidence of a bacterial infection. Antibiotics do not work for viral infections.
- Never give antibiotics to your pet without the direction of your vet.
- Always give the full antibiotic course, as prescribed. Do not stop early. Your pet may look better but the infection could still be there.
- Never save antibiotics for 'future use'. If your pet develops another infection, you need to have your pet evaluated by a vet to determine if antibiotics are needed, and what antibiotic would be best.
