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Volume 115, Issue 10
Advice from a veterinary dermatologist for treating drug-resistant skin infections in your patients while protecting their owners and your staff from possible exposure.
Reports about methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant Staphylococcus pseudintermedius (MRSP) have been raising eyebrows and anxiety levels for years. Drug-resistant Staphylococcus infections aren’t just affecting dogs and cats. Horses, cattle, pigs, and even exotic species also get them. According to Alison Diesel, DVM, DACVD, clinical associate professor of dermatology at Texas A&M University College of Veterinary Medicine and Biological Sciences, these infections aren’t going away anytime soon. In fact, research shows they are on the rise.
Initial reports dealt with drug-resistant S aureus infections in humans. Yet Diesel warned, “We’ve been seeing an increasing number of these drug-resistant infections in our companion animals as well. These include methicillin-resistant S aureus, S pseudintermedius, Staphylococcus schleiferi, and other species.” During a session at the AVMA Virtual Convention in August, Diesel highlighted the need for clinicians to understand how to diagnose, manage, and help prevent these infections.
According to Diesel, there are several possible reasons for the rising number of cases, including more frequent culture testing and increased (or inappropriate) exposure to antibiotics. Diesel remarked that the strains of methicillin-resistant microbes being identified in humans are being detected in veterinary patients as well, so the increasing trend of MRSA in humans may be a factor in the rising numbers of veterinary cases of MRSP and MRSA.
Identifying drug-resistant pyoderma begins with the workup. However, Diesel cautioned that physical exam findings cannot tell a clinician whether an infection is drug-resistant. It is incorrect to assume that a more “virulent-looking” dermatitis is more likely to be caused by a drug-resistant pathogen, she said.
Staphylococcal bacteria are part of the normal skin and mucosal microflora in dogs, cats, and large animals. The most important skin pathogen in dogs is S pseudintermedius. However, other Staphylococcus species, such as coagulase-negative S schleiferi and S aureus (which is more traditionally a pathogen in humans), can also play a role in canine skin infections. Besides staphylococcal species, other microbes can have a role in canine and feline skin infections.
The diagnostic steps that focus on identifying the species of bacteria involved in the infection should not be dismissed. Papules, pustules, crusts, and epidermal collarettes are commonly featured in canine pyoderma. However, pustules are less common in cats. Papules and crusts may be identified in cats, but increased dander, exfoliative dermatitis, and scaling are more likely. Eosinophilic plaques and indolent ulcers can also be featured in feline pyoderma. Diesel reminded attendees that most veterinarians use glucocorticoids to treat eosinophilic plaques, granulomas, and indolent ulcers in cats, but these lesions may respond to antibiotics, too. “Consider treating with antimicrobials first, prior to reaching for those steroid medications.” She advised.
In addition to selecting an appropriate antibiotic for pyoderma, the dose and duration of treatment can both have important implications for treatment success. For calculating an antibiotic dosage to treat skin infections, Diesel advised, “Go big or go home. If you aim for the higher end of the dose range, you reduce the risk of developing resistance down the line.” This is important because some dogs that develop drug-resistant infections have previously received antibiotics near the low end of the dose range. Underdosing can increase the risk for developing a drug-resistant infection later.
Diesel also recommended treating superficial bacterial skin infections for 3 to 4 weeks initially. Deep bacterial pyoderma should be treated for a minimum of 6 to 8 weeks. “Ten to 14 days of antibiotics (for superficial pyoderma) is generally insufficient,” she said. “You kill off the majority of the organisms, but some remain, so the infection comes right back.”
Bacterial culture and susceptibility testing can be costly, so it’s understandable that some pet owners and veterinarians delay this step. However, postponing it for too long can lead to more serious problems in the future. Diesel recommended culture testing when empirical therapy doesn’t fix the problem. This includes patients presenting with lesions that don’t resolve, new lesions that appear during therapy, or lesions that recur after antibiotics are discontinued. Deep pyodermas should be cultured earlier in the process. The presence of nodules, draining tracts on the skin, and/or identifying rod-shaped bacteria on cytology are all good reasons for culture testing, Diesel said. “However, rods on a skin cytology tell me that I’m not dealing with a Staph infection,” she added. “Identifying what that organism is will help guide my treatment recommendations for that patient.”
Ear infections warrant different considerations when it comes to culturing. Diesel cultures ears if a middle ear infection is suspected, in which case she obtains a culture sample from the tympanic bulla. In general, some veterinarians don’t culture pyoderma until 2, 3, or maybe even more different antibiotics have been tried, yet the infection is still unresolved. She noted that this sequential use of stronger and stronger antibiotics can lead to drug resistance. Instead, she urged attendees to culture earlier in the process. “If you put a dog on cephalexin, see the pet back in 3 weeks. If the infection is still unresolved, that’s the time to culture.” She also recommended culturing an intact pustule or non-ruptured crust to obtain a good, representative sample. For deep infections, find an intact nodule and submit a small tissue/biopsy sample for culture testing.
Interpreting the laboratory’s culture report isn’t always straightforward, either. If the culture report identifies “coagulase-positive” Staphylococcus, that still doesn’t confirm whether the organism is S aureus or S pseudintermedius. In that case, Diesel suggested, before you decide what to do, call the lab to ask if they can identify the species further. Clinicians may also have to ask the lab to test separately for susceptibility to specific antimicrobials, as some antibiotics aren’t included in standard susceptibility panels.
You’ve cultured a pustule and received a laboratory report that confirms a methicillin-resistant infection, so what’s the next step? A methicillin-resistant microbe will be resistant to beta-lactam antibiotics, notably penicillins, cephalosporins, and carbapenems. Not only are veterinarians seeing methicillin-resistant infections, they are also seeing multidrug-resistant infections that are resistant to beta-lactam antibiotics and other drug classes like fluoroquinolones. Most of the “reliable” oral antibiotics veterinarians normally choose empirically for pyoderma won’t work for these infections.
Fortunately, topical therapies fill that void. Diesel noted that her practice relies much more on topical therapy for these resistant infections. “For superficial Staph bacterial folliculitis, you can treat them 100% with topical therapy,” she advised. Diesel recommended bathing the pet every 1 to 2 days, or at least 3 times per week. Leave-on sprays, conditioners, or mousses may extend the time between baths, due to their residual effects. (Some of these leave-on products can also be used in between baths.) Chlorhexidine, benzoyl peroxide, Tris-EDTA, and sodium hypochlorite are topical ingredients that may be effective. Mupirocin, silver sulfadiazine, and even some natural-ingredient preparations can also be options. Owner compliance may be an issue (especially with cats), so if the owner is unwilling or unable to bathe the pet, another option, such as wipes or sprays, should be considered.
According to Diesel, numerous studies have shown that MRSA is transmissible between humans and animals. She also cautioned attendees that “as veterinary professionals, our chance of being carriers for MRSA is about 4 times higher than in the general population. It’s even higher for large animal veterinarians.”
Currently, S pseudintermedius is not considered a pathogen in humans. Reports of human infections are rare and usually occur following bite wounds or in people who are immunocompromised. However, over the past couple of years there are increasingly reports of identification of this organism in people and consideration of S pseudintermedius as an “emerging human pathogen”. This may be related to improvements in molecular diagnostics in both human and veterinary laboratories. Coagulase-negative S schleiferi is being investigated, but its zoonotic potential is still in question. This organism has been isolated from pustules in cases of canine dermatitis. Diesel cautioned that over 90% of coagulase-negative Staphylococcus species are methicillin-resistant or multidrug-resistant, so this is something to keep in mind when counseling clients and determining treatment for patients.
Important preventive measures for veterinary professionals include using antimicrobials judiciously in patients and performing bacterial skin culture testing earlier instead of waiting, Diesel said. Additional recommendations for veterinary teams include the following:
It is also important to educate clients so they understand the zoonotic potential. For pets receiving antibiotic therapy, Diesel advised sharing the following recommendations with clients:
The role drug-resistant microbes play in veterinary medicine is an ongoing area of investigation. Diesel referred attendees to the consensus guidelines from the World Association for Veterinary Dermatology1 for more information about diagnostic testing, patient management, and preventive measures.
Dr. Todd-Jenkins received her VMD degree from the University of Pennsylvania School of Veterinary Medicine. She is a medical writer and has remained in clinical practice for over 20 years. She is a member of the American Medical Writers Association and One Health Initiative.