MRSP: Assessing the threats to patients and practices

NATIONAL REPORT — When it comes to methicillin-resistant Staphylococcus pseudintermedius (MRSP), resistance is increasing and it could signal new therapeutic challenges for veterinarians.

The infection rates in human hospitals have been well-documented for methicillin-resistant Staphylococcus aureus (MRSA), but new data is suggesting MRSP may emerge as a similar threat to dogs, but without the same zoonotic potential to people.

A study published in the Journal of Antimicrobial Chemotherapy (Perreten V, et al.) in late March suggests that two major clonal MRSP lineages "have disseminated in Europe and North America. Regardless of their geographical or clonal origin, the isolates displayed resistance to the major classes of antibiotics used in veterinary medicine and thus infections caused by MRSP isolates represent a serious therapeutic challenge."

How serious? These isolates are showing resistance to many popular antibiotics including: trimethoprim (90.3 percent), gentamicin/kanamycin (88.3 percent), kanamycin (90.3 percent) streptomycin (90.3 percent), streptothrycin (90.3 percent), macrolides and/or lincosamides (89.3 percent), fluoroquinolones (87.4 percent), tetracycline (69.9 percent), chloramphenicol (57.3 percent) and rifampicin (1.9 percent).

The issue, according to Paul Morley, DVM, professor of biosecurity and epidemiology at Colorado State University's (CSU) College of Veterinary Medicine & Biomedical Sciences, is that methicillin-resistant staph infections have gone from being relatively uncommon to a health concern most practitioners will encounter.

"It does appear that the resistance to all the beta-lactam drugs in association with the resistance that causes MRSA is dramatically increasing in methicillin-resistant Staphylococcus pseudintermedius."

Overall, 25-30 percent of people are considered colonized with Staphylococcus aureus. In general, MRSA colonization is about 1 percent to 2 percent. For dogs, about one-third of Staphylococcus aureus isolates are methicillin resistant strains. And the colonization numbers for Staphylococcus pseudintermedius for dogs is about the same — one-third.

"But the problem is we are looking at rates of somewhere between 5 percent or 6 percent of animals with MRSP," Morley explains.

Diagnosis of MRSA or MRSP has implications for treatment of infected animals, but also for potential zoonotic transmission, experts say.

Spreading the bug

Transmission of MRSA and MRSP is integrally tied to human contact. Whether people's hands are contaminated by a dog that has colonized or whether the workers are colonized themselves, "human contact is incredibly important," Morley says.

MRSA, on the other hand, likes people, explains VCA/Antech expert David P. Aucoin, DVM, at the recent Western Veterinary Conference. MRSP loves dogs, and it's the most common isolate for them. In referral hospitals, one-third of every staph infection is methicillin resistant, Aucoin adds. While outbreaks have occurred throughout the country, Colorado is the hot spot right now, according VCA/Antech data.

"The concern we have is this problem is not abating, and it will increase. Everyone in this room will see it," he says. "We culture about 25,000 MRSP samples a year at Antech, and about 10 percent to 15 percent are methicillin resistant," Aucoin adds.

Veterinarians are at greater risk for colonizing and even spreading the organism because they are more frequently exposed to these populations. "There are some really nice studies that show that if veterinarians come in contact from colonized and infected animals, they have an increased risk. And they have an increased risk of colonization infection over the general public because of their likelihood of contacting animals that are colonized or infected."

In a study published in the Journal of the American Veterinary Medical Association in 2008 titled "Characteristics of biosecurity and infection control programs at veterinary teaching hospitals" (Benedict, Morley and Van Metre), about 82 percent of the hospitals reported outbreaks of nosocomial infections during the five years prior to the interviews. Nearly 45 percent of the teaching hospitals reported more than one outbreak, and 32 percent had completely closed sections of the hospital to stamp out disease spread.

"What we also found was that MRSA was the second most common cause of those outbreaks."

Looking for the source

If you have a clinical infection, look for the nidus of infection, Morley advises. Veterinarians should explore less traditional treatment options if antibiotic therapy fails. This could include chlorhexidine baths or other non-traditional, non-antimicrobial remedies. Suspect MRSP or MRSA in the face of clinical infection of a non-healing injury, particularly after antimicrobial therapy fails.

And if a patient is on antibiotics, clinicians need to understand the susceptibility of the bug you are fighting. "You need to achieve optimal concentrations at the target site. And removing any underlying factors for not clearing up the infection is a really, really important thing," Morley says. Antibiotic therapy should be based on culture and sensitivity.

Veterinarians need to be more than just passively aware of the potential for infection and treatment of infected patients when it occurs, Morley adds.

"Both of these organisms can be spread within a hospital through the care that is provided. There needs to be awareness about the zoonotic potential. There also needs to be some method for tracking the occurrence. Some hospitals may want to do some microbial surveillance."

Morley suggests that if an outbreak occurs, consult with experts in infectious disease control just to determine whether or not more serious intervention steps need to happen.