SAN DIEGO – In households of children with methicillin-resistant Staphylococcus aureus (MRSA) infection, pet dogs and cats often were colonized with S. aureus. In addition, the S. aureus strains colonizing the pets were likely to be concordant with those found on humans and/or their environmental surfaces within the household.
Those are key findings from a study that set out to determine the molecular epidemiology of S. aureus colonization of pets in the context of their human contacts and household environments, in households of children with community-associated MRSA infections.
“S. aureus is a significant pathogen in both health care and community settings and causes a spectrum of infections ranging from superficial skin and soft tissue infections (SSTIs) to invasive, life-threatening infections,” Ryley M. Thompson said at an annual scientific meeting on infectious diseases. “Due to the enormous clinical and economic burden posed by S. aureus, transmission prevention is essential.”
According to traditional dogma, “humans are the source of S. aureus for their pets and … pets are not a natural reservoir for S. aureus,” said Mr. Thompson, a clinical research study assistant in the Clinical and Translational Research Laboratory of Dr. Stephanie Fritz in the department of pediatrics at Washington University, St. Louis. “This is supported by the fact that pets often clear colonization without antimicrobial treatment. Risk factors for pet colonization include veterinary health care contact, contact with children, and using their mouths to interact with their environment. To date, directionality of S. aureus transmission between humans and pets is unclear.”
Between 2012 and 2015, the researchers enrolled 100 households of children with active or recent community-associated MRSA SSTIs who had been treated at St. Louis Children’s Hospital or other pediatric practices in the area. Over the course of 1 year, five study visits were conducted in each of the patient’s homes. Every 3 months, cultures were obtained from index patients and their household contacts, indoor dogs and cats, and 21 household environmental surfaces. The index patients and household contacts were swabbed at their axillae, nares, and inguinal folds; indoor dogs and cats were swabbed at their nares and dorsal fur; and household surfaces thought to be frequently touched by multiple household members were swabbed, such as TV remote controls, refrigerator door handles, and toilet seats. Researchers also administered a detailed survey to evaluate health, hygiene, and activities that may be associated with S. aureus infection transmission.
Molecular typing of all S. aureus strains was performed by repetitive-sequence polymerase chain reaction to determine strain relatedness, and staphylococcal cassette chromosome mec (SCCmec) characterization was performed by multiplex PCR.
Of 100 households, 49 had a total of 89 pets: 63 dogs and 26 cats. Of the 63 dogs, 13 (21%) were colonized with S. aureus (9 with MRSA) and 2 of 26 cats (8%) were colonized with MRSA. Eleven isolates were SCCmec type IV (MRSA), one was type II (MRSA), and two were type III (MRSA). At baseline, the researchers recovered 16 S. aureus isolates from 15 pets: 13 from the nares and 3 from pet dorsal fur.
One dog was colonized at both sites with concordant strains. In the three households that had two colonized pets, one household had two colonized dogs with matching strain types, the second had two dogs with nonmatching strain types, and the third had a dog and a cat with nonmatching strain types, Mr. Thompson reported at the combined annual meetings of the Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the HIV Medicine Association, and the Pediatric Infectious Diseases Society.
Pet characteristics significantly associated with S. aureus colonization at study enrollment were older age (P = .04) and advanced number of years living in the home (P = .03), but sleeping in the same bed as a household member was not (P =. 96).
Molecular analysis revealed that the primary caretaker for 10 of the 15 colonized pets (67%) also was colonized with S. aureus, and 70% of these strains were concordant with the pet strain. In addition, seven of eight humans (88%) who shared a bed with a colonized pet also were colonized with S. aureus, and 43% of these strains were concordant with the pet strain.
Mr. Thompson also presented the longitudinal molecular epidemiology results in pets. In this analysis, 37 of the 89 pets were colonized with S. aureus at some point over the period of 12 months. Of these, 24 were colonized just once, while 13 were colonized at more than one of the samplings over time. Among these 13, two (15%) had concordant strains at all samplings, five (39%) had concordant and discordant strains, and six (46%) had discordant strains over the longitudinal study period.