The environment as an unrecognized reservoir for community-associated methicillin resistant Staphylococcus aureus USA300: a case-control study

Abstract

Background: Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections are spreading, but the source of infections in non-epidemic settings remains poorly defined.

Methods: We carried out a community-based, case-control study investigating socio-demographic risk factors and infectious reservoirs associated with MRSA infections. Case patients presented with CA-MRSA infections to a New York hospital. Age-matched controls without infections were randomly selected from the hospital's Dental Clinic patient population. During a home visit, case and control subjects completed a questionnaire, nasal swabs were collected from index respondents and household members and standardized environmental surfaces were swabbed. Genotyping was performed on S. aureus isolates.

Results: We enrolled 95 case and 95 control subjects. Cases more frequently reported diabetes mellitus and a higher number of skin infections among household members. Among case households, 53 (56%) were environmentally contaminated with S. aureus, compared to 36 (38%) control households (p = .02). MRSA was detected on fomites in 30 (32%) case households and 5 (5%; p<.001) control households. More case patients, 20 (21%) were nasally colonized with MRSA than were control indexes, 2 (2%; p<.001). In a subgroup analysis, the clinical isolate (predominantly USA300), was more commonly detected on environmental surfaces in case households with recurrent MRSA infections (16/36, 44%) than those without (14/58, 24%, p = .04).

Conclusions: The higher frequency of environmental contamination of case households with S. aureus in general and MRSA in particular implicates this as a potential reservoir for recolonization and increased risk of infection. Environmental colonization may contribute to the community spread of epidemic strains such as USA300.

Figure 1. Flow chart enrollment of cases and controls.

Figure 2. Comparison of frequency of contamination of household surfaces with S. aureus overall, MRSA, MSSA or USA300 by case (blue) or control (red) group status.

* denotes p < 0.05 and ** p < 0.01.

Figure 3. Distribution of S. aureus spa-CC in environmental (panel A) and nasal (panel B) colonization between case (blue) and control (red) households.

Genotyping of all isolated nasal and environmental S. aureus strains yielded 83 different spa-types that were clustered into spa-CC by BURP analysis. Spa-types with < 5 repeats were excluded from clustering and are summarized as singletons. Individual households may be represented more than once, which reflects that they harbored multiple strains.

Figure 4. Comparison of S. aureus infectious isolate and environmental contamination between cases with reinfection (blue) or without recurrent infections (red).

Shown are the frequencies of USA300 as the clinical infectious isolate, the colonized environment, the houses with clinical and environmental strain concordance (Clinical-enviro concordant), the clinical and environmental concordant strain being USA300 (Clinical-enviro USA300), or the clinical and environmental strain being a clone other than USA300 (Clinical-enviro other strain). * denotes p < 0.05.