Swine Farming Is a Risk Factor for Infection With and High Prevalence of Carriage of Multidrug-Resistant Staphylococcus aureus

Abstract

Background: Livestock-associated Staphylococcus aureus (LA-SA) has been documented worldwide. However, much remains unknown about LA-SA colonization and infection, especially in rural environments.

Methods: We conducted a large-scale prospective study of 1342 Iowans, including individuals with livestock contact and a community-based comparison group. Nasal and throat swabs were collected to determine colonization at enrollment, and skin infection swabs over 17 months were assessed for S. aureus. Outcomes included carriage of S. aureus, methicillin-resistant S. aureus (MRSA), tetracycline-resistant S. aureus (TRSA), multidrug-resistant S. aureus (MDRSA), and LA-SA.

Results: Of 1342 participants, 351 (26.2%; 95% confidence interval [CI], 23.8%-28.6%) carried S. aureus. MRSA was isolated from 34 (2.5%; 95% CI, 1.8%-3.5%) and LA-SA from 131 (9.8%; 95% CI, 8.3%-11.5%) of the 1342 participants. Individuals with current swine exposure were significantly more likely to carry S. aureus (prevalence ratio [PR], 1.8; 95% CI, 1.4-2.2), TRSA (PR, 8.4; 95% CI, 5.6-12.6), MDRSA (PR, 6.1; 95% CI, 3.8-10.0), and LA-SA (PR, 5.8; 95% CI, 3.9-8.4) than those lacking exposure. Skin infections (n = 103) were reported from 67 individuals, yielding an incidence rate of 6.6 (95% CI, 4.9-8.9) per 1000 person-months.

Conclusions: Current swine workers are 6 times more likely to carry MDRSA than those without current swine exposure. We observed active infections caused by LA-SA. This finding suggests that individuals with livestock contact may have a high prevalence of exposure to, and potentially infection with, antibiotic-resistant S. aureus strains, including LA-SA strains.

Keywords: Staphylococcus aureus; colonization; livestock-associated Staphylococcus aureus; multidrug resistance; swine.

Figure 1.

Distribution of Staphylococcus aureus (SA) compared to concentrated animal feeding operations (CAFOs) in Iowa. A, Locations of participants with positive S. aureus colonization (red). SA strains testing positive for drug resistance by type: methicillin (B), tetracycline (C), and multidrug (D). E, Locations of livestock-associated S. aureus (LA-SA) strains. All panels include distribution of swine CAFOs within Iowa.

Figure 2.

Prevalence ratios for colonization by any Staphylococcus aureus or by methicillin-resistant S. aureus (MRSA), tetracycline-resistant S. aureus (TRSA), multidrug-resistant S. aureus (MDRSA), or livestock-associated S. aureus (LA-SA) specifically, based on occupational exposure to swine (A), cattle (B), or chickens (C). The prevalence of colonization among participants without livestock exposure was selected as the reference for each group. Prevalence ratios are adjusted for age and clustering within households. Open and closed symbols represent past and current occupational livestock exposure, respectively. Error bars indicate 95% confidence intervals. Dashed lines indicate a prevalence ratio of 1.