Our monthly collaboration with the National Hog Farmer continues; this month Dr Peter Davies from the University of Minnesota, College of Veterinary Medicine, explains what is livestock-associated MRSA and if it affects people living near pig farms.
“Livestock-associated” MRSA first isolated in 2004
Most people are likely familiar with MRSA (methicillin resistant Staphylococcus aureus), a flagship “superbug” that is a major concern to human medicine. And just about everybody in the pig industry has heard that certain variants of MRSA are very common in some livestock populations (including pigs), and these are referred to as “livestock-associated” MRSA (LA-MRSA).
A novel variant of MRSA (labelled ST398 using a DNA typing method) was first found in pigs in the Netherlands in 2004. Subsequently, ST398 MRSA and several other types (e.g., ST9 in Asia, ST5 in North America) were reported in pigs in numerous countries, and often in their caretakers as well. The discovery that pigs may be a large MRSA reservoir created some justifiable panic and confusion, raising questions about the implications for human health, particularly for industry workers (e.g., farmers, veterinarians, processing plant workers), pork consumers and, last but not least, people living in the neighborhood of pig farms.
Generally, LA-MRSA lack most of the key “virulence factors” that enable the bacteria to cause clinical infections in people.
Human clinical infections by “livestock-associated” MRSA are rare
Although workers on MRSA-positive farms often harbor LA-MRSA in their nose, significant clinical infections in healthy workers have been rare. Human clinical infections with LA-MRSA do occur, but most cases tend to be of relatively mild disease (such as skin infections), with more severe infections typically limited to elderly and medically compromised patients.
Remembering that about 2% of healthy U.S. citizens carry human adapted variants of MRSA, the relative clinical importance of LA-MRSA appears to be minimal in most countries. Globally since 2004, there have been around 10 fatal cases of LA-MRSA infections reported, compared with about 50 fatal MRSA cases per day (18,650 per year in 2005) in the United States alone.
A 2016 study of Iowa hospitals found probable livestock variants in only 0.24% of MRSA cases, and 1% of S. aureus infections. In North Carolina, another leading swine-producing state, there were no LA-MRSA variants among more than 1,200 MRSA isolated from human bloodstream infections between 1995 and 2015 (Dr. Vance Fowler, Duke University, personal communication).
Living next to pig farms does not increase the risk of exposure
Although MRSA can be isolated from meat products, there is little evidence to suggest cause for concern about food-borne transmission. In contrast, conclusions of studies looking at the neighborhood risk of exposure to LA-MRSA from pig farms are conflicting. We will focus on the findings of studies that have compared pig workers and neighbors directly, measured the distance from pig barns to residences directly; and used laboratory testing to confirm the presence of LA-MRSA in the study populations.
Across three early studies in Europe, LA-MRSA prevalence (nasal carriage) was greater than 180 times higher in 352 pig industry workers (44%) than in 2,094 rural residents without farm exposure (0.24%).
- A similar study in Holland in 2017 showed similar prevalence of nasal colonization (0.56%) in people without livestock contact, but also found the positive people on average lived closer to farms (of any type). Importantly, the authors noted that routes of transmission underlying this were not known.
- A very detailed study of cases of MRSA infection in Denmark showed that overall MRSA risk did not differ between pig-dense regions versus other regions. However, the likelihood that a MRSA infection would be a LA-MRSA type was higher in the pig-dense regions, confirming some “spillover” from the industry to the community.
- Notably, a follow-up study published in 2018 found that, within pig-dense areas, the patients with LA-MRSA infections did not live closer to pig farms than population controls. The authors conclude that direct environmental spread from neighboring pig farms was unlikely and suggested that community spread through contact with people working with livestock, might be the predominant mechanism.
In summary, the overall impact of LA-MRSA relative to human variants remains very small in most countries including the United States. There is no evidence that residence in rural areas increases overall MRSA risk.