MRSA and Livestock Production
Paul Ebner and Yingying Hong - Purdue Animal Sciences
The connection between Methicillin Resistant Staphylococcus aureus
(MRSA) and livestock production is among the most debated issues
related to CFOs. (Go here for a definition of CFO.) Unfortunately, it is
also one of the least understood. This is largely due to the fact that the
connection between MRSA and livestock is still a new field of research. In
recent years, however, there has been a substantial amount of progress
in understanding, and much more information is known regarding
MRSA, animal agriculture, and human health. Below are several of the key
updates on the state of science related to MRSA and livestock production.
(See link for further reading.)
Classifications. MRSA infections were first discovered in hospitals
and were generally thought of as nosocomial infections, or infections
that occurred in the hospital itself. As new infections were seen in
individuals with no connection to health care settings, it became clear
that MRSA was circulating outside of hospitals as well. Epidemiologists
began separating MRSA strains and infections into two groups:
hospitable acquired (HA); and community acquired (CA). While there
were some genetic differences between the two types of MRSA, there
were also some differences in disease patterns, with HA-MRSA being
more associated with pulmonary infections and CA-MRSA being more
associated with skin infections.
In the mid-2000s, researchers in Europe found MRSA circulating in swine
farms, both in pigs and humans who worked or were in regular contact
with pigs. The strain of MRSA in this case was different from other HAMRSA
or CA-MRSA. This type of MRSA was deemed livestock-associated
(LA) MRSA. Several labs in the US have identified this strain in US pigs,
among other animals.
It is worth noting that bacterial communities do not live in silos. There is
a considerable amount of movement of these strains from the hospital,
the community, and from animals to humans (and vice versa). So much
so that some scientists no longer use groups (e.g., HA, CA, LA) to describe
strains, but rather use more specific genetic characterizations.
ST398. The original LA-MRSA was referred to as
ST398. This name is based on a DNA fingerprinting
technique used to identify different types of
bacteria. ST398 distinguished itself from other MRSA
based on a few other characteristics; namely it was
also resistant to tetracycline, but lacked some (not
all) genetic components that were responsible
for pathogenesis, or the ability to cause disease.
With time, it became clear that ST398 was able to
move from pigs to humans, and it was also isolated
from other types of livestock. As DNA sequencing
techniques improved, different groups have
shown that ST398 was likely a human variant of
Staphylococcus aureus that morphed to infect pigs.
Most agree it acquired methicillin resistance in pig
populations prior to being re-introduced to human
While ST398 in livestock herds received a great
amount of attention, several research groups have
since shown that it is not alone, as numerous MRSA
strains are able to cross over from livestock to
humans and vice versa.
LA-MRSA and Humans. At the onset, one major
concern was whether the newly identified LA-MRSA
could transmit to human populations and, if so,
whether it could cause disease. Research over the
past 10 years has shown that different LA-MRSA
strains are certainly able to colonize humans, even
those with no livestock contact. These strains,
however, have also been found in cats, horses,
dogs, etc. Humans can “carry” a bacterium without
it causing harm or infection. This is true for the
majority of Staphylococcus aureus, which regularly colonize our skin or nasal membranes. Thus, one
major question remained unanswered: Do LA-MRSA
cause human disease on the level of HA-MRSA or
CA-MRSA? It is now clear that LA-MRSA, and ST398
in particular, can cause infections in pigs as well as
humans. While most of this research has taken place
in Europe, there is no reason to think that those
results could not be extrapolated to the US. There
is, however, still disagreement as to the virulence of
LA-MRSA. Some research groups concluded that it
is generally lower than HA- or CA-MRSA; others felt
that all MRSA should be treated as equals.
MRSA and CFOs. LA-MRSA is not necessarily
an issue specific to CFO-size farms. As stated
earlier, MRSA strains classified as LA-MRSA have
been isolated from a variety of animals, including
companion animals. At the same time, the European
farms where LA-MRSA was originally isolated
would likely not be considered CFOs in the US
based on their smaller size. While larger volumes of
animals could certainly increase the volume of any
pathogenic bacteria, the bacteria itself would not
discriminate between a farm with 599 pigs (not a
CFO) vs. a farm with 600 pigs (CFO).
Bosch T, Schouls LM. 2015. Livestock-associated MRSA: innocent or serious health threat? Future Microbiology. 10:445-447.
Mole B. 2013. Farming up trouble. Nature. 499:398-400.
Smith T. 2015. Livestock-Associated Staphylococcus aureus: The United States Experience. PLoS Pathog 11(2): e1004564.