MRSA in animals
Methicillin-resistant
Staphylococcus aureus
A critical point of note is that the specific strain of
the MRSA organism referred to as ST398 found in pigs and some other
livestock (poultry and horses) is different from that found in humans in
hospital and community infections.
Introduction
Methicillin-resistant Staphylococcus aureus, or MRSA, has
been known for a long time as a major human health problem due to its
resistance to most traditional antibiotic classes. Originally, human MRSA
infections were only found in hospitals (hospital-acquired or HA-MRSA).
Nowadays, MRSA infections are increasingly found in the general community
(community-acquired or CA-MRSA), although they still represent <1% MRSA in
the UK (Elston et al, 2009).
These same strains can be found in companion animals,
usually dogs and occasionally cats, and horses, probably as a result of
transfer from humans coming into contact with them. Thus, pets can act as a
reservoir for these bugs.
Recently, a new clone known as NT MRSA , (or ST398) has
been found on food animals, mostly in pigs and poultry. So far, no
connection between this new emerging strain and the more familiar human MRSA
has been established. ST398, whilst resistant to the penicillin family of
antibiotics, is sensitive to many other antibiotics so allowing successful
treatment of an infection. It is also less virulent than CA-MRSA.
It is known that ST398 MRSA can be transmitted between
animals and humans by direct contact without causing disease but so far the
experts and regulatory authorities agree that eating meat from those animals
that were carriers does not pose a threat to human health (EFSA Report,
March 2009). Similarly, only a few sporadic cases of humans with ST398 MRSA
have been reported.
There are a number of publicly funded projects on-going
in Europe and the USA helping to provide a better understanding of the
epidemiology of ST398. There is still much to learn.
What is MRSA?
Staphylococcus aureus is a common bacterium that lives
primarily on the skin of people and is commonly associated with infections
there such as spots and boils (skin and soft tissue infections). It can also
be carried in the human nasal cavity without producing signs of disease. In
fact, most of the time, it cause no problems in healthy individuals.
In hospital situations, the great concern is that it may
cause infections in surgical wounds. Transplant and joint replacement units
are particularly risky areas. Similarly, people whose immune system is
weakened, for instance after major surgery (particularly transplant surgery)
or chemotherapy, those with immunosuppressive disease such as HIV/AIDS or
the elderly and very young can also be particularly susceptible.
In the past, S aureus infections were relatively easily
treated with penicillins. But with time, the bacteria became resistant,
initially to first generation penicillins, then later to stronger versions
such as methicillins (hence the M in MRSA) and most recently to the first,
third and fourth generation cephalosporins, the newest of the penicillins
The impact of this is that the range of antibiotics
available in hospitals to treat infections may be much reduced and
practitioners are forced to use ‘last resort’ antibiotics like vancomycin,
which require IV administration.
What about animal ST398
MRSA?
ST398 MRSA has been found in pigs and some other
livestock species (poultry), and is especially prevalent in Germany and
Netherlands. In dogs, cats and horses it is more common to find CA MRSA
(although horses have been found with ST398). In pets, this is probably due
to transfer from man to animals and transfer back to man.
ST398 MRSA is clearly different from the human hospital
and community epidemic strains. Nevertheless, it can cause disease in humans
if wounds become infected or it gets into the blood stream. This is why, in
the countries where ST398 is more common, authorities responsible for
hospital hygiene are extremely cautious about admitting individuals who are
regularly in close contact with livestock.
The organism is encountered quite frequently amongst
specific sectors of the population in The Netherlands and Germany. In a
study conducted amongst Dutch farmers, 50% of those tested using nasal swabs
carried animal MRSA (Wulf et al, 2008) versus only 0.03% in the general
public. Similarly in a German study, 30% of pig vets, 14% of meat inspectors
and 38% of diagnostic workers who also visited pig farms, were positive for
animal MRSA (Blaha et al, 2008).
Why is the incidence in
Germany and Holland so high?
First of all, it is important to note that these studies
relate to a specific sample of the population, those who work in pig
production, and not to the public at large. Possibly part of the answer lies
in the intensity of their livestock systems, the proximity of the two
countries and the regular movement of livestock and people between these two
countries. This would be supported by the fact that Sweden, with a low
livestock density and its relative geographical isolation, reported the
absence of ST398 MRSA (Eliasson et al, 2008). Similarly, the UK has
negligible incidence and Ireland recently indicated a very low 2% infection
level. The organism has very little clinical impact in livestock and it is
not considered to be a primary pathogen - it is not a major cause of disease
(van der Wolf et al, 2008).
Can infection be caught
from eating meat from animals carrying MRSA?
The available evidence suggests that eating or handling
contaminated food does not give rise to a greater risk of humans becoming
either infected with or becoming 'healthy carriers' of MRSA. (EFSA Press
release March 2009).
Where has the ST 398
MRSA come from and how does it spread to man?
Although originally it was thought that the ST398 emerged
as a resistance phenotype (strain) independent of human MRSA, the current
thinking is that a swine-adapted MSSA (Methicillin susceptible S. aureus)
acquired a gene (mecA) from a human CA-MRSA by direct gene transfer to the
swine MSSA. Thus, the swine MSSA became a swine MRSA. However, further
studies are needed to confirm this hypothesis.
The most likely way that ST398 MRSA transfers from
animals to man is via the environment in the pig house. The organism can
survive for some time in dust and pig houses inevitably are sources of dust.
In 2010 in the UK, we can expect a report on a major survey containing the
results of tests from over 300 pig herds. This should give a valuable
insight into possible incidence and reasons for its occurrence, as well as
helping design new prevention programmes and possible additional biosecurity
measures.
References
Elston J W T, Barlow G D Community–associated MRSA in the
UK, J Infect (2009) doi: 10.1016/j.jinf.2009.07.001 (in press)
European Food
safety Authority, Press release following publication of a report from its
Biological Biohazard Panel on the risk of MRSA in food and animals, March
2009
Wulf, M et al (2008) Proceedings of the 20th IPVS Congress, Vol 1, 162
Blaha, T et al (2008) Proceedings of the 20th IPVS Congress, Vol 2, 310
Eliasson-Selling et al (2008). Proceedings of the 20th IPVS Congress, Vol 2,
308
Van der Wolf, P. et al (2008) Proceedings of the 20th IPVS Congress. Vol
1, 166
October 2009
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