More on MRSA and meat
While it shouldn't come as a surprise considering other studies, a recent study in PLoS One (O'Brien et al 2012) has caused a bit of a stir in the US. This study, headed up by Dr. Tara Smith's research group in Iowa, looked for methicillin-resistant Staphylococcus aureus (MRSA) in retail pork. They bought pork from different stores in Iowa, Minnesota and New Jersey, and tested it for the presence of MRSA. They focused on pork because MRSA can be found widely in pigs internationally, including in the US.
Not surprisingly, they found MRSA. Overall, they tested 395 pork samples from 36 stores, including both "conventional" pork (300 samples) and "alternative" pork (95 samples). The latter consisted of samples labelled "raised without antibiotics" or "raised without antibiotic growth promotants." MRSA was found in 6.6% of samples; 6.3% of conventional pork samples and 7.4% of alternative pork samples.
When they looked at the MRSA types that were present, 27% were the ST398 "livestock-associated" MRSA that's most commonly found in pigs. However, like our earlier Canadian studies, they found common "human-associated" MRSA strains more often. These strains can also be found in pigs, albeit less commonly than ST398, and it's unclear whether meat contamination with these strains comes from pigs or from people who handle the meat throughout the processing chain.
The fact that there was no difference between conventional and antibiotic-free pork isn't surprising to me, although it catches some people off-guard because of some basic over-assumptions about the relationship between antibiotics and MRSA in food animals. We can find MRSA quite commonly on both regular and antibiotic-free farms. While it's reasonable to assume that antibiotics were a key factor in driving the emergence of MRSA in pigs, there's not much evidence showing that ongoing antibiotic use is an important factor in determining whether MRSA is present on specific farms or in specific pigs. One potential explanation is that in order to control infections, farms that stop using antibiotics start using other substances such as zinc in feed to help control overgrowth of certain intestinal bacteria, and these compounds may be just as effective at selecting for certain resistant bugs as classical antibiotics. That's just one possible explanation, but it shows how complex the issue of antibiotic-resistance is, and it shows that simply saying "stop using antibiotics," without really looking at the overall problem, won't necessarily reduce MRSA.
What does the presence of MRSA in food mean? Who knows? MRSA is a pretty high profile bug, and with good reason, because it's a very important cause of infection in people. A key aspect of MRSA in food is that cooking food will kill the bacteria (as well as many of the other harmful bacteria that often contaminate raw meat). So proper attention to food safety, including thorough cooking, cleaning of surfaces, prevention of cross-contamination and hand hygiene, should greatly reduce any risk (the problem is a lot of these things aren't usually done very well).
Tularemia alert in Texas
When it comes to handling microorganisms, there are 4 biosafety levels.
- Biosafety level 1 (BSL-1) organisms are harmless.
- BSL-2 organisms include most of the commonly encountered bugs, including things like E. coli, Salmonella and Staphylococcus aureus. They can cause serious disease but infections are often treatable and they can be handled safely with standard lab protocols.
- BSL-3 organisms are a relatively small group of bad guys that require more extensive facilities and protocols to handle them. These include Coxiella burnetii (the cause of Q-fever) and Francisella tularensis, the cause of tularemia.
- BSL-4 organisms are the really bad guys that require high level containment like you see in the movies. There are a limited number of BSL-4 facilities in the world and they deal with bugs like Ebola virus.
One of the BSL-3 pathogens I mentioned was a bug called Francisella tularensis, the bacterium that causes tularemia, which is also listed as a potential bioterrorism agent. Tularemia is a very nasty disease. It's uncommon but human infections occur sporadically in many regions, typically associated with wildlife exposure. It's often associated with contact with rabbits, but the bacterium can be found in a wide range of animals (including insects) and in the environment.
Recently, people in Bell and Coryell counties in Texas (between Dallas and San Antonio) were warned about the potential for tularemia exposure from wild hogs, since 15-50% of tested feral hogs in those areas had evidence of current of past infection. While evidence of past infection (the presence of antibodies against the bacterium in their blood) does not mean that they are actively infectious, it indicates that the bacterium is circulating in the area and that hogs are being exposed. If a hog was actively infected, it could be a source of human infection if there was direct contact (i.e. hunting and butchering).
Because of the potential risk of exposure, the following recommendations have been made:
- Always wear rubber gloves and eye protection when dressing (i.e. skinning & gutting) wild game.
- Ensure that game meats are handled carefully and thoroughly cooked.
- Use insect repellent to keep ticks, biting flies and other insects at bay.
- Look for rabbit nests in tall grasses before mowing. (As unusual as it sounds, running over rabbits with a lawnmower has been associated with development of tularemia).
The risk of tularemia is pretty low, but it's a very serious disease and you don't want it. Using these basic precautions should help reduce the risk.
Livestock-associated MRSA in dogs
A study we just published in the journal Veterinary Record (Floras et al 2010) described an MRSA outbreak in a dog breeding kennel. That's a little unusual in itself, but considering how MRSA is spreading amongst the dog population, it's not really astounding. What was unique about this outbreak was the strain of MRSA that was involved, sequence type 398 (ST398).
ST398 MRSA is commonly referred to as livestock-associated MRSA, since this strain seems to have originated in pigs, and is commonly found in pigs and calves in some regions of the world. It can also infect people, and high rates of carriage of this MRSA strain can be found in pig farmers, pig vets and other people with close contact with livestock. In some areas of Europe, this strain is a big problem, accounting for a large percentage of human MRSA infections. Interestingly, it seems to be a rare cause of illness in people in North America (at least at the moment).
Dogs seem to be innocent bystanders when it comes to MRSA. The vast majority of MRSA strains found in dogs are common human strains, indicating that, ultimately, MRSA in dogs originated in people. There are only two other reports of dogs with ST398, both from Europe. One was a dog with a skin infection. The other was a healthy dog (a carrier) who was owned by a pig vet.
This kennel outbreak involved a larger number of dogs, including both healthy carriers and sick dogs. Overall, MRSA was isolated on at least one occasion from 23/42 (55%) dogs in the kennel. In a couple of litters, most of the puppies were identified as carriers, but fortunately most stayed healthy. MRSA caused skin infection in one puppy and mastitis in a mother dog, and was also found in the respiratory tract of a puppy that died (although it may or may not have been the cause of death).
The source of the ST398 was not identified. One of the owners worked on a pig farm, but MRSA was not isolated from either owner. It's most likely that the owner did bring MRSA home from the farm, either as a transient carrier (in their nose) or as a contaminant on their skin. Regardless, once it got into the kennel, it was able to move between dogs, either from dog-dog contact or with the help of human hands. Fortunately, ST398 MRSA carriage by dogs seemed to be transient in this situation, which is consistent with what we know about carriage of other strains. MRSA is not really adapted for long-term survival in dogs, so they only carry it for short periods of time. That's a big advantage when it comes to trying to control this pathogen.
While we have to be careful to not over-interpret data from only a few studies, this report indicates that ST398 can cause disease in dogs and it can be present in apparently healthy dogs. It can also be spread relatively easily amongst dogs in a breeding kennel situation. While a pig-link was not confirmed, it's reasonable to suspect that dogs with contact with pigs (and perhaps other livestock) might be at higher risk of developing ST398 infections, as is the case with people.
This is a perfect example of the one medicine concept, and why we need to think about infectious diseases in broad terms, not just focusing on specific populations or species. This situation involved a pig Staphylococcus aureus that somehow acquired methicillin-resistance, spread widely around the world (most likely in pigs, initially), spread to people, and then likely spread to another species, in this case dogs.
