I’m kidding, but ScienceBlogling Tara Smith has co-authored a PLoS One article about the emergence of the MRSA strain ST398 in Iowan pork farms. Pig farms are a tremendous reservoir of bacteria: as far as I can tell, there are about six pigs for every person in Iowa.
MRSA ST398 a methicillin resistant S. aureus bacterium that has spread epidemic through various European animal populations (particularly pigs). It recently jumped from the animal population to the human population in Europe and has begun to establish itself there in hospitals. The last thing we want is for another MRSA strain which is good at colonizing clinical setting to increase in the U.S.: we have enough problems with MRSA USA200 and USA300, thank you very much….
Which leads us to Tara’s article. I’ll get to my thoughts and questions in a moment, but I’ll let the article’s abstract summarize the findings first (unlike too many articles, the abstract actually does its job; I’ve added a couple of notes to explain some technical things):
Recent research has demonstrated that many swine and swine farmers in the Netherlands and Canada are colonized with MRSA. However, no studies to date have investigated carriage of MRSA among swine and swine farmers in the United States (U.S.).
We sampled the nares of 299 swine and 20 workers from two different production systems in Iowa and Illinois, comprising approximately 87,000 live animals. MRSA isolates were typed by pulsed field gel electrophoresis (PFGE) using SmaI and EagI restriction enzymes, and by multi locus sequence typing (MLST). PCR was used to determine SCCmec type [Mad Biologist: methicillin resistant gene type] and presence of the pvl gene [Mad Biologist: pvl stands for Panton-Valetine leukocidin, which kills blood cells; it may or may not result in increased disease severity].
In this pilot study, overall MRSA prevalence in swine was 49% (147/299) and 45% (9/20) in workers. The prevalence of MRSA carriage among production system A’s swine varied by age, ranging from 36% (11/30) in adult swine to 100% (60/60) of animals aged 9 and 12 weeks. The prevalence among production system A’s workers was 64% (9/14). MRSA was not isolated from production system B’s swine or workers. Isolates examined were not typeable by PFGE when SmaI was used, but digestion with EagI revealed that the isolates were clonal and were not related to common human types in Iowa (USA100, USA300, and USA400). MLST documented that the isolates were ST398.
These results show that colonization of swine by MRSA was very common on one swine production system in the midwestern U.S., suggesting that agricultural animals could become an important reservoir for this bacterium. MRSA strain ST398 was the only strain documented on this farm. Further studies are examining carriage rates on additional farms.
One interesting observation is that younger piglets were more likely to be colonized than older piglets and adults:
It might be that as the immune system of pigs matures, they are more able to fight off MRSA ‘infections’ (which, in this case, don’t cause disease in pigs). Antibiotic resistance in these Iowan ST398 isolates is unusual too: many are resistant to clindamycin, but not erythromycin. This is a very odd phenotype and suggests a novel genetic resistance mechanism.
A final interesting section from the paper:
Additionally, a portion of the sows at PSA were imported from Canada, while those from PSB originated in Michigan. Canada is the most important exporter of live hogs to the U.S. Thus, it is possible that ST398 may have been brought into the U.S. via live swine or pork products. However, this study was not designed to identify the source of the MRSA and additional research should further examine this question.
Blame Canada? Actually, some detailed genomic sequencing could get at this. Too bad that there’s no one who’s interested in this question who is in a position to attempt to get projects like this funded.
Anyway, the whole paper is worth a read–and I think is very accessible to the non-specialist reader.
Cited article: Smith, T.C. et al. 2009. Methicillin-Resistant Staphylococcus aureus (MRSA) Strain ST398 Is Present in Midwestern U.S. Swine and Swine Workers PLoS ONE. 2009; 4(1): e4258. doi: 10.1371/journal.pone.0004258.