Constant readers: Fresh from the journal Emerging Infectious Diseases — posted AOP (electronic publication/ahead of print) this afternoon — comes more news of MRSA ST 398, the "pig strain," in food animals. This time, it's chickens, in Belgium.
The authors (from Ghent University and the Veterinary and Agrochemical Research Center in Brussels) took swabs from living chickens — laying hens and broilers — from 24 farms, 50 layers and 75 broilers total; one broiler-raising farm was sampled twice. They found no MRSA in the layers, which is important for reasons I'll get to in a moment, and ST 398 in 8 broilers. From each chicken, they took two samples, nasal and cloacal, and in the 8 positive chickens, they got 15 MRSA isolations; one cloacal swab was negative. Of the positive chickens, several (I deduce three, but the math is a bit cloudy) were spread across the two visits to the farm that was sampled twice. Since chicken farms are depopulated between batches — yes, just what it sounds like, farms sell/kill all the birds and clean the place — that finding suggests that MRSA is persisting in the environment on that farm.
Important point: This strain was ST 398, which we here have been calling the pig strain from many previous findings, most of them in pigs. However, ST 398 is an identification using a particular technique called MLST (multi-locus sequence typing), which is used for this strain because the standard typing method, PFGE (pulsed-field gel electrophoresis), did not return a readable result when the strain was first identfied back in 2004. (Trivia: That's why the initial reports of this strain called it NT, for "nontypeable.") It's becoming increasingly clear, though, that ST 398 is actually a category, not a single strain. And within that category, today's research is a new find: a strain with the unusual spa type t1456, which has only been found 10 times in the past three years, in Germany and the Netherlands, not in Belgium. The author suggest that this particular strain may be adapting to poultry in the same manner that the ST 398 we have been talking about (different spa type — sorry, I will have to look it up) has adapted to pigs.
So, as before: Why do we care? We care for two reasons: First, because since this strain is in a food animal, the possibility exists that it could contaminate the chickens' meat during slaughter and pass to humans. As has happened with some ST 398, the humans could be only colonized, and not become ill. But, second, any increase in colonization is a bad thing: The more strains out there, the greater the chance that they will exchange virulence and resistance factors and become something unpredictable.
Now, about those layers, here's an interesting factor that the authors call out in their paper: Layers, unlike broilers, do not receive antibiotics. The layers did not carry MRSA. The broilers did. It's a pretty potent argument, in case anyone needed convincing, of the effect of the selective pressure that antibiotic use in food animals exerts on these strains.
The site is: Persoons D, Van Hoorebeke S, Hermans K, Butaye P, de Kruif A, Haesebrouck F, et al. Methicillin-resistant Staphylococcus aureus in poultry. Emerg Infect Dis. 2009 Mar; [Epub ahead of print] DOI: 10.3201/eid1503.080696