Are Deer the Reservoir for Shiga-Toxinogenic E. coli (STEC)?

oldie-whitetail-deer-face
THIS IS THE FACE OF THE ENEMY!
(from here)

A recent paper in Applied and Environment Microbiology describes the frequency of STEC–Shiga toxin producing E. coli–in wild animals. STEC can cause organ failure and death and are found throughout E. coli–there is no one lineage of STEC, largely because shiga toxin is encoded by a bacterial virus (‘bacteriophage). This means that this dangerous trait can spread from E. coli to E. coli, as if it were an infectious agent (I discuss some of the possible evolutionary implications here).

One of the confusing things about STEC is that the typical conduit into the human food supply is through cattle (improperly washed cow carcasses), although wild animals pooping on vegetables, which are then improperly washed and uncooked (enjoy that salad!) are another major source*. One of the odd things people have noticed is that, in cattle, the frequency of STEC is associated with two things. First, ranging herds (those that eat grass outside of the barn) typically have higher frequencies of STEC. Second, this is seasonal: STEC is less frequent in winter than summer. One hypothesis is that cattle aren’t really the reservoir, but instead are being ‘infected’ by wild animals–or their poop–that they contact (I put infected in scare quotes, since adult cows don’t seem to suffer any ill effects of having STEC, unlike humans).

Anyway, back to the paper. I previously discussed a study that reported high levels of STEC in deer in the U.S. The AEM paper found that 53% of roe deer in Spain carried a STEC.

It’s worth describing the methods, because ‘found’ can be a very misleading term. This is not a case of using PCR to show that one out of million E. coli in a given deer has the shiga toxin gene (‘stx‘). The authors isolated between twenty to forty isolates (colonies on a plate) of E. coli and a deer was considered positive if one or more isolates were STEC (they would isolate twenty, and if no STEC were seen, then they isolated twenty more colonies). In some cases, multiple isolates from a deer were STEC. This means that, per gram of deer poop, we’re talking about one million to ten million cells of STEC. Deer are big mammals and produce a lot of poop.

It really does seem that deer are a major reservoir of STEC. From what I’ve been able to read, deer populations in the U.S. are exploding (the size, not the deer themselves…). In 1900, less than 500,000 deer occurred in the U.S. Now, estimates run between fifteen to thirty million deer, and many areas previously lacking deer now have them.

Somebody should probably start surveying deer E. coli. Might be important. Just saying.

*Actually, food epidemiologists have a very simple rule of thumb: if mostly men are ill, then check the meat; if women, then check the vegetables. It’s not fool-proof but it works reasonably well.

Cited article: Mora A, López C, Dhabi G, López-Beceiro AM, Fidalgo LE, Díaz EA, Martínez-Carrasco C, Mamani R, Herrera A, Blanco JE, Blanco M, Blanco J. 2012. Seropathotypes, Phylogroups, Stx Subtypes, and Intimin Types of Wildlife-Carried, Shiga Toxin-Producing Escherichia coli Strains with the Same Characteristics as Human-Pathogenic Isolates. Applied and Environmental Microbiology 78(8):2578-85. doi: 10.1128/​AEM.07520-11

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