Emerging Disease and Zoonoses #12–Salmonella from fish

So, I’ve had this research article on multiple drug-resistant Salmonella in the drafts section for about a week now, waiting for me to do a bit of background research before commenting on it. (Anything involving Salmonella always makes me a bit hesitant–one almost needs a PhD on the bacterium just to keep up with the nomenclature). This morning I’m doing my quick glance-through of my blogroll, and lo and behold, what do I find but these posts by Mike the Mad Biologist, who just happens to have been quoted in the New York Times write-up of the research.

Michael Feldgarden, who helps track resistant bacteria, said he agreed that the blanketing doses of antibiotics necessary to create that deep pattern of resistance were probably given much earlier in the distribution chain of the fish — probably on fish farms in Southeast Asia.

He is part of a Boston-based surveillance network, the nonprofit Alliance for the Prudent Use of Antibiotics, that, among other activities, works to identify the reservoirs of drug resistance among nonpathogenic bacteria.

One long range concern, Dr. Feldgarden said, is that now that the five resistance genes seem to be established in that strain of salmonella, they may fairly easily jump as a unit to another strain — or even to a completely different, nastier organism living in the same soup.

And those new changes may prove even more dangerous to people, he said.

“I’d be interested to culture some of those fish and see what else is infecting them,” Dr. Feldgarden said.

To back up a moment, the authors carried out the study because previous outbreaks of the bacterium–Salmonella paratyphi B–had been linked to ownership of aquariums, but the bacteria hadn’t been molecularly characterized to see if the isolates that came from disease in humans were identical to any isolates of the bacterium taken from the aquariums. In the current study, they found that they were, indeed, indistinguishable, suggesting that the people probably acquired the infection from their fish tank. The authors note this has big public health significance:

These findings identify home aquariums containing tropical fish as the most important, although not necessarily the only, source of multidrug-resistant S. Paratyphi B dT+. The fact that 12%-14% of Australian households have ornamental fish and as many as 12 million American and 1 million Canadian families own domestic aquariums, together with the young age of most affected patients, indicate that multidrug-resistant S. Paratyphi B dT+ in home aquariums is a risk factor for Salmonella infection and thus becomes a public health issue.

We often think of our animals as a source of infection, but fish are generally pretty far down on the list as reservoirs of zoonotic pathogens. Outbreaks of disease from them, however, happen every once in awhile. For example, outbreaks of Streptococcus iniae in humans happen occasionally. These types of outbreaks are more commonly associated with the handling of live fish, however, than with consumption of cooked fish.

Finally, as Mike notes, the Times story missed a major message:

But one important message–that antibiotic use in agriculture or aquaculture has influenced the evolution of a bacterial pathogen–didn’t really make it into the story. From my perspective, that, and not drinking water that your fish have crapped in, is the important part.

I wouldn’t suggest, however, that you drink the water your fish crapped in, either. 🙂

10 Replies to “Emerging Disease and Zoonoses #12–Salmonella from fish”

  1. Anything involving Salmonella always makes me a bit hesitant–one almost needs a PhD on the bacterium just to keep up with the nomenclature.

    Even Salmonella researchers don’t agree on the taxonomy. Some call the whole genus S. enterica, and each of the seven groups is a subspecies (I think that’s the official designation). Others use species names for each group. I’ve always just referred to them by the group number (e.g., I, III, etc.). It’s a disaster.

  2. So, how widespread is the use of antibiotics in aquaculture? I have been well aware of their use in livestock – growing up on a cattle farm will do that – and the dangers of resistance.

    Presumeably they are used for the same reasons: weight gain and disease prevention? What is the danger to water supply?

    I don’t eat farmed fish anymore, largely because of the PCBs, but I guess this is one more reason…

  3. That’s a good question, and I’m not exactly sure of the answer, though I think they are generally used more for actual disease prevention/treatment rather than as a growth promotant. Antibiotics in the water are certainly a concern as well, since they can persist and no one knows exactly what effect they may have at very low concentrations (Mike can correct me if I’m wrong about that one). I thought Seed actually had an article on it recently, but I can’t find it…

  4. “Antibiotics in the water are certainly a concern as well, since they can persist and no one knows exactly what effect they may have at very low concentrations”

    Yeah, that seems to me to be quite dangerous – if they persist, but aren’t sufficiently high in concentration, it seems as if they’d promote selection for resistant strains faster than large doses that would clobber a larger percentage of the bacterial population (catostrophic). Are there mathematical models for this sort of thing (seems like there must be), or are they specific to the antibiotic?

  5. I would imagine persistence of the drug is very much a question of the composition & stability of the individual compound.

  6. Montepellier,

    antibiotics is aquaculture are used to prevent disease outbreaks, not as growth promoters. You’re right about the concentration effect: lower concentrations can make the problem far worse than high concentrations (both in the animal and the environment). To the best of my knowledge, no one has seriously modelled this in soil or water; as you point out, there are a lot of details that have to be parameterized. There is a lot of work done in animals (pharmokinetics).

  7. Thank you guys for the primer.

    BTW – Montpellier is a pseudonym for posting on a few other sites…the email address pretty well gives that all away…so, it’s Scott.

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