At the very beginning of the year 1981, the United States saw an unusually large boost in Salmonella infections across the country. Incidences of the food-borne illness had risen by nearly 20% from the previous year, surprising health officials not only with the unexpectedly high number of cases but its odd timing during the winter season.
Ohio, Michigan, Georgia, and Alabama saw most of the action from the Salmonella outbreak but from an unusual serotype of the microbe, Salmonella muenchen, and CDC investigators were unable to pinpoint its edible source. Michiganders, however, provided local investigators with an interesting lead in the case - 76% of those infected reported personal usage of or "household exposure" to marijuana (1).
Isolates of Salmonellamuenchen from cases of infection in the United States, from January 1978 through August 1981. Note the massive spike in infections in January and February of 1981; not only were these cases above the expected baseline number of infected but they also occurred at an unusual date.Image: Taylor DN et al. (1982) Salmonellosis Associated with Marijuana — A Multistate Outbreak Traced by Plasmid Fingerprinting. N Engl J Med.306: 1249-53
This wasn't a food-borne but a marijuana-borne outbreak.
Laboratory tests of the marijuana in question found staggering quantities of the bacteria on the drug, in some instances as high as 10^7 or ten million bacteria per gram of bud. Investigators found that the green stuff had reached its way across the country from California to Massachusetts, concomitantly spreading the S. muenchen outbreak.
How did the Salmonella end up in the skunk? Investigators determined that the marijuana was imported from either Jamaica or Colombia and distributors there seem to have deliberately adulterated the product with manure prior to shipment so as to increase its weight and, in turn, raise its consumer costs (2). Not only were people falling ill from their illicit drug, but they were also paying more for less!
A CDC physician, Dr. David Taylor, described the method of transmission thusly,
The salmonella was in the marijuana. When a marijuana smoker rolled a cigarette, his hands became contaminated, and when he put the cigarette in his mouth his lips became contaminated. Then a touch or a kiss or any sort of contact could spread the infection … And not only that. Pot decreases the gastric acid, and gastric acid is an important defense against infections of all kinds (3).
Potheads weren't the only ones susceptible to this salmonella smoke up. Family members, including children, and roommates also became infected indirectly through person-to-person contact with those toking up or from touching contaminated household surfaces. (Infected people spreading diseases from their hands and contaminating stuff, you ask? See here.)
There's an aphorism, "when you hear hoofbeats, think horses not zebras". In public health and medical school, students are taught to think of the most likely scenario, the most common culprit behind a disease or outbreak. An organism commonly associated with food outbreaks? Show me your spinach! Show me your cantaloupe! Show me your ground beef! And, indeed, this often holds true with many of the microbes we humans encounter regularly - the E. coli nestled in our bagged salad greens and the TB bacterium sprayed from a cough are two good examples. But this marijuana outbreak is a lovely exception, an interesting case that I like to think shows the unpredictably and caprice of microbes. Every now and then we do need to think of horses, zebras andeven giraffes.
Some background history on the "medical zebra" aphorism. I also quite like the "KISS principle": Keep It Simple, Stupid.
Wanna look at past foodborne outbreaks in the US? Of course you do! The CDC's Foodborne Outbreak Online Database (FOOD) can be accessed here but sadly has only data up to the year 2010.
References (1) Taylor DN et al. (1982) Salmonellosis Associated with Marijuana — A Multistate Outbreak Traced by Plasmid Fingerprinting. N Engl J Med.306: 1249-53 (2) Associated Press. (1982, May 27) Marijuana Linked to Salmonellosis. The New York Times [Online]. Accessed on September 13, 2012 here. (3) Thompson A. (2008, June 13) Smoking Salmonella. The New Yorker [Online]. Accessed on September 13, 2012 here.
Taylor DN, Wachsmuth IK, Shangkuan YH, Schmidt EV, Barrett TJ, Schrader JS, Scherach CS, McGee HB, Feldman RA, & Brenner DJ (1982). Salmonellosis associated with marijuana: a multistate outbreak traced by plasmid fingerprinting. The New England journal of medicine, 306 (21), 1249-53 PMID: 7070444