A cautionary tale of mice, microbes, and mechanical harvesting

Short: A recent medical report indicates that fresh juice from mechanically harvested grapes can (on very rare occasions) carry infectious diseases from animals picked up and crushed by harvesting machinery.

Long: Winery work is dangerous. Especially (but not only) during harvest, wineries are production facilities with heavy objects, slippery floors, potentially hazardous chemicals, machinery with moving parts and, often, forklifts. The positive side of that situation is that winemaking professionals know that winemaking — like any similar form of factory or food production work — is dangerous, and they take steps to mitigate risks. An enormous part of mitigating risks is knowing what those risks are, which is why a recent letter to the editor about mechanical harvesting in the New England Journal of Medicine is worth knowing about.

Potential dangers of mechanical harvesting are usually discussed in terms of dangers to the grape vines (of being beaten up by the machines), to the wine (though mechanical harvesting is far from being always a bad thing for quality) or, maybe, to vineyard soil compressed by heavy machinery or animals caught by harvesting equipment. Endangering animals, however, might on rare occasion also endanger humans — and not just if a harvester has an inadvertent run-in with a neighbor’s cat. Some critters commonly found around vines can carry diseases that affect humans.

When six of 29 harvest workers employed at a vineyard in Rhineland-Palatinate, Germany, came down with the same symptoms — trouble swallowing, swollen lymph nodes in the neck, diarrhea, general aches and chills — regional specialists worked them up for infectious disease and came down with a diagnosis of tularemia. Tularemia is what happens to humans when infected by a bacteria, Francisella tularensis, carried by some rabbits and rodents. Most cases occur in hunters. Symptoms vary with how the bacteria entered the body, but the bacteria are never transmitted from human to human, only from animal to human. Or, in this case from animal to grape juice to human. All six workers who became sick remembered drinking fresh juice from the same load of mechanically harvested grapes before becoming sick. Epidemiologists found DNA from F. tularensis and from common field mice in the freshly fermented wine made from that batch of grapes.* Their concluding hypothesis was that an infected mouse or two had been picked up by the harvester and crushed along with the fruit, contaminating the fresh juice.

The wine was confiscated and prohibited from sale, needless to say, though probably without real cause. Infectious disease transmission via wine is unheardof, thanks to the combination of high alcohol and low (acidic) pH that makes wine an inhospitable environment for could-be pathogens. Though I don’t know (and I’m not sure anyone knows) about F. tularensis‘s ability to survive in wine, it’s important to note that the epidemiologists found F. tularensis DNA in the wine, not intact infectious bacteria, and that workers suffered from drinking grape juice, not wine.

The authors of the NEJM letter — most of them German public health officials — concluded that “raw food stuff should be treated before consumption.” If that means “we recommend that you don’t drink fresh juice from mechanically harvested grapes,” that sounds pretty reasonable. Like recommendations about not eating raw cookie dough, many will choose to accept the risk; a lot of us will accept the very small chance of getting ill from bacteria in raw eggs or raw flour for the sake of the certain pleasure of enjoying delicious cookie dough. But, importantly, we know that the risk exists, and if we become sick we might even think to tell our physician about what we ate, which could speed up receiving appropriate treatment.

Tularemia is endemic but rare across the United States and Europe (it’s mostly a Northern Hemisphere disease), with only a few hundred cases per year across the United States. The lesson here would NOT seem to be “don’t drink fresh mechanically harvested grape juice or you might get tularemia” but, rather, “know enough about your environment to have a clue about what might have happened on the very, very rare occasions that something goes wrong.” Also, watch out for mice.


*Did any of the harvest workers remember seeing field mice in the vinyard, and did anyone in the cellar see a dead mouse or two pour into a fermenting vat? Unfortunately, the public health officials who wrote the report didn’t say.