Infected Mice Found in Pressed Grapes Leads to Oropharyngeal Tularemia Outbreak

A small outbreak of oropharyngeal tularemia that occurred in grape harvesters at a German vineyard in October 2016 has been linked with the consumption of pressed grapes contaminated with wood mice DNA.

The retrospective study findings made by a team of German researchers have been published in the New England Journal of Medicine.

Antibodies to Francisella tularensis lipopolysaccharide, the bacterium that causes tularemia, were detected in 6 of 29 harvesters at the vineyard and was confirmed on Western blot analysis, following complaints of swollen cervical lymph nodes, fever, chills, difficulty swallowing, and diarrhea, within 4 to 8 days after the suspected exposure.

Tularemia is a disease for which rabbits, hare, and rodents are susceptible, but human infections are possible and can occur through insect bites, skin contact with infected animals, inhalation of air-borne bacteria, or consumption of contaminated food and water, according to the Centers for Disease Control and Prevention (CDC).

Through a retrospective cohort study, the researchers were able to determine that the 6 harvesters, 2 women and 4 men, had each consumed grape must—a fruit juice made up of pressed grapes including seeds, skins and stems—from a batch of grapes that had been collected mechanically. Consumption of this particular grape must was revealed to be the only significant predictor of acquisition of tularemia in a multivariate analysis.

Furthermore, a batch of grapes collected manually from the same vineyard was pressed after the contaminated batch and were found by the researchers to contain traces of the bacteria, suggesting the pressing machine served as a source of cross-contamination. Additionally, the mechanical harvester was utilized later on in the day at a separately owned vineyard and the batch was also found to be contaminated with F. tularenis, suggesting that the mechanical harvester was an additional source of cross-contamination.

The presence of wood mice DNA was determined through sequencing analyses of wine made from the must from the same batch that the sickened harvesters consumed. The researchers hypothesize that an infected mouse was collected by the mechanical harvester and pressed with the grapes, causing contamination to the must which was consumed by the 6 harvesters, leading to further contamination.

In speaking with the vineyard owner, researchers learned that it is not uncommon for rodents to occasionally be discovered amongst grapes collected by mechanical harvesters. As such, the researchers conclude that the results indicate that mechanical harvesting can be a risk factor for the transmission of zoonotic borne diseases.

The authors report that all contaminated products were confiscated, and sales were prohibited by health officials.

If left untreated, tularemia can lead to serious infections including pneumonia, meningitis, pericarditis, osteomyelitis, and in severe cases, death. The disease has proven difficult to diagnose due to the fact that symptoms can be mistaken for other common illnesses.

The CDC has indicated that the sole prevention strategy is to reduce exposure to F. tularenis, but researchers are trying to overcome the difficulties and costly nature of developing a vaccine for tularemia, to fight back against the rare disease.