METHODS: A confirmed case was defined as an individual who tested positive for Salmonella species and participated in Oozeball. A probable case participated in Oozeball and subsequently developed diarrhea. Human isolates were tested by the Rhode Island State Health Laboratory and the Massachusetts Department of Health. Using an electronic questionnaire, a case control study was conducted of 19 ill and 39 healthy Oozeball registrants. The source of the topsoil was investigated; dirt was sampled by the Rhode Island Department of Environmental Management and tested for Salmonella by the Rhode Island State Health Laboratory.
RESULTS: Three students met the confirmed case definition, testing positive for Salmonella: two with Salmonella Typhimurium (matching by PFGE) and one with Salmonella Thompson. Sixteen students were probable cases. Nine students visited health care providers; one was hospitalized. Getting mud in one’s mouth while playing Oozeball was significantly associated with illness (14 of 18 cases vs. 16 of 39 controls; odds ratio, 5.03; 95% CI, 1.40 - 18.12; p=0.01). Four samples of the Oozeball dirt tested positive for three different serotypes of Salmonella: Kentucky, Newport and Schwarzengrund. A background soil sample tested negative, indicating that Salmonella was not present in the ground near the purchased dirt.
CONCLUSIONS: Although none of the Salmonella serotypes in the dirt matched the human serotypes, the multiple serotypes present in both soil and patients indicate that the soil was heavily contaminated with Salmonella species. The laboratory evidence combined with the strong epidemiological evidence point to the conclusion that getting dirt in one’s mouth was the cause of this outbreak. The dirt used in the Oozeball game had many opportunities to become contaminated with Salmonella species, as it originated from multiple unknown construction projects and was stored outdoors before being transported to the university. Collaboration between multiple state agencies and the university was essential to this outbreak investigation.
While there have been known outbreaks of campylobacter and norovirus in the last several years associated with mud sports, to our knowledge, this is the first reported mud sport-associated Salmonella outbreak.