BACKGROUND: Giardiasis is a reportable condition in Wisconsin caused by the protozoan parasite Giardia. Infection occurs via consumption of infected cysts excreted in the stool of infected people and animals that have contaminated water, food, or soil. Wisconsin public health staff interview patients with enteric diseases using a standardized questionnaire to collect demographic, clinical, and exposure information.
METHODS: In order to hypothesize risk factors for giardiasis in Wisconsin, a case-case comparison was performed using surveillance data from 3,202 confirmed giardiasis cases and 4,932 confirmed salmonellosis cases with illness onset during 2010-2015. Cases involved in point-source outbreaks were excluded from analysis. Analysis of exposure frequencies between comparison groups was performed; odds ratios, 95% confidence intervals, and Pearson p-values were calculated using SAS v9.4. Significant associations (p<0.05) of risk factors with giardiasis were reported.
RESULTS: During 2010 to 2015, incidence of giardiasis ranged from 8.3–11.2 cases per 100,000 persons. Case-patient age ranged from 0–96 years and had a bimodal distribution with peaks at ages 1–9 years (18.9%) and 50–59 years (15.9%). Symptom onset occurred most frequently during July–September. Compared to salmonellosis case-patients, giardiasis case-patients were more likely to be male, and reside in the northern region of the state. Giardiasis case-patients were also more likely to report exposure to standing water, a river, lake or pond, and drinking water from private or common/rural well systems. Individuals with giardiasis were more likely to buy organic food and consume food from farmer’s markets, their own gardens, and food obtained by hunting, fishing, or trapping. Exposure to adults or children in diapers and to manure or compost was also more likely among giardiasis case-patients. Attending daycare, living or working on a farm, and exposure to reptiles/amphibians and poultry were more likely among individuals with salmonellosis.
CONCLUSIONS: Annual incidence of giardiasis in Wisconsin during 2010-2015 was 1.5–1.9 times the national incidence. Compared to salmonellosis case-patients, giardiasis case-patients reported significantly higher rates of exposures associated with the outdoors, suggesting environmental reservoirs may be an important source of infection in Wisconsin. Wisconsin’s northern region contains a high concentration of surface water; water may facilitate transportation, survival, and transmission, of Giardia cysts. Findings suggest current national guidance emphasizing control of giardiasis in daycare settings should also address control of environmental exposures. Additional research into the relationships between giardiasis, surface water and land use is warranted.