BACKGROUND: Giardiasis is a nationally notifiable gastrointestinal illness caused by the parasite Giardia intestinalis. Recognized as a leading cause of waterborne illness in the U.S., the Ohio Department of Health uses surveillance to understand the epidemiologic characteristics of giardiasis in the state, design prevention strategies, and establish research priorities. Previously, confirmed giardiasis cases required only laboratory demonstration of Giardia organisms, antigen, or DNA in stool or another biological specimen. In 2011, the case definition was revised, requiring that confirmed cases exhibit gastrointestinal symptoms such as diarrhea, abdominal cramps, or bloating in addition to positive laboratory results. As a result, misclassification among traditionally asymptomatic populations (i.e., immigrants, refugees, and international adoptees) is thought to have increased. The purpose of this evaluation was to 1) determine if there has been a significant change in the number of confirmed cases of giardiasis among foreign-born populations compared to U.S.-born cases; and 2) ascertain the extent of case misclassification due to the change in case definition.
METHODS: Giardiasis case report data were extracted from the Ohio Disease Reporting System (ODRS) for the years 2008–2014 (N=5101). To identify immigrant, refugee or international adoptee populations, case notes were searched for text containing key words “immigra*,” “refuge*,” and “adopt*” (n=1341). Changes in case classification were calculated, as were the number of confirmed cases potentially misclassified due to lack of symptomology.
RESULTS: Over the study period, the number of confirmed giardiasis cases among the foreign-born fell by 73.3%; conversely, the number of non-cases in this population increased by 2100%. The proportion of confirmed giardiasis cases among U.S.-born individuals decreased by 55.8% during this same time period; however, there were zero non-cases among this population over the 7-year span. Post-2011, it is estimated that 40.2% of foreign-born non-cases were misclassified as confirmed due to lack of symptomology, compared to 15.6% of U.S.-born non-cases. Other contributors to misclassification included use of stool culture as a sole laboratory detection method (15.5%) and patient loss to follow-up (2.5%).
CONCLUSIONS: The 2011 change in case definition has substantially impacted case classification for refugees, immigrants and international adoptees, as these populations are often asymptomatic for giardiasis. Recommendations include: (1) adding a question to the ODRS disease report form that would allow identification of cases by national origin; and (2) partnering with the Ohio Department of Job and Family Services Refugee Services Program for improved reporting of giardiasis screening results, symptomology, and patient follow-up.