Outbreak of Giardiasis Associated with Multiple Asymptomatic Food Handlers, New York State, 2015

BACKGROUND:  Giardiasis is the most common intestinal parasitic disease in the United States. The protozoa that causes giardiasis, Giardia lamblia, is commonly spread through contaminated water or food. From 1998 to 2014, foodborne outbreaks of giardiasis represented <1% of foodborne outbreaks in the United States. In August 2015, New York State Department of Health (NYSDOH) was alerted of an increased incidence of giardiasis cases in the Long Island area. Initial case interviews found multiple exposures to a single gourmet grocery store.

METHODS:  An outbreak case was defined as a laboratory confirmed case of giardiasis with onset between 6/1/2015 and 10/31/2015 and a history of shopping at or consuming food from the grocery store, or an affiliate grocery store with shared staff, within 30 days before symptom onset. An environmental assessment was conducted and various food samples were collected for testing from the grocery store.  All grocery store employees were asked to provide a stool specimen and complete a questionnaire about work history.  Giardiacontaining stools recovered from employees and consumers were sent to the NYSDOH Wadsworth Laboratory for confirmation testing and genotyping.

RESULTS:  Twenty outbreak cases with symptom onsets from June 15^th to September 10^th were identified.  No concerns with water quality were identified.  While Giardia was not detected in any of the food sources tested, Cryptosporidium was identified in two food items. Of the 43 grocery store employees tested, three asymptomatic food handlers had Giardia detected in their stool and one was co-infected with Cryptosporidium. The food handlers with Giardia reported occasionally sharing ready-to-eat food preparation duties and one occasionally worked at the affiliated grocery store. Sequencing results indicated a match of G. lambliaassemblage BIII for all three food handlers and the two consumer cases where sequence was available.

CONCLUSIONS:  This outbreak presented a non-typical transmission of Giardia, as foodborne outbreaks of giardiasis in the United States are less frequent than waterborne giardiasis outbreaks. Genetic assemblage data support transmission of Giardia from the infected asymptomatic food handlers to the outbreak cases via the handling of ready-to-eat foods. These results show the importance of considering foodborne transmission of Giardia and assessing exclusion and testing of asymptomatic food workers with possible intermittent shedding of cysts in a protracted outbreak scenario.