An outbreak of cryptosporidium that affected 40 people in Ireland this past year has been linked to salad from one farm.

A cluster of cryptosporidiosis cases was reported from a South Dublin laboratory in July 2020. All sick people were interviewed, and environmental health officers did 110 inspections to collect food and water samples at businesses in which suspected cases had food in the 14 days before onset of symptoms.

Forty patients were linked to the outbreak, of which 33 met the confirmed case definition. Twenty five were female and 15 male. In total, 31 cases were aged 20 to 40 years old and 14 required hospital treatment. Dates of symptom onset were from mid to late July and those sick ranged in age from 3 to 74 years old.

No microbiological link
Cross-referencing of food exposures identified a common salad box served in the implicated restaurants, sourced from a single farm, which led to a product recall of Gold River Quality Mixed Salad Leaves (Seasonal Salad Box) sold in 1-kilogram packages. Testing of water and salad leaf samples failed to detect Cryptosporidium oocysts, according to a study published in the Irish Medical Journal.

“Given the three day shelf life of the salad box and the symptom onset date of cases, negative food sample testing cannot rule out contamination of previous salad batches which were not captured in these samples,” said researchers.

In late July, a cluster of nine cases of Cryptosporidium parvum were reported to the public health department by a laboratory in the Wicklow area.

All reported patients were contacted by phone to discuss precautions and complete a standardized exposure questionnaire that focused on potential high-risk exposures, such as salad leaves, unpasteurized dairy, private water supplies or contact with animals.

A likely foodborne source was suspected from the preliminary outbreak investigation, based on the frequency of reported restaurant exposures, and an initial cluster of six cases involving a single outlet.

Cryptosporidiosis is a notifiable disease in Ireland. Since 2012, the country has consistently reported the highest annual rate of the parasite in the European unio.

Researchers said PCR screening of stool samples for Cryptosporidium helped early detection and management of the outbreak but routine surveillance remains “inconsistent” at Irish and European levels.

Farm investigation
Of 40 cases, 31 ate at restaurants that sourced salad from a common supplier farm. Five food sites were linked to at least three cases each.

A seasonal salad box was identified as having been supplied to all premises associated with the 31 cases. It was produced by Gold River Farm and shipped by two suppliers.

This farm primarily produced baby leaf salad but also had livestock in adjoining fields. Water testing results from a private well onsite were negative for Cryptosporidium, E. coli and Enterococci. Investigators were told the well water was not used for washing salad leaves but was used in steam cleaning of the sorting bench and product containers and crates after production. Officials said there were no obvious sources of contamination during an inspection.

The farm owner reported no irrigation of the crops since early June because of adequate rainfall. There had been a number of heavy days of rainfall in June following minimal rain in May. Including within the 28 to 35 day growing period of the implicated salad leaves.

Recommendations to the farm included fencing off the salad crop field; remediation work to the well head that was vulnerable to contamination; installation of UV disinfection equipment; and washing of salad by food premises before use. A product recall of the seasonal salad box was issued in early August.