Researchers find Italian Giardia outbreak linked to tap water

Researchers have provided more details about the first documented outbreak of giardiasis in Italy that was linked to tap water, according to recently published research.

More than 200 people were affected during several months and two needed hospital treatment for the parasitic infections.

Consumption of raw vegetables and fruits was also associated with lower odds of giardiasis. This could be because of exposure to low levels of Giardia cysts on such items, which can trigger protective antibodies, according to the study. Giardia duodenalis can withstand standard chlorine treatment.

From November 2018 to April 2019, people fell sick in a municipality of the Bologna province, in Northeast Italy. Tests identified cysts and antigens of the Giardia parasite in stool samples of 228 individuals. Molecular typing of 136 samples revealed a vast predominance of Giardia duodenalis assemblage B.

Giardiasis is part of mandatory surveillance in 24 European Union countries but Italy, Austria, Denmark, France and the Netherlands do not yet have compulsory reporting systems. Giardiasis is not a reportable disease in Italy, according to the study published in the journal Eurosurveillance.

Patient interview findings
On one day in January 2019, the parasitology section of the microbiology unit in a hospital in Bologna identified 10 Giardia infections. Based on this high number in one day, the local public health office was informed and an outbreak declared.

Up to 80 percent of cases fell ill between the end of November and late December. The last case, likely due to secondary transmission, was in early May 2019. Overall, 199 patients were confirmed. The average time between onset of illness and diagnosis was 42 days but ranged from two to 125. Women were more affected than men with those aged 45-64 and 20-44 years old making up most of the patients.

Drinking tap water from the local municipal supply and having close contact with individuals diagnosed with giardiasis had the most frequent association with the infection.

A case-control study with 60 cases and controls indicated that drinking tap water was associated with Giardia, and infection was strongly correlated with the amount of water consumed.

Cases consumed an average of five glasses of tap water per day, whereas controls had an average of 2.7 glasses daily; for each additional glass of water per day, the probability of infection doubled.

Gardening, cultivating a vegetable garden at home, and living in certain streets were associated with higher odds of giardiasis. Those who used domestic water filters had reduced odds of infection. These cases may have used tap water to rinse vegetables or had it in places outside the home.

Not found in water samples
Investigations into potential sources indicated tap water as the most likely vehicle of infection, although cysts were not detected in water samples.

Giardia cysts and fecal coliform bacteria were not found in any water samples collected from the distribution network on two days in January 2019. Giardia contamination was also not recovered in 20 samples during additional monitoring plan of the water supply network.

A review of data for the year before the outbreak showed no contamination of the water network by sewage from residential areas, industrial activities or farms. No heavy rainfall occurred before or during the estimated exposure period. During this period, plumbing maintenance of the supply network was documented and it is possible water contamination may have occurred, said researchers.

The waterworks serves four other municipalities, but cases of giardiasis were only recorded in part of the Bologna province.

As Giardia contamination was not detected in water samples and the chemical-physical and microbiological standards were compliant with legislation, a water avoidance notice was considered unnecessary by authorities.

Control measures mostly aimed at preventing secondary transmission by informing citizens about the outbreak, and treatment of patients with anti-parasitic drugs.

Passive surveillance of laboratory-confirmed cases did not allow timely detection of the outbreak. This highlights the need to increase awareness of giardiasis among doctors and pediatricians and to encourage routine diagnosis of the parasite in patients with persistent diarrhea, according to scientists.

Researchers said the investigation showed the difficulties in detection and management of the parasite, which is often overlooked as a cause of gastroenteritis.

“The long and variable incubation time, absence of specific symptoms and a general lack of awareness about this pathogen contributed to delay in diagnosis.”

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