Researchers in France have described an outbreak of Cryptosporidium linked to unpasteurized, raw milk white cheese. In November 2017, a gastroenteritis outbreak was reported at a school in Loire-Atlantique, western France, with more than 150 sick people.

Cryptosporidiosis appears to be largely underdiagnosed in the general population in France and Cryptosporidium is not always covered by labs when testing for parasites, according to the researchers.

Finding the source
Drinking water was not associated with disease but eating in the canteen was a significant risk factor. After negative bacteriological and virological analysis of 15 samples, a molecular method (Polymerase Chain Reaction) detecting 22 pathogens, found the parasite Cryptosporidium parvum.

Among foods served six days before to all people eating at the canteen, there was an organic unpasteurized white cheese. A survey at the producing farm showed the presence of calves infected with the same virulent zoonotic genotype close to the production room.

Cryptosporidium is a single celled parasite which, if ingested, can cause cryptosporidiosis. The main symptom is watery diarrhoea, which can range from mild to severe. It is often accompanied by stomach pain, nausea or vomiting, fever and sometimes dehydration and weight loss. The infective dose is 10 to 100 oocysts and incubation time is six to seven days.

Norovirus was initial suspect
A retrospective cohort study of students and adults used standardized questionnaires posted online the next day on the school website. This was conducted under the initial hypothesis of norovirus contamination, incubation time one to two days. Almost 300 people completed the questionnaire: 264 students and 29 teachers and employees.

A total of 167 school students and 13 adults met the case definition. An age-response relationship was observed, with the attack rate decreasing with age. Clinical signs began between Nov. 19 and 26, 2017 with a peak of 75 cases on November 22.

Fecal samples were analyzed for bacterial, viruses and parasites and environmental specimens were also tested. Canteen meals were prepared by three employees who reported no vomiting or diarrhea.

After all samples were subjected to a specific search for Cryptosporidium spp, 12 were positive for Cryptosporidium parvum. The survey was then expanded to food served five to seven days before the outbreak peak including the white cheese served on Nov. 16 and a coleslaw salad available the day after.

Oocysts of Cryptosporidium parvum were found in calf feces at the farm near the production laboratory in samples taken two months after the outbreak and were of the same genotype found in patients. Of feces samples taken on Jan. 18, 2018, for four calves, three were Cryptosporidium parvum positive.

Researchers said the outbreak was a reminder that manufacturers of dairy products must be subject to special hygiene considerations, especially in agricultural areas.