Researchers have shared findings from Cryptosporidium surveillance in Sweden from 2018 to 2022.

A national microbiological surveillance program was implemented in 2018 to increase knowledge of the molecular epidemiology of human cryptosporidiosis to better understand transmission patterns and potential sources.

Cryptosporidium-positive fecal and DNA samples from domestically acquired infections were collected. Between 2018 and 2022, 1,654 samples were analyzed, and 11 species were identified, with the majority being Cryptosporidium parvum. All 21 counties in Sweden reported cases of Cryptosporidium parvum.

Cryptosporidiosis has been a notifiable disease in humans since 2004. Incidence increased from 0.8 per 100,000 inhabitants in 2005 to 6.8 cases per 100,000 in 2022. The rise is due to better diagnostic tools and more awareness and knowledge of the disease.

Cryptosporidiosis was the most common notifiable parasitic disease in 2022. According to a study published in the journal BMC Infectious Diseases, the notification rate is high compared to other European countries.

Between 2018 and 2022, 3,684 cryptosporidiosis cases were reported to the national mandatory notifications system (SmiNet), of which 2,639 were domestic, 950 had travel history, and 95 had no information. During this period, 1,850 samples were sent to the Public Health Agency of Sweden (Folkhälsomyndigheten) for typing, and 1,654 were further analyzed.

The majority of cases were reported between July and December. The submitted samples ranged from 3 months to 98 years old. A total of 57 percent were from women and 43 percent from men. Adults aged 25 to 44 had the highest notification rate from 2018 onwards.

Outbreak details

More than a dozen outbreaks were identified between 2018 and 2022. Most were foodborne, and some were due to contact with infected animals. Two large outbreaks affected more than 100 people. In 2019, unpasteurized contaminated juice was behind 122 illnesses, and in 2022, frisée salad was the suspected source of 107 illnesses.

All outbreaks except one were caused by Cryptosporidium parvum. A Cryptosporidium mortiferum outbreak affected three people in October 2019.

Sweden is currently experiencing an outbreak of Cryptosporidium with 76 cases. Officials believe the source of infection is a type of fresh food that is no longer available in stores.

Since mid-December 2023, infections have been reported from 14 regions, mainly from Halland and Jönköping. Typing of 16 samples showed that 13 belonged to the same type of Cryptosporidium, which indicates that cases have a common source of infection. Of outbreak cases, 73 percent are women, the median age is 41, and 81 percent are in the 20 to 59 age group.

In the study, subtyping revealed seven subtype families of Cryptosporidium parvum and 69 different subtypes, including 11 new ones. Several Cryptosporidium parvum subtypes and many different subtypes and subtype variants were commonly detected, suggesting a high level of diversity.

During the COVID-19 pandemic in 2020, when travel abroad was restricted, no domestic Cryptosporidium hominis infections were found, suggesting that many cases are contracted abroad and occasionally cause secondary domestic transmission.

Researchers said the approach to detect cryptosporidiosis may need to change from suspicion of parasitic infection to more symptom-based diagnostics, which may increase disease detection in Sweden and other countries.