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MARIJE BOKS - Long-term effects of an outbreak of Cryptosporidium hominis in Östersund, Sweden, 2010

PhD project participating in the National Research School in General Medicine.

In 2010, the Swedish city of Östersund was hit by an outbreak of the parasite Cryptosporidium hominis, caused by contaminated drinking water. An estimated 27.000 inhabitants (45% of the city’s population) showed symptoms of gastroenteritis. This research project focuses on the long-term effects of the Cryptosporidium outbreak. Do those who contracted clinical cryptosporidiosis report post-infectious symptoms a decade after the outbreak? Has the incidence of inflammatory bowel disease increased? We will also conduct a long-term health economic evaluation of the outbreak.

PhD Student

Marije Boks
Research student

Project overview

Project period:

Start date: 2023-01-01

Project description


Cryptosporidium hominis is a protozoan that causes gastroenteritis. Sequelae from the abdomen, joints and eyes might occur up to 5 years after the acute infection, but longer follow-up studies are lacking. Gastroenteritis can trigger inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). In 2010, Östersund was hit by a waterborne outbreak of Cryptosporidium hominis with approx. 27,000 people (45% of the city's residents) reporting symptoms of acute infection.

During the outbreak, a questionnaire was sent to 1,524 randomly selected residents and to an additional sample of 500 children between the ages of 0-5, with questions about new abdominal symptoms, joint pain and anxiety. The response rate was around 70%. Follow-ups were conducted after 6 months, 2 and 5 years. Individuals who reported clinical cryptosporidiosis in 2010, were more likely to report sequelae, mainly in the form of IBS-like and joint-related complaints. Local physicians also perceive that the incidence of IBD has increased since the outbreak.


The overall aim is to study the long-term effects of the Cryptosporidium outbreak. The main focus is on the occurrence of post-infectious symptoms 10 years after the outbreak. In addition, the incidence of IBD, and the long-term health economic consequences of the outbreak will be evaluated.


Studies 1 and 2 are prospective cohort studies. In January 2021, a new questionnaire was sent to all individuals who answered the outbreak questionnaire (n=1288). The response rate remained about 70%. Via logistic regression, it is investigated whether those who reported symptoms of acute infection during the outbreak, are more likely to report possible long-term symptoms, compared to those who were not infected, and whether these symptoms persist over time. For children 0-5 years of age, health care consumption and other diagnoses will also be examined. The results for adults and children will be published separately.

Study 3 is an observational study. In this study, the incidence of IBD before and after the outbreak is calculated using negative binomial regression analysis. Data is collected from SWIBREG, the national registry for IBD, and the electronic patient records.

Study 4 is a health economic analysis. For the years 2011-2019, data on individuals who responded to the outbreak questionnaire is linked to the Social Insurance Agency's database for sick leave and VAB (care of a sick child), the National Board of Health and Welfare's patient register and the health care region's database for care events in primary care. The costs are calculated using salary data from Statistics Sweden and prices from www.norrasjukvardsregionforbundet.se and compared between the group of individuals reporting acute symptoms at the time for the outbreak and those that did not report acute symptoms.


The project's goal is to contribute to a better understanding of the consequences of a Cryptosporidium hominis infection for both individual and society and in that way be able to influence the treatment of today's and future patients around the world. This is particularly relevant in light of climate changes, as waterborne diseases are expected to increase with rising temperatures.

University affiliation

Umeå University

Main supervisor

Malin Sjöström, associate professor, Umeå University.

Latest update: 2023-03-10