New research highlights that even low levels of human contact with open freshwater sources are associated with an increased likelihood of infection with water-borne parasites. The results are published today in PLOS Neglected Tropical Diseases.
Schistosomiasis is a poorly understood tropical disease that is estimated to affect over 250 million people with approximately 779 million people at risk globally. Over 90% of these individuals live in sub-Saharan Africa.
The disease is caused by a parasitic fluke (worm) that lives in the blood stream and is transmitted via human contact with freshwater sources. Water bodies such as lakes, rivers, and streams are inhabited by freshwater snails which release the infectious stage of the parasite. Infections can cause severe liver and spleen damage or bladder cancer, depending on the species of the parasite.
The World Health Organization (WHO) focuses treatment on high-risk groups such as school-age children and fishermen. However, it is not clear to what extent water contact can explain who is at high infection risk. Populations in areas where the infection is common may acquire some immunity through past exposures. The water contact activities people engage in, such as fishing, bathing, and doing laundry, vary between age groups and genders and different activities carry different risks. Infection also depends on environmental conditions that influence the number of snails in the water.
Researchers from the University of Oxford’s Big Data Institute carried out a systematic review of 101 studies involving 192,691 participants in 22 countries across three continents (Africa, Asia, and South America), published between 1984 and 2022. Among the 101 studies, 98 provided data on associations between water contact duration, frequency, and activities with schistosome infection. By analysing these data, the researchers were able to quantify the overall association between water contact and infection, whether the association varied across age groups, and whether different activities carried different risks.
Key findings:
- People who had any current water contact were 3.14 times more likely to be infected when compared to people without any water contact irrespective of frequency of water contact;
- Almost all water contact activities were significantly associated with infection and there was no evidence that engaging in occupational activities such as fishing was linked to higher infection likelihood than conducting domestic or occupational activities, suggesting that the categories of people currently considered high-risk should be reconsidered and that community-wide treatment and prevention may be more appropriate;
- Even though children routinely have the highest schistosome infection prevalence, this review found that water contact was not more strongly associated with infection in children compared to other age groups, suggesting that levels of exposure alone may not explain the high prevalence in children;
- Water contact patterns were only associated with an increased likelihood of infection in areas where more than 10% of the population had a schistosome infection;
- The type of water body (stagnant versus flowing or natural versus artificial) which people used had no significant influence on the association between water contact and schistosome infection.
Associate Professor Goylette Chami, the senior author of the study, said ‘This study is, to our knowledge, the most comprehensive assessment of exposures to schistosome parasite acquisition and infection likelihood and provides a detailed database for assessing the burden of schistosomiasis prevalence. I anticipate these results will inform future research into the spread of the infection and updates to the Global Burden of Disease.’
Fabian Reitzug, DPhil student and the study’s first author, said ‘This review summarises what we know about exposure and infection from the published literature, but critically, also highlights gaps in our understanding. For instance, despite this being a longstanding hypothesis, it remains unclear whether higher levels of exposures are associated with a higher intensity of schistosome infection. Very few studies reported on this.
‘But, what is exciting is that wearable GPS loggers, a relatively novel technology, will provide new ways to revisit this old question. The SchistoTrack project, a collaboration between the Uganda Ministry of Health and Oxford Population Health, has collected detailed human movement data on 600 participants and will explore this avenue further.’
The study authors suggest that Water Sanitation and Hygiene (WASH) infrastructure may need to be targeted at all areas where at least 10% of people are infected (as water contact was significantly associated to infection risk above this prevalence threshold). As almost all water contact activities were associated with increased infection likelihood, WASH interventions may benefit from broadening their scope beyond the provision of safe drinking water. They may need to aim at minimising all different types of open water contact (domestic, recreational, and occupational) to successfully suppress community-level transmission.