Holiday photos and specimens trace source of disease outbreak

The research appears this week in Lancet Infectious Diseases and was led by the University of Perpignan.

Schistosomiasis, also known as bilharzia, is an infection caused by a parasitic worm that is spread in lakes and rivers by specific fresh water snails. The urogenital form of the disease can be spread by urine from infected people contaminating fresh water. The vast majority of cases occur in Africa.

The European outbreak was first reported in 2013, when doctors in France and Germany diagnosed the disease in holidaymakers who had been swimming in rivers in Corsica. To identify the source of the outbreak researchers used holidaymakers’ photographs and questioned local Corsicans who had also been infected. They narrowed down the source to popular bathing areas of the river Cavu in southern Corsica.

By comparing  DNA from parasite eggs and larvae from urine samples of infected patients against specimens in the Museum’s collection, researchers traced the parasite to Senegal. This suggests it was introduced to the island via infected individuals bringing it from West Africa.

Dr Bonnie Webster, researcher at the Museum, whose work uncovered the origins of the parasite, says that the incident serves as a warning about emerging diseases in Europe:

“This recent outbreak in Corsica demonstrates the potential for forms of this debilitating and potentially deadly infectious disease to become rapidly established in Europe.

“Our findings clarify how this outbreak happened and highlight freshwater areas that should be monitored closely for further transmission, as well as the need for increased awareness among medical professionals of the potential presence of infection in Europe.”

In the short term schistosomiasis can cause symptoms such as bloody urine and fever, but if left untreated it can cause permanent organ damage. More than 200 million people worldwide are infected, 85 per cent of whom live in Africa.

The Natural History Museum holds the SCAN collection, one of the world's largest archives of schistosomiasis material. Dr Webster explains: “Each worm species is found in particular geographical areas with some exhibiting specific geographical genetic differences that can help us to trace where they have come from.”

The DNA of parasites collected from the infected patients showed that the species present in Corsica was an extremely close match to the species found in Senegal.

As a former French colony, there are many links between France and Senegal, with people frequently moving between the two countries. Infected humans can therefore introduce the parasite from Senegal to Corsica, where the necessary host snail is present and can support the lifecycle.

The outbreak has lessons for Europe to be on the watch for diseases to become established in Europe. The Corsican outbreak first occurred in 2013 with over 120 infections diagnosed, and was followed by an awareness campaign to diagnose infections and eliminate transmission. However, at least one case was identified in 2015, signaling that the disease was still present in Corsica.

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Schistosomiasis life cycle

The urogenital schistosomiasis parasite has a complex lifecycle, involving both a species of freshwater snail and humans as its hosts. Infected humans release eggs into water through urination. The eggs hatch and infect suitable snails, where they develop and replicate. The larvae are then released back into the water, where they can infect bathing humans by burrowing though the skin.

The host snail is prevalent in many areas around the Mediterranean, including in the suspected locations of the outbreak in the Cavu river.