Researchers collecting snails along the banks of Lake Victoria

Deadly disease hotspots identified by four-year study

By James McNish

First published 18 July 2017Last updated 18 July 2017

A study of aquatic snails that carry the parasitic disease schistosomiasis has identified infection hotspots around part of Africa's Lake Victoria that could be targeted to reduce infection.

The four-year project was carried out by Museum scientists in conjunction with the National Institute for Medical Research in Mwanza, Tanzania.

Dr Anouk Gouvras, a Museum research scientist who worked on the project, says, 'Prioritising snail control at places such as fish markets, ferry and commuting sites, and major fishing locations could reduce transmission of the parasite in these high-risk areas.'

A deadly disease

Schistosomiasis, also known as bilharzia, is a parasitic disease that can cause intestinal or urinary problems, with long-term infection leading to irreversible damage to vital organs.

The disease makes washing, bathing or swimming in natural water a real risk to the health of people who live in affected areas.

Over 200 million people worldwide are estimated to be affected, with over 90% of those living in sub-Saharan Africa.

The Museum is part of an international effort to tackle the disease, with the World Health Organisation (WHO) aiming for schistosomiasis to be eliminated as a global health problem by 2025.

Strategies to control the disease include improving sanitation and access to safe freshwater, treating at-risk communities with medicine and engaging and educating communities. Snail populations can also be controlled by a variety of methods, from targeted snail pesticides and snail predators to environmental modifications such as draining marshy habitats and using concrete-lined irrigation canals for crop cultivation.

A cycle of infection

Schistosomes, the parasitic flatworms that cause the disease, have a complex life cycle that involves living in both human and water snail hosts.

Infected snails shed the schistosome larvae into rivers and lakes. Humans bathing in the water are then infected through their skin. The flatworms develop inside the human and lay eggs, which are passed out through the person's urine or faeces.

Poor sanitation can lead to human waste to enter the water supply, where the flatworm eggs hatch and infect snails. So the cycle continues.

Studying the snails

The researchers studied an area in the Mwanza region of Tanzania, around the south shore of Lake Victoria. This area has a high prevalence of schistosomiasis and local communities are part of a programme to control the disease through preventative medicines.

Map showing the sites sampled around Lake Victoria — The research sampled 26 sites at 15 villages on the southern shore of Lake Victoria

The scientists investigated two species of snail that carry the parasite in the region - Biomphalaria sudanica, which lives on the banks of the lake and surrounding marshes, and Biomphalaria choanomphala, which lives in the lake itself.

The researchers wanted to see how the total snail population, and the percentage of infected snails, varied throughout the year, in wetter and drier seasons.

Over four years, the researchers collected almost 43,000 snails from 26 sites and examined them to see how many had the parasite.

The field work team sifting for aquatic snails — Dr Anouk Gouvras and field team member James Kubeja collecting shore-dwelling snails

A tale of two snails

The scientists found that though the total number of snails varied with the season, the percentage of infected snails did not significantly change across a year. On average, just over 1% of the snails collected were infected.

They found the shore-dwelling Biomphalaria sudanica at all the sites, but the lake-living Biomphalaria choanomphala at only a few.

According to Dr Gouvras, this is good news, if it is decided to control the snail population.

'Biomphalaria sudanica would be the more important snail to focus on as it was found at all sites. As it lives on the shore of the lake, it will be much easier to reach than the deeper dwelling Biomphalaria choanomphala.

'It would be best to use snail control technologies in the dry season, as that is when we found the snail population to be at its greatest.'

Snails found by dredging the lake — The field team sorting through deeper-dwelling snails, collected by dredging in the lake

Targeting schistosomiasis hotspots

The study discovered that the research sites with most infected snails were also the sites with the highest levels of human population movement - from fishing, trading or commuting.

This could be because areas with high human activity are more likely to have more infected human waste entering the water supply and, as a result, more infected snails.

Dr Gouvras thinks a two-level strategy could be used to tackle the disease in the region:

'Local communities in low-risk areas could continue to be treated with preventative medicines. Meanwhile the full range of control measures - medicines, community engagement, improved sanitation and snail control - could be used in the high-risk areas.'

The aquatic snail Biomphalaria sudanica — The shore-dwelling Biomphalaria sudanica snail, found at all survey sites

Are control measures helping?

While the numbers of infected snails was constant in the first three years of the study, in the fourth year there was a significant reduction.

According to Dr Gouvras, this may be due to the preventive medicine programme hindering the cycle of transmission.

'If this is the case, it is interesting that it has taken at least three years for a decline to be seen. Further research could see if there is a clear link between local preventative strategies and the number of infected snails.'

The project was funded by the Schistosomiasis Consortium for Operational Research and Evaluation (SCORE), who work to answer strategic questions about schistosomiasis control and elimination.