Schistosomiasis, also known as bilharzia, is the second most socio-economically devastating parasitic disease after malaria, affecting millions of South Africans. Caused by parasitic worms which rely on specific snails as intermediate hosts, recent research has explored how climate change might impact the distribution of these snails, and, consequently, the spread of the disease.
Dr Nisa Ayob, a researcher at the North-West University’s (NWU’s) Geography and Environmental Science subject group, has conducted a study to predict future changes in the habitats of these snails, using ecological models and climate data.
"Our goal was to understand how shifts in temperature and rainfall due to climate change could affect where these snails live and thrive," she explains.
"This study provides crucial insights for public health planning," says Dr Ayob. "Understanding these shifts can help in developing strategies to prevent the spread of schistosomiasis into new areas."
In South Africa about four million people are at risk, mainly children. According to the National Institute for Communicable Diseases, the prevalence in children in some places is up to 95%, but the total number of infected people is unknown, even though schistosomiasis is a notifiable disease.
The disease is mainly found in Limpopo and Mpumalanga; the north and east of Gauteng; the lower-altitude areas of KwaZulu-Natal, extending along the coast into the Eastern Cape to Gqeberha. All neighbouring countries, except Lesotho, have transmission areas.
What the research results revealed
The findings of the study revealed that temperature and rainfall have historically influenced the distribution of schistosomiasis-transmitting snails in South Africa. Using the random forest model, the study effectively mapped the habitats of three snail species, which are currently found mainly in the eastern and coastal regions of South Africa.
Future projections indicate a shift in the habitats of these snails. The species known as Bulinus africanus may see a decrease in suitable habitats across several provinces, but could expand into the south-west coast and central Free State. The most commonly found species, Biomphalaria pfeifferi, might face reduced suitability in the eastern regions, but could find new habitats in the Free State, Northern Cape and western areas. The third species, Bulinus globosus, could experience habitat declines, but might adapt to suitable climates in the Free State and KwaZulu-Natal regions.
Study enables better monitoring
One significant achievement of the research was the creation of a geospatial database of historical snail occurrences, digitising over 15 000 records. "This digital archive supports continuous research and long-term monitoring of snail distribution changes," notes Dr Ayob.
Moreover, by integrating fine-scale historical snail data with high-resolution climate data, the study improved the accuracy of predicting snail habitat suitability. "This approach enhances the precision of our models, ultimately guiding targeted interventions to control and manage schistosomiasis."
This research highlights the importance of considering climate change in disease management strategies. The projected changes in snail habitats suggest that warming climates may create new environments for these snails, potentially altering the transmission dynamics of schistosomiasis. By anticipating these changes, public health officials can better prepare and implement measures to protect at-risk populations from this disease.
How the disease spreads
The disease is transmitted by contact with contaminated fresh water, inhabited with swimming forms of the parasite called cercariae which emerge from the snails. If they come into contact with human skin, they attach and penetrate. Once inside the body they live in the blood vessels of the urogenital organs or the large intestine.
Female worms release thousands of eggs that are passed out of the human body in the urine and faeces. If infected urine or faeces contaminates bodies of fresh water, the eggs hatch, releasing the parasite stage that seeks out and infects snails, and the cycle begins again.
However, many schistosome eggs remain in the body and cause inflammation and damage in organs such as the bladder, female reproductive system, intestines, liver, spleen, lungs, and sometimes the brain and the spinal cord. Fortunately, it can be treated with medication.
Dr Nisa Ayob.
