Of the neglected tropical diseases (NTDs) affecting the African continent, Schistosomiasis, also known as Bilharzia, remains a significant public health challenge endemic in more than 40 countries, Dr Nada Abdel Aziz, Fellow of the African Research Initiative for Scientific Excellence (ARISE) programme and Lecturer at the Faculty of Science, Cairo University, Egypt explains more about the disease.

Q: How widespread is schistosomiasis as a public health problem in the African region?

A: Schistosomiasis is a significant public health challenge in Africa, where more than 90% of the global burden is concentrated. Over 200 million people worldwide are affected, with at least 700 million more are at risk of infection. The disease is endemic in more than 40 African countries, especially in sub-Saharan Africa, where many communities rely on contaminated freshwater sources for daily activities such as bathing, fishing, and farming. The prevalence is highest in regions with poor sanitation and inadequate access to clean water, which facilitate continuous transmission. Efforts such as mass drug administration (MDA) programs have been implemented, but reinfection remains a major issue due to persistent exposure to infected water bodies.

Q: How is schistosomiasis transmitted?

A: Schistosomiasis is transmitted when individuals come into direct contact with freshwater contaminated with Schistosoma larvae (cercariae). These larvae are released by infected snails, which act as intermediate hosts. When people bathe, swim, fish, or collect water in these environments, the larvae penetrate their skin and migrate through the bloodstream, eventually settling in blood vessels around the bladder (S. haematobium) or intestines (S. mansoni and S. japonicum). Inside the body, the parasites mature into adult worms, mate, and lay eggs, which are excreted through urine or faeces. If human waste containing eggs contaminates freshwater, the cycle continues as the eggs hatch into larvae that infect This mode of transmission explains why poor sanitation, lack of clean water, and frequent human-water interactions are key risk factors for the disease.

Q. What’s the relationship between schistosomiasis parasites and water bodies?

A: Schistosomiasis is closely linked to water bodies, as these serve as the primary habitat for the freshwater snails that act as intermediate hosts. The disease is commonly found in regions with large lakes, rivers, irrigation canals, and reservoirs, where infected snails release infectious cercariae into the water. People living near Lake Victoria, the Nile River, and the Senegal River Basin have a high risk of infection due to frequent contact with contaminated water for domestic, agricultural, or recreational activities.

Q: How is schistosomiasis different from other neglected tropical diseases (NTDs)?

A: Schistosomiasis is unique due to its requirement for an aquatic environment for transmission and its lifecycle involving an invertebrate host—specific freshwater snails. The disease also leads to chronic conditions with systemic effects, contrasting with some NTDs that might focus more on localized infections. The ecological aspect of control, such as the necessity for snail management, sets it apart from other neglected tropical diseases.

Q. What are the long- and short-term impacts on vulnerable groups such as children and pregnant women?

A: Schistosomiasis has severe short- and long-term effects, particularly on children and pregnant women. In the short term, children experience fatigue, anaemia, malnutrition, and developmental delays, which affect their ability to concentrate and perform well in school. Chronic infections lead to stunted growth and cognitive impairment, making it difficult for affected children to reach their full potential. For pregnant women, schistosomiasis increases the risk of anaemia, low birth weight, preterm labour, and pregnancy complications. The disease is also linked to female genital schistosomiasis, which can cause infertility, chronic pain, and increased susceptibility to sexually transmitted infections, including HIV. In the long term, untreated schistosomiasis can result in severe liver fibrosis, kidney damage, bladder cancer (S. haematobium), and even death due to complications from organ damage.

Q. How is schistosomiasis managed so far since most neglected tropical diseases don’t have a cure?

A: Management strategies include snail control programs, which use molluscicides to reduce the snail population, as well as improved water, sanitation, and hygiene (WASH) initiatives to prevent freshwater contamination. The primary treatment for schistosomiasis is praziquantel (PZQ), a highly effective drug that kills adult worms. To control the disease, Mass Drug Administration (MDA) programs distribute praziquantel to school-aged children and high-risk populations. However, a major challenge is that PZQ does not eliminate juvenile parasites, prevent reinfection, or control schistosomiasis-induced tissue damage. Therefore, there is an urgent need to develop an effective vaccine and therapeutic interventions that can prevent reinfection and mitigate tissue destruction, respectively.

Q. Have there been vaccine developments for schistosomiasis, and what has been their progress so far?

A: Vaccine development for schistosomiasis is ongoing, with several candidates in the early stages of clinical trials. Research has focused on identifying antigens that can invoke strong and protective immune responses, showing promising results in preclinical studies. However, as of now, no schistosomiasis vaccine has been commercially licensed for use.

Q. What are the challenges affecting vaccine development for the disease?

A: Vaccine development for schistosomiasis faces multiple challenges. One major obstacle is the complexity of the parasite’s life cycle, as Schistosoma species have evolved sophisticated immune evasion mechanisms that make it difficult to develop a lasting immune response. Another challenge is limited funding, as schistosomiasis is a neglected tropical disease (NTD) that does not attract as much investment as diseases like malaria or tuberculosis. Additionally, the genetic variability of different Schistosoma species also poses a challenge, as an ideal vaccine would need to protect against multiple strains.

Q. Is there any prospect for a lasting cure?

A: Although praziquantel is an effective treatment, it does not provide lifelong immunity or prevent reinfection. A lasting cure would require either a highly effective vaccine or a new drug capable of eliminating both adult and juvenile parasites and mitigating tissue inflammation. Research on host-directed therapies and immunomodulatory treatments is ongoing, offering potential avenues for a more comprehensive cure.

Q. With regards to your work on this disease, what have been some of the key findings?

A: My research focuses on vaccine development and host-directed therapies to combat schistosomiasis, particularly Schistosoma mansoni (parasites leading to intestinal schistosomiasis). Our work has shown a promising vaccine formula that significantly protected mice from S. mansoni infection. In particular, we have examined papain-based vaccination, which has demonstrated potential in enhancing protective immune responses against schistosomiasis.

Additionally, one of my key findings is the role of Foxp3+ T regulatory (Treg) cells (white blood cells that control the immune system’s response to harmful substances) in controlling inflammation during infection. By understanding how these immune cells modulate host responses, we can explore therapeutic strategies that minimize tissue damage caused by chronic infection. Another significant finding is the identification of key immune signalling pathways and cellular responses that contribute to host protection and pathology, providing insights into possible targets for drug development. These discoveries contribute to the broader effort to develop effective treatments and vaccines for this debilitating disease.

Q. What can be done to help in the eradication of the disease from research, community, and political levels?

A: At the research level, efforts should focus on accelerating vaccine development, identifying new drug candidates, and improving diagnostics to detect low-burden infections. At the community level, increased investment in health education, behavioural change campaigns, and improved sanitation infrastructure is essential to break the cycle of transmission. At the political level, governments must allocate more funding for control programs, strengthen healthcare policies, and collaborate with international organizations such as WHO. A multi-sectoral approach combining research, community engagement, and strong political commitment is crucial for the ultimate eradication of schistosomiasis.

About Dr Nada Abdel Aziz
Dr Aziz is a Lecturer at Faculty of Science, Cairo University, Egypt, and a Fellow of the African Research Initiative for Scientific Excellence (ARISE) programme. 
ARISE is an innovative research and innovation support programme of the African Academy of Sciences (AAS), implemented by the AAS in partnership with the African Union (AU) the European Union (EU). ARISE is funded by the European Union and co-funded by the Carnegie Corporation of New York.

Original article written by Sharon Atieno and published in Science Africa.