Schistosomiasis vaccine
Schistosomiasis is a major health concern affecting approximately 200 million people globally, primarily in regions with contaminated freshwater sources. This disease, caused by parasitic worms known as schistosomes, is transmitted through contact with infected water, particularly affecting children and women who engage in activities like playing or washing in these areas. Current treatment relies on the drug praziquantel, which alleviates symptoms but does not prevent reinfection, highlighting the need for a vaccine. Several vaccine candidates have been explored, focusing on antigens from schistosomules and schistosomes, with some clinical trials showing promising immune responses. However, no vaccine has yet demonstrated consistent efficacy in providing lasting immunity. The continuous research into Schistosoma genomes and the potential for combination therapies offers hope for future vaccine development. Addressing schistosomiasis with a successful vaccine could significantly reduce morbidity and mortality associated with this disease, which ranks just behind malaria in terms of prevalence and impact.
Subject Terms
Schistosomiasis vaccine
Definition
Several vaccines have been developed for the prevention of schistosomiasis, a snail-transmitted, waterborne, parasitic disease. The World Health Organization (WHO) estimated in 2023 that roughly 251.4 million people required preventative treatment for the disease in 2021. No vaccine, however, has been able to produce consistent immunity. Antigens from schistosomules and schistosomes show some promise.
Clinical trials by the Pasteur Institute have resulted in good immune response for volunteers in Niger and Senegal. The late discovery of several Schistosoma genomes continues to expand opportunities for schistosomiasis vaccine research. Cocktails (combination therapies) of recombinant antigens also offer hope for a vaccine.
Immunization
The control of schistosomiasis infection depends on drug treatment, which has effectively reduced morbidity. Immunization is still much needed to complement drug therapy because of a high reinfection rate. Hope for successful immunization through vaccination is based on some degree of natural protection seen in endemic human populations and the partial success of prototype vaccines.
Pathology
Schistosoma is a parasitic helminth (worm) carried by snails and is transmitted to humans through contact with contaminated freshwater sources. There are three major species. S. mansoni is found in North Africa, Arabia, and parts of South America. S. haematobium is found in the Middle East and Africa. S. japonicum, which causes the most severe infections, is endemic to China and Southeast Asia. Children who play in contaminated water and women who wash in contaminated water are most at risk. Chronic infections may cause anemia, kidney failure, liver damage, growth retardation in children, and bladder cancer.
Pathogenicity
Freshwater snails infected with schistosomiasis release larvae that enter the body through breaks in human skin. The free-swimming larvae then lose their tails and become schistosomules that migrate through the venous system and lay their eggs. Eggs that remain in the body stimulate the formation of scar tissue in the form of fibrosis or granulomas that may lead to organ failure. Other eggs exit the body through urine or feces to hatch in freshwater and continue the cycle of infection and reinfection.
Impact
There is a definite need to develop a vaccine to complement drug therapy for schistosomiasis. Schistosomiasis continues to be a major health problem in endemic areas around the world and is second only to malaria in disease occurrence. The WHO estimated in 2023 that schistosomiasis causes at least 11,792 deaths worldwide each year. The treatment drug of choice, praziquantel, reverses pathology, but it comes with concerns of the emergence of drug-resistant strains of schistosomiasis.
Bibliography
"Schistosomiasis." World Health Organization, 1 Feb. 2023, https://www.who.int/news-room/fact-sheets/detail/schistosomiasis#. Accessed 4 Feb. 2025.
McManus, D. P., and A. Loukas. “Current Status of Vaccines for Schistosomiasis.” Clinical Microbiology Reviews 21 (2008): 225-242.
Plotkin, Stanley A., Walter A. Orenstein, and Paul A. Offit. Vaccines. 5th ed. Philadelphia: Saunders/ Elsevier, 2008.