Cattle hybrid schistosomes complicate disease control in humans

Effective control of bovine schistosomiasis will be required to achieve goals of eliminating the disease in humans, new research shows.

A study led by the Liverpool School of Tropical Medicine (LSTM) and the Malawi Liverpool Wellcome Research Program (MLW), published in One Health, shows that human schistosome hybrids – the parasites that cause the disease – regularly emerge from cattle.

This threatens the World Health Organization’s (WHO) goals to eliminate genitourinary schistosomiasis as a public health problem by 2030 in sub-Saharan Africa. Hybrid human schistosomes can also mutate rapidly and change their genetic makeup, increasing the risk of transmission and reinfection.

The study focused on genitourinary schistosomiasis in Malawi. It is the first to demonstrate the scale of the bovine schistosomiasis problem and, with the application of new molecular diagnostic tests, presents cattle as a major source of hybrid schistosomiasis infection.

These are important findings. In summary, we have shown that without effective future disease control in animals, sustainable human disease control will be difficult. Our One Health approach to finding schistosome hybrids between Schistosoma haematobium and Schistosoma mattheei adds to our scientific understanding, not only in Malawi but also in nearby countries. The findings will have important implications for revising policy debates and demonstrate the complex links between disease, agriculture and food production.”

Dr. Alexandra Juhasz, animal veterinarian, lead author of the paper and Postdoctoral Research Associate at LSTM

Schistosomiasis is a neglected tropical disease that affects over 240 million people. In 2021, the WHO released a new schistosomiasis roadmap, which aims to eliminate the disease as a public health problem by 2030.

Current control strategies rely on providing regular access to drugs distributed in the community, but if infections are from non-human sources, such disease control strategies are unlikely to be effective.

Study

The new study was funded by the Wellcome Trust and led by Professors Russell Stothard from LSTM and Janelisa Musaya from MLW. The latest report is the result of two years of detailed field surveillance using the careful application of new DNA diagnostic tests and the application of ground-breaking GPS animal data recording. These methods can track infections in cattle and their movements. It also highlights the evolutionary potential of schistosomiasis to adapt to expanding cattle production.

The multidisciplinary team of UK-US-Malawi researchers studied several cattle herds in three areas where almost half (49.3%) of the animals were found to have bovine schistosomiasis. Although hybrid human schistosome infections were present in only 1.8% of these animals, such cattle infect local aquatic snails which in turn infect a significant proportion of individuals with genitourinary schistosomiasis.

The team used a particularly novel real-time GPS satellite surveillance strategy to track and trace cattle movements over a three-month period on the Lake Malawi shoreline. This showed that even when cattle are treated with a deworming drug, they become reinfected within three months.

These spatial maps of cattle movements better revealed seasonal watering and grazing practices, identifying where Schistosoma hybrida infections were acquired by lake users. This precision mapping will become increasingly important in the context of zoonotic schistosomiasis research and control in sub-Saharan Africa.