Hybrid schistosomes from cattle complicate human disease control

Effective control of bovine schistosomiasis will be required to meet disease elimination targets in humans, new research shows.

A study led by Liverpool School of Tropical Medicine (LSTM) and the Malawi Liverpool Wellcome Research Programme (MLW), published in One Health, shows that hybrid human schistosomes - the parasites that cause the disease - are regularly emerging from cattle.

This threatens the World Health Organization's (WHO) targets to eliminate urogenital schistosomiasis as a public health problem by 2030 in sub-Saharan Africa. Hybrid human schistosomes can also abruptly mutate and change their genetic make-up, increasing the risk of transmission and reinfection.

The study focused on urogenital schistosomiasis in Malawi. It is the first to demonstrate the scale of the bovine schistosomiasis problem and, by applying new molecular diagnostic tests, show cattle as a primary hybrid schistosome infection source.

These are important findings. In short, we have shown that without effective future disease control in livestock, sustainable disease control in humans will be difficult. Our One Health approach to find hybrid schistosomes between Schistosoma haematobium and Schistosoma mattheei adds to our scientific understanding, not only in Malawi but also in countries nearby. The findings will have important consequences in revising policy-level discussions and demonstrate the intricate links between disease, agriculture and food production." Dr. Alexandra Juhasz, livestock veterinarian, lead author of the paper and Post Doctoral Research Associate at LSTM

Schistosomiasis is a neglected tropical disease that affects over 240 million people. In 2021, the WHO launched a new road map for schistosomiasis, which targets the disease for elimination as a public health problem by 2030.

Current control strategies are based on providing regular access to community-distributed medicines, but if infections are acquired from non-human sources, however, such disease control strategies are unlikely to be effective.

Study

The new study was funded by the Wellcome Trust, led jointly 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 novel DNA diagnostic tests and implementation of cutting-edge GPS animal datalogging. 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-USA-Malawi researchers studied several herds of cattle across three districts 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 people with urogenital 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 shoreline of Lake Malawi. This demonstrated that even when cattle are treated with deworming medicine they become reinfected within three months.

These spatial maps of cattle movements better revealed seasonal watering and grazing practices, pinpointing where hybrid schistosome infections were acquired by people making use of the lake. Such precision mapping will become evermore important within research and control of zoonotic schistosomiasis in sub-Saharan Africa.