Pesticides increase the risk of schistosomiasis outbreaks

in Uganda, where schistosomiasis is endemic, insecticides are constantly sprayed on coffee trees. Photo Source: REUTERS/Hereward Holland
Pesticides are a double-edged sword: they make agriculture more productive, but if used improperly, they can harm wildlife and humans. Now ecologists have identified a new threat from pesticide use in developing countries. By killing the predators of worm parasites, they increase the risk of an outbreak of schistosomiasis, the second most common parasitic disease after malaria.
, an ecologist at the University of South Florida, and colleagues created simple ecological models in 60 open water tanks. After loading each tank into 800 litres of pond water, they put in two snails that spread the schistosomiasis parasite, seaweed for snails, and two predators, crayfish and ostentails. Finally, the researchers added three different dose combinations of agrochemicals - fertilizers, herbicides and pesticides - to the tank. Their concentrations are characteristic of streams and ponds near Cornfields in the United States.
, fertilizer increases the amount of seaweed in the tank, which in turn increases the number of snails. Herbicides also bring more food to snails because they kill large amounts of tiny algae shrouded in water. When these algae die, the water clears, allowing more light to reach the larger algae, or snail's food, that grow at the bottom of the pond. Epidemiological patterns of schistosomiasis show that this typical amount of fertilizer leads to an increase in the number of snails, which increases the risk of the disease spreading to humans by 20 per cent.
the insecticide "poisoned ticks" played a bigger role by killing the two predators of snails. The oscarp will insert its head into the snail's shell, bite the moppy, spray the digestive enzyme, and eat the rest. The 20cm-long crayfish relies on violence to crush the 2cm-long snail directly. "They're absolutely very greedy." Rohr said. However, when these predators disappear, the number of snails increases dramatically. In this case, the risk of schistosomiasis spreading to humans increased tenfold. The team recently reported the findings on a
life sciences preprinted website.
although the researchers added only one concentration of insecticide to the tank, the model showed that lower concentrations of insecticides in ponds could still have a substantial impact on the spread of the parasitic disease. (Source: Science Network Zonghua)