Anionic detergent and chlorine concentrations in polder ditches in a former malarious area: The island of Walcheren, province of Zeeland, the Netherlands
Abstract Since 1958, malaria is no longer autochthonous in The Netherlands. One of the factors contributing to this fact must have been the pollution of water with detergents in the breeding sites of the anopheline mosquitoes. In 1968 it was found that this mosquito population had decreased considerably compared with the years just after the second world war, except on the island of Walcheren, located in the province of Zeeland, and a former malarious area. In June 1969 about 400 water samples from the island were analysed for detergent concentration, and enough suitable breeding sites were found to support the anopheline population. However, in October 1969, 153 pig farms were searched for adult mosquitoes semi-hibernating in these farms. The number of mosquitoes had decreased compared with the mosquito population in the same pigsties in 1968. This could not be caused exclusively by the intensified use of insecticides. The amount of rain in July, after the water samples were taken, was much lower than normal, so that the detergent concentration in the water of the breeding sites must have risen to a degree in which the larvae could not survive. The decrease of the malaria vector is so far the only positive side-effect to which water pollution by detergents has contributed.