Guidelines for laboratory and field testing of molluscicides for control of schistosomiasis
The purpose of these guidelines is to provide specific, standardized procedures and criteria for efficacy testing and evaluation of molluscicides for schistosomiasis control. The aim is to harmonize the testing procedures carried out in different laboratories and institutions in order to generate comparable efficacy trial data required for registering and labelling such products by the national regulatory authorities.
Schistosomiasis, also known as bilharziasis, is caused by trematode worms of the genus Schistosoma. The lifecycle of the parasites involves mammalian hosts, including humans, and freshwater snails as intermediate hosts.
Molluscicides are chemicals, of synthetic or biological origin, used primarily to kill various species of molluscs, including intermediate host snails involved in the lifecycle of schistosomes. In the past, numerous molluscicides have been used in schistosomiasis control programmes (WHO, 1992). However, during the past 35 years niclosamide has been the most widely used compound. Nevertheless, there are certain limitations with the use of formulations based on niclosamide including restrictions on their potable water usage immediately after application and temporary high toxicity to non-target aquatic organisms including fish, amphibians and invertebrates (USEPA, 1999; WHO, 2002).
There is a need for new and safer compounds for use as molluscicides, especially with the renewed interest in control and elimination of schistosomiasis as recommended by resolution WHA65.21 adopted by the Sixty-fifth World Health Assembly in 2012. Although currently there is little evidence for chemical resistance in snail populations, it is necessary to search for new chemicals to replace those that may become less effective after several generations of applications in the field, or there could also be development of new active ingredients (AIs) that are more specific to snails hosting schistosomes.