Comparison of methods for counting third stage larvae of gastrointestinal nematodes of small ruminants in tropical pastures.

The accuracy and the precision of a simple and reliable technique for the extraction and the counting of third stage larvae densities of gastro-intestinal strongyles of ruminants from pasture samples are assessed in tropical conditions, i.e. for pangola pastures and for the genera of Haemonchus and Trichostrongylus. To separate the larvae from herbage samples, water washings obtained by a centrifuge spin-dryer were put to sediment in disposable plastic bags. The extraction and counting of larvae from sediments were achieved by exhaustion of sediments with repeated sucrose/water interface procedures. The recovery rate of third stage infective larvae (L3) added to the sediment was 79.1% (+/- 9.49%). The accuracy, estimated as recovery rate of L3 added to herbage samples, was 76.5 (+/- 11.31%). The recovery by the sedimentation method was 3.5 times higher than that of a filtering method through a 20 microns sieve. The detection limit was estimated to be 130 L3 kg-1 dry matter. Precision, estimated as the residual standard deviation of duplicate assays, was 1857 L3 kg-1 dry matter (mean 9164 L3 kg-1 dry matter). Results obtained in routine epidemiological surveys of gastro-intestinal strongylosis of small ruminants in West Africa and in the West Indies demonstrated the usefulness of the technique.