The length of strongylid nematode infective larvae as a reflection of developmental conditions in faeces and consequences on their viability.

Isolates from eight gastro-intestinal nematode species of sheep, cattle and deer parasites were maintained in sheep and faecal cultures were processed at different temperatures (for four species) and moistures (for the eight species). For all the tested species, the third-stage larvae (L3) were longer at temperatures optimal for development and their length increased linearly with faecal moisture contents (FMC). These variations in length mainly concerned the volume of intestinal cells. The length of L3 had no consequence on their survival in water at room temperature (five species tested), but the short L3 migrated more slowly on agar gel than did the longer ones (verified for three species) and exsheathed more slowly (verified for two species). The establishment rate observed for short L3 of Teladorsagia circumcincta in lambs was lower than that seen for long L3 in three other lambs, but the developed worms were similar in length and fecundity. These results are relevant to our understanding of how the different species could adapt to and/or compensate for adverse conditions of development.