Survival of first-stage Neostrongylus linearis larvae in ovine faeces under environmental conditions in Galicia (north-west Spain).

A study was made on the survival of first larval stage of Neostrongylus linearis, from November 1990 to October 1991, under natural conditions in an inland locality in Galicia (North-West Spain). The faeces were obtained from a sheep naturally infected with this nematode. Once a month, faeces were placed on a 0.5 x 0.5 m plot, in natural conditions, until we had 12 deposits. Samplings were done weekly until there was no more faecal matter in the plots. Larval survival was determined using the Baermann migration technique, and we calculated the number of larvae per gram after the transformation of the weight of the faecal sample, in order to discard weight variations owing to climatic conditions. Over the whole period of this study, the percentages of larval survival in the faeces of the 12 deposits changed from the first week onwards. In those samples that were left in the pasture more than three weeks, survival was less than 50%. After the seventh week only very small numbers of active N. linearis larvae were obtained. We observed a positive influence of relative humidity on survival during the first week's presence of the faeces in the pasture. In the same way, a statistically significant negative correlation was proved between larval survival in faeces and temperatures during the first three weeks post-deposit.