Seasonal development and survival of equine cyathostome larvae on pasture in south Louisiana

Cyathostome development and survival on pasture in subtropical climates of the US have yet to be completely defined and available data on seasonal transmission are minimal. In an attempt to study this phenomenon, a group of pony mares and their foals was maintained on a naturally contaminated pasture in southern Louisiana. Fecal egg counts (FEC) and numbers of infective third stage larvae (L₃) kg^-1 dry herbage were recorded biweekly during two time periods, from January 1986 through December 1988, and September 1996 through October 1997. A FEC rise occurred during the late summer-early autumn which preceded the peak of L₃ on pasture during the winter season. The numbers of cyathostome L₃ were reduced during the hottest months of the year due mainly to daily minimum temperatures above 18°C, and in winter during short freezing spells when daily minimum temperatures dropped below 0°C. Tilling of the pasture reduced the number of cyathostome L₃ during the early winter months but this is an efficacious measure only if horses are given an effective anthelmintic treatment prior to being returned to pasture. The data collected suggest that parasite reduction in southern Louisiana is possible using a treatment program with treatment beginning at the end of September and continuing through the end of March. Copyright (C) 2000 Elsevier Science B.V.