Seasonal transmission of equine cyathostomes in warm climates

Few studies investigating the seasonal transmission of equine cyathostomes have been done in warm climates. Two Australian studies used experimentally -infected plots to determine hatching, development and survival of free liv ing stages of equine cyathostomes, Four studies in the southern United Stat es used pasture larval counts, and in some instances tracer animals, to det ermine seasonal availability of infective cyathostome larvae on naturally-i nfected pastures. With the exception of the dry Australian tropics, a gener al pattern of peak transmission of cyathostomes during the cooler seasons o f the: year and minimal transmission during the warmest seasons was observe d. Infective larvae and developing stages survived poorly in hot weather, a lthough the rate of development was most rapid during that time. In contras t, infective larvae and developing stages survived well in cool weather, al though the rate of development was slower. Adequate moisture was crucial to cyathostome transmission in warm climates, thus hot, dry weather effective ly sterilized a pasture, whereas cool, moist weather was optimum for transm ission. These data suggest that suppression of cyathostome egg output in fe ces of horses beginning shortly before the onset of cooler and/or more mois t weather, and continued through the favorable period for development and s urvival of larvae on pasture - usually the autumn and winter should provide adequate control of these parasites. However, the efficacy of such seasona l control programs has yet to be adequately tested against that of traditio nal year round treatments. (C) 1999 Elsevier Science B.V. All rights reserv ed.