One hundred and seventeen ponies were surveyed for the seasonal prevalence of strongyloid parasites, particularly cyathostomes, and for host- and age-related differences in these infections. For 56 ponies, all stages of the cyathostome life cycle, both mucosal and luminal, were enumerated. Total numbers of cyathostomes and percentage of developing larvae (DL) encysted in the mucosa remained constant in all 4 seasons of the year, whereas a significant increase in the percentage of adults in the cyathostome population occurred in fall. In yearling ponies, encysted early third-stage larvae constituted a significantly lower percentage of the cyathostome population, and DL and adults formed a significantly higher percentage, compared with those stages in older ponies, 2–5 yr of age. More species of cyathostomes were present in yearling ponies than in older ponies. Significant differences occurred in fecal egg counts at different seasons of the year, even though adult cyathostome and large strongyle numbers remained constant. Twenty-four species of cyathostomes were found year-round, and 2 rare species were found in only 2 or 3 seasons of the year. Prevalences for these 24 species were not significantly different during any season, although 5 species had significant differences in intensity levels of infection in certain seasons of the year. Three species of large strongyle (strongylinae) adults had significant seasonal variations in intensities. These were Strongylus edentatus and S. vulgaris, which occurred in significantly higher numbers in summer and fall, and Triodontophorus brevicauda, which was more numerous in spring and summer. Fourth- and fifth-stage larvae of S. vulgaris recovered from the mesenteric vasculature were significantly more numerous in winter and spring than in other seasons.

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