MORPHOLOGICAL POLYMORPHISM IN THE NEMATODE TELADORSAGIA-CIRCUMCINCTA IN RELATION TO AGE OF LARVAE, INFECTION MODE AND LAMB CHARACTERISTICS IN EXPERIMENTAL CONDITIONS

Variations in the proportion of the less-frequent morph of Teladorsagi a circumcincta, T. c. trifurcata, were studied under experimental cond itions. Infections with older larvae (over 7 months of age at 4 degree s C) yield a lower proportion of T. c. trifurcata compared with those obtained from fresh larvae (15 days). This phenomenon could account fo r the reduction in the proportion of the morph trifurcata in two isola tes that were acclimated to laboratory rearing for several generations . The infection mode (single or trickle infections) did not significan tly modify the proportions of both morphs. The morph trifurcata was fo und in higher proportions (>5.5%) in more susceptible male and/or Roma nov lambs, and in lower proportions (<2.5%) in less susceptible female and/or Merinos d'Arles lambs.