Experiments on inhibited development of Haemonchus contortus and Ostertagia circumcincta in sheep in The Netherlands.

Inhibited development of Haemonchus contortus and Ostertagia spp was studied in sheep. Exposure of infective Haemonchus contortus larvae for five weeks to 15 degrees C or 16 degrees C very effectively induced inhibited development but exposure to lower temperatures was much less effective. Infection with fresh larvae cultured for seven or 12 days at 25 degrees C in the dark resulted only in very low rates of inhibited development. This was not much influenced by the age of the host or the month of infection. Exposure of infective Ostertagia spp larvae for five weeks to 4 degrees C was fairly effective in inducing inhibited development while exposure to 16 degrees C was not effective. On two occasions inhibited development of Ostertagia spp was observed in lambs after a primary infection with fresh infective larvae cultured for seven days at 25 degrees C in the dark. This implies that a conditioning for inhibited development of Ostertagia spp is possible within this seven day culture period.