The thermotolerance of some parasitic nematodes of sheep and the role of the sheath

摘要

The nematode parasites of sheep can be expected to experience temperatures in excess of 40 deg C in faeces on pasture and, perhaps, in the host. Third stage larvae (L_3) of Ostertagia (Teladorsagia) circumcincta, Haemonchus contortus and Tnchostrongylus colubriformis survived for at least 90 min at 45 deg in vitro in water, but the larvae were inactivated rapidly by slightly higher temperatures. After 30 min incubation, the temperatures at which 50% of the larvae were immotile during the observation period (LT_(50)) were 47.2+-0.1 deg , 48.1+0.2 deg and 47.7+-0.1 deg for sheathed O. circumcincta, H. contortus and T. colubriformis, respectively. Exsheathing reduced the LT_(50) up to 1 deg for O. circumcincta (46.5+-0.1 deg) and H. contortus (47.1+-0.1 deg ), but the LT_(50) of T. colubriformis was unchanged. Extending the incubation period systematically reduced the LT_(50) of exsheathed O. circumcincta and H. contortus compared with the sheathed L_3, but exsheathing did not affect the LT_(50) of T. colubriformis. These data imply that O. circumcincta is slightly less thermotolerant than H. contortus or T. colubriformis and that the sheath may confer some protection against high temperature or that the exsheathing affects the larvae in some way.