Laboratory for Biochemical Parasitology, Institute of Food, Nutrition and Human Health, Massey University, Private Bag 11222, Palmerston North, New Zealand.
*Corresponding author, present address: School of Human Life Sciences, University of Tasmania, Locked Bag 1320, Launceston, Tasmania 7250, Australia, Phone: +61 3 63245467; Fax: +61 3 63243995; E-mail: simon.brown@utas.edu.au
The nematode parasites of sheep can be expected to experience temperatures in excess of 40°C in faeces on pasture and, perhaps, in the host. Third stage larvae (L3) of Ostertagia (Teladorsagia) circumcincta, Haemonchus contortus and Trichostrongylus colubriformis survived for at least 90 min at 45°C 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 (LT50) were 47.2±0.1°C, 48.1±0.2°C and 47.7±0.1°C for sheathed L3O. circumcincta, H. contortus and T. colubriformis, respectively. Exsheathing reduced the LT50 up to 1°C for O. circumcincta (46.5±0.1°C) and H. contortus (47.1±0.1°C), but the LT50 of T. colubriformis was unchanged. Extending the incubation period systematically reduced the LT50 of exsheathed O. circumcincta and H. contortus compared with the sheathed L3, but exsheathing did not affect the LT50 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.
Thermotolerance, Cuticle, Ostertagia circumincta, Haemonchus contortus, Trichostrongylus colubriformis
