Brown Besier

Haemonchus contortus is a widely-distributed nematode parasite of ruminants, whose life-cycle involves a free-living phase vulnerable to the external environment. This thesis examines the proposition that intraspecific differences in characteristics of the non-parasitic stages have evolved as a consequence of exposure to different climatic conditions, and that this indicates the adaptation of H.contortus populations to local environments. Ecological studies by earlier workers indicate that the major environmental factors which determine the global and seasonal distribution of H.contortus are the requirement for moisture, and the relatively poor cold tolerance by the free-living stages. In field studies which defined the seasonal pattern of H.contortus populations on pasture at Albany, Western Australia, both egg development and larval survival were also severely limited by dry conditions over summer. However, during winter, egg development continued and infective larvae survived in large numbers. Comparisons between literature reports and these studies suggest differences between H.contortus isolates from different regions in the ability of their eggs to develop to infective larvae at low temperatures. Experiments comparing the effect of low temperatures on eggs of H.contortus isolates from different regions within Australia showed that isolates from relatively cool areas (Albany, in Western Australia, and King Island, near Tasmania) hatched and survived at significantly lower temperatures than isolates from warmer locations in Queensland. Cold tolerance by the eggs, and possibly other free-living stages, would offer an important survival advantage for H.contortus populations in cooler regions such as Albany, where hypobiosis (developmental arrest by parasitic larvae, to escape hostile external conditions) does not occur routinely. It is suggested that cold-tolerance by the free-living stages is an adaptive device which permits H.contortus to survive in temperate regions where winters are cold; in very cold climates, hypobiosis represents a further adaptive feature. Intraspecific variation in low temperature tolerance by the free-living stages therefore appears to be a significant factor which enables H.contortus to occupy a wide climatic range.