However, immature worms are smaller than adults. Distributions are shown on the maps. Conditions which favour each parasite are included. Sheep are not usually raised in very arid areas because of lack of water and not in areas of high rainfall because of the risk of disease, including parasitism. Sheep-rearing areas of Australia fall into three major rainfall zones.
Rainfall affects parasite distribution.
The summer rainfall zone occurs in the north of N.S.W. and in Queensland. The winter rainfall zone which is further south, is represented in all southern states and is where the majority of Australian sheep are raised. The uniform rainfall zone is found in contiguous parts of N.S.W. and Victoria. Parasite distributions are only indicative as the maps are low scale, there is little actual parasite survey data, local conditions exist which are favourable for parasites and distribution varies between years. Aspects of the disease associated with the parasite are listed.