Larvae may be locally concentrated where defaecation is frequent, near sheep camps, under trees or around water troughs. While there are L3 on pasture, sheep are exposed to infection. Treatment of sheep does not remove the reservoir of larvae on the pasture. The number of larvae present on pasture when grazing starts is the most important factor in determining future parasite levels. There is a delay in establishment of successive generations because it takes time for eggs from the initial infection to be produced, develop on pasture and infect sheep.
Weather patterns are important, especially after winter.
Eggs may lie dormant until the arrival of conditions favourable for development. After rain and warmth of larvae may develop resulting in a rapid increase in larval availability. H. contortus deserves special mention as it is so prolific and pathogenic. Egg deposition can be enormous. Sheep may pass 30,000,000 eggs per day which, under the right conditions, build up to high concentrations of eggs and larvae.