Studies on Nematodirus battusCrofton and Thomas, 1951, have shown a marked seasonality in the pattern of transmission. The life cycle normally involves only one parasitic generation per year. The infective third-stage larvae develop over the summer within eggs deposited on pasture. Eggs are sensitized by low temperatures during fall and winter; larval hatching follows in the presence of adequate moisture and proper osmotic conditions as the temperature rises above 10°C. This set of conditions, conducive to hatching, is regularly present only during spring; thus, hatching of eggs is concentrated into a few weeks during this time. Outbreaks of clinical nematodiriasis are determined by the timing and magnitude of the seasonal hatch of larvae and the presence of young, susceptible lambs.
On occasion, deviations from this typical pattern have been reported. Completion of the life cycle has been recorded during other times of the year, particularly in the fall. Larval availability during fall has been attributed to three different mechanisms: (1) eggs hatching within the same year as deposited; (2) eggs deposited during fall, overwintering and hatching the following fall; and (3) eggs hatching in spring, followed by long-term survival of the larvae through the summer
