The number of available larvae (infection rate) varies with the seasons. This burden is generally heavier in warm months and tends to decline in autumn and winter. This is largely due to the requirement of parasites for warmth and moisture in their development outside the host. Eggs hatch more readily and immature parasite forms develop faster with high humidity and warm temperatures. In summer months, the worm´s life cycle is shorter and a higher proportion of larvae survives. Therefore, worm burdens in horses tend to increase during the warmer seasons.