The number of parasites infecting animals ("parasite load") varies with the seasons. This burden is generally heavier in warm months, declining in late 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 cattle increase tremendously during the warm, wet seasons.

Seasonal variations affect the activity of parasites within animals. The commonly found parasites such as Cooperia, Haemonchus, and Oesophagostomum may be most numerous during late summer and autumn, while Ostertagia and Trichostrongylus are usually at highest levels from autumn through spring.

In the specific case of Ostertagia ostertagi, a small proportion of the larvae slow their development such that infective larvae that enter hosts in autumn may stop developing and remain in larval stages until spring. This inhibited or arrested development is called hypobiosis. Arrested development is advantageous to the parasite as it is a means of delaying egg production until the external environment permits larval development.

Because of their location in the glandular tissue, a possible lower state of metabolism, and other little-known factors, the inhibited (hypobiotic) larvae of O. ostertagi are not effectively controlled by many anthelmintic drugs currently in use except the avermectin/milbemycin compounds.