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 form 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, although under NZ conditions reasonable burdens of the nematode worms infecting cattle are found to a greater or lesser extent throughout the year. The commonly found parasites in beef and cattle are Ostertagia spp, Trichostrongylus spp, Cooperia spp, and to a much lesser extent Dictyocaulus viviparus or lungworm. For dairy-beef cattle however all four of these parasites are commonly found.

In the specific case of Ostertagia ostertagi, considered the most dangerous of the nematode parasite a small proportion of the larvae slow 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. If these arrested larvae recommence development to adults in large numbers, serious problems can occur in cattle.

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.