1.   The further one can extend the grazing season in spring and autumn, without damage to soil or sward, the better.

2.   Acquired immunity in ostertagiasis is slow to develop and calves do not achieve a significant level of immunity until the end of their first grazing season.

3.   The disease is characterised by bronchitis and pneumonia and typically affects young cattle during their first grazing season on permanent or semi-permanent pastures.

4.   Generally only calves during their first grazing season are clinically affected, since on farms where the disease is endemic older animals have a strong acquired immunity.

5.   Small numbers of adult worms can survive in the bronchi of infected animals, particularly yearlings, until the next grazing season.

6.   The dispersal of larvae from the faecal pat during the grazing season appears to be effected by a fungus rather than by simple migration.

7.   As with the other trichostrongyloids it seems likely that only two cycles of the parasite occur during each grazing season.

8.   Thus there are two sources of infection during the grazing season in temperate areas.

9.   First there are infective larvae which developed during the previous grazing season and have survived on pasture over winter.