1.   Infective larvae which have developed from eggs deposited by ewes in the spring are ingested by ewes and lambs in early summer.

2.   Most sheep graze throughout their lives so that pasture contamination with nematode eggs and the intake of infective larvae is almost continuous and modified only by climatic restrictions.

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

4.   Although the infective larvae can penetrate the skin, the infection rarely matures and there is no evidence as yet of transmammary infection.

5.   Skin penetration by infective larvae may cause an erythematous reaction which in sheep can allow the entry of Bacteroides nodosus, the causative organism of foot-rot.

6.   Strongyloides infective larvae are not ensheathed and are susceptible to extreme climatic conditions.