Arrested development of Nematodirus species in grazing lambs

Spring born lambs grazing in initially clean pasture for two successive years acquired low levels of Nematodirus species infection which showed a progressive increase in the level of arrested development, reaching 70 to 90 per cent of the total worm burden in lambs slaughtered in November. Similar levels were recorded in worm free tracer lambs allowed only one month access to contaminated pastures. Results suggested that both seasonal environmental conditions and host age may play an important role in larval inhibition for this genus and that this phenomenon was not density dependent, nor associated with host immunity.