Studies on the immunity of sheep to Oesophagostomum columbianum: effects of different and successive doses of larvae on worm burdens, worm growth and fecundity

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Abstract
The effects of different and repeated doses of Oesophagostomum columbianum larvae on the survival of the adult parasite, its length and the fecundity of the female worm in sheep were studied. Adult worm burdens were dependent on the numbers of larvae administered when sheep were infected once, but not when they were infected twice. It was concluded that these effects were caused by non-specific elimination of worms following a single infection, related to competition for space among larvae and to inflammatory reactions stimulated by them in the gut mucosa which increased with the numbers of larvae given. When sheep were infected once with large numbers of larvae or when they were infected twice the surviving adult population was also influenced by the adaptive immune responses of the host. Parasites recovered from sheep given a single infection of 5000 larvae were stunted compared with worms from sheep given fewer larvae. After a second infection all the worms were stunted and the females produced fewer eggs than did female worms following a single infection; but the numbers of larvae administered did not influence these effects. Egg production was also delayed after a second infection. The reduced fecundity of female worms in the immune host was not related to stunting of the worms, because stunted female worms from sheep infected once with 5000 larvae produced as many eggs as females from sheep infected once with smaller numbers of larvae. It is suggested that the different manifestations of protective immunity in sheep were elicited when threshold levels of infection were exceeded. These levels were represented by between 2000 and 5000 larvae in first, and fewer than 500 larvae in second infections.

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