Factors regulating tapeworm populations: estimations of the duration of acquired immunity by sheep to Taenia hydatigena.

Lambs reared from birth in a 'tapeworm-free' unit were immunised at three months old by a parenteral injection of the embryos of Taenia hydatigena. Subsequently, they were grazed with unimmunised control animals on an egg-free pasture. At one, three, six, nine, 12 or 15 months after immunisation, groups of the treated and control animals were grazed for eight weeks on an experimental egg-contaminated pasture. Strong immunity was shown up to three months after immunisation by the presence of only a few pinhead lesions in the lamb's liver. At six and nine months after immunisation, the lesions were similar in size to those in the control animals. While a few organisms reached the abdominal cavity in animals grazed six and nine months after immunisation, no viable cysts were found until 12 months. Seven-year-old sheep, which had been naturally infected with T hydatigena at an early age but had rarely ingested eggs since, were also grazed on the egg-contaminated pasture. Populations of original and superimposed metacestodes were observed in the abdominal cavity of some of them. The first experiment investigated the duration of immunity to the embryo, reorganising larva and metacestode in the absence of further egg infections. The second experiment confirmed that original and superimposed cyst populations can coexist.