Why is it so difficult to develop vaccines against parasites?

Depite many effords, there is still no vaccine against onchocerciasis. Success is only seen, when irradiated live L3 are injected as a live vacine.

Epidemiological data had suggested, that crosstransmission of human and animal filariae can protect the human population from severe onchocerciasis (Renz et al. 1994, Wahl et al. 1998). This protection is afforded, at least partially, by a cross-reacting concomitant immunity by infective larvae of O. ochengi ‘vaccinating’ the human host. Much in the same way, as cowpox are used to ‘vaccinate’ man against smallpox. In order to test the hypothesis, that premunisation can be afforded by heterologeous larvae, we vaccinated naive cattle by injecting live L3 larvae of O. volvulus and challenged  them by injecting live O. ochengi L3 (Achukwi et al. 2007).  The protection  thus affored was calculated to be about 80 %:

data from Achukwi et al. 2007, design A. Renz