Apart from the fact that vaccines began to be developed much later than chemotherapeutic drugs, a number of additional factors have affected the progress of parasitic vaccine development. Not least was the implementation in the 1990s of legislation on the authorisation of veterinary medicinal products in Europe (50). Moreover, and in contrast to viruses and bacteria, even the simplest parasites and their life cycles are highly complex, and there is a general lack of precise understanding of the host/parasite interaction