The market size for products that control these parasites is often not impressive. The commercial viability of a vaccine depends on such factors as development and production costs, and specific characteristics, such as storage/transport conditions and shelf life. Perhaps the biggest barrier is the fact that current drugs have efficacies approaching 100%. It will not be easy to persuade users that a vaccine which is less than 100% effective can usefully control the disease. In addition, as patents expire on many anti-parasiticides, there is a market trend in favour of generic drug companies, which spend little on research and development and essentially do not invest in discovering new drugs or vaccines (18). Reasons for this are many and varied, with the demand for quick, high returns on investment reducing the opportunity for long-term discovery projects. As a result, very few animal health companies are currently committed to the discovery and development of antiparasitic vaccines.