THRESHOLD QUANTITIES FOR HELMINTH INFECTIONS

For parasites with a clearly defined life-cycle we give threshold quan tities that determine the stability of the parasite-free steady state for autonomous and periodic deterministic systems formulated in terms of mean parasite burdens. We discuss the biological interpretations of the quantities, how to deal with heterogeneity in both parasite and h ost populations, how to incorporate the effects of periodic discontinu ities, and the relation of the threshold quantities to the basic repro duction ratio Ro. Examples from the literature are given. The analysis of the periodic case extends easily to 'micro-parasitic' systems.