Parasitism: Life Cycle and Host Defences against Parasites

Abstract

Abstract Parasites must reach new hosts and have evolved a variety of strategies to achieve this. For some there is a direct life cycle, with the parasite passing from one member of the host species to another via the infective stage of the life cycle. Other parasite species have more complex life cycles, known indirect life cycles, involving intermediate host species, before final transmission to the definitive host. The host's defence against parasitisation broadly takes two forms. First there are nonimmune mechanisms such as the protection given to infection with the human malaria parasite Plasmodium falciparum by the presence of the abnormal ‘sickle’ haemoglobin. Secondly, parasites can be attacked by the full panoply of the host's acquired immune defence system. A feature of most host/parasite relationships is survival of the host but equally survival of the parasite: parasites have evolved a variety of mechanisms for evading or down‐regulating the host's immune response to their advantage. The pathological consequences to the host of parasitic attack include physical damage from the host's activities to indirect consequences of the host's immune response. The problems of developing antiparasite vaccines are discussed. Key Concepts: Finding a new host is a formidable challenge for a parasite: variety of strategies evolved to achieve this end. A detailed understanding of parasite life cycles facilitates development of strategies for control of parasites of medical and veterinary importance. Host species can be very restricted for some parasites, but for some parasites many host species can be infected. Some parasites have direct simple life cycles whereas others have indirect complex lifecycles involving a vector or intermediate host. In complex life cycles the parasite's development and progression through the life cycle is adapted to the physiology and behaviour of the host. Prevalence of parasites in the host population is often over dispersed – that is most individuals have a few parasites but a few have many. In general the higher the parasite load the greater the severity of disease in the host. Defence to parasitic infection involves both nonimmune and acquired immune mechanisms. In many host/parasite relationships the host defences reduces the parasite load to low levels but fails to eliminate the parasite completely and transmission continues. Parasites have evolved a variety of strategies to survive or evade the host's defence mechanisms.