BIOLOGY Lecture Notes - Golden Pages 2003

10. MEDICAL PARASITOLOGY

Parasite Life Cycle

The ontogeny of parasites is more complex than The Development of free-living species. In most parasites, development involves a complex metamorphosis comprising numerous larval stages that inhabit different environments and perform distinct Functions, such as dispersal, active growth, passive waiting for transition to a new habitat, and so forth.

The sequence of all ontogenetic stages of a parasite, along with the pathways of its transmission from one host to another, is referred to as its life cycle. Larvae may lead either a free-living or a parasitic lifestyle. The host they inhabit is known as the intermediate host. Intermediate hosts play a crucial role: they serve as sources of infection for definitive hosts and often act as vectors for parasite dispersal, thereby ensuring the survival of the parasite population.

The life cycle of parasites may sequentially involve two, three, or even more intermediate hosts. The host in which the sexually mature parasite develops and reproduces sexually is called the definitive host. Infection of the definitive host occurs either by ingestion of the intermediate host or through contact with it within the same habitat.

Parasite dispersal can take place at various stages of their life cycle. Typically, this occurs during dormant stages. In Protozoa, this happens at the cyst stage; in helminths, at the egg stage and occasionally at the encapsulated larva stage.

Cysts, eggs, and larvae can be dispersed not only via a specific host, but also transported through the environment by wind, Water currents, or other animals.

The active mobility of an intermediate host increases the probability of contact with the definitive host, while the movement of the definitive host ensures the efficient dissemination of cysts, eggs, and larvae.