Passive Dispersal by Parasitic Stages

Parasitic stages are important for dispersal because usually the host can move much greater distances than the parasite would, even if the parasite has a motile stage. This type of dispersal is passive on the part of the parasite, and is successful only if the host is fed upon by another suitable host. The longer the period between when the host is infected and it is eaten (i.e. when transmission occurs), the greater the chances for dispersal of the parasite to new locations. Many parasites are capable of living extended periods.


Gametocyte

The gemetocyte of malarial parasites is produced in the vertebrate host, but is taken up by the mosquito vector during feeding and then transported by the mosquito.


Sporozoite

Sporozoites of malarial parasites are produced in the mosquito vector, and then introduced to the vertebrate host passively as the mosquito feeds.


Microfilaria

Microfilariae of filarial worms are produced in the vertebrate host, and then introduced to an insect during feeding, and transported by the insect as it searches for a new host to feed on.


Metacercaria

Many Digenea produce a metacercariae that encysts in an animal host and is transported wherever that animal travels. In this photo the parasite (P) has encysted in the dermis (D) of its host, between the epidermis (E) and musculature (M).

Some metacercariae do not encyst, but are nevertheless carried passively by their hosts.


Procercoid

Many Cestoda have a plerocercoid stage that is present in aquatic invertebrates and transported by them.


Plercoercoid

The plerocercoid stage of Cestoda occurs in a vertebrate intermediate host, usually a fish or mammal, and is transported by that host.


Cysticercoid

Some Cestoda produce a cysticercoid that is carried within the body of an arthropod intermediate host.

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Cysticercus

Some Cestoda produce a cysticercus that is carried within the body of an mammalian intermediate host.


Hydatid

Some Cestoda produce a hydatid that is carried within the body of an mammalian intermediate host.


Cystacanth

Acanthocephalan cystacanths parasitize an invertebrate intermediate host. They may alter the behavior of the host causing it to move to a location where it will be more easily preyed upon.


Adult

Adult parasites are dispersed by the hosts they are infecting, but with the exception of some ectoparasites, this dispersal is of little consequence to the adult parasite because it cannot leave its host. However, the eggs or cysts produced by the adult parasite are dispersed by these host movements.