Transmission by Vectors and Intermediate Hosts

The use of vectors and intermediate hosts is a common method for parasites in terrestrial habitats to transmit to the next host in the life cycle. Parasites enter vectors passively, as the vectors are feeding. The parasite then undergoes some development or replication in the vector. When the vector seeks out a new host to feed on, it also introduces the parasites it is harboring. Parasites may enter intermediate hosts actively or passively. Once inside they usually transform into a stage that can resist the defense responses of the intermediate host, and persist for long periods, until that host is eaten by another animal that can serve as a second intermediate host or definitive host.


Blood-feeding arthropods are the most common vectors in terrestrial habitats. The best vectors are those that feed many times on several hosts and who live long enough for any parasite development to occur.

Mosquitoes transmit many apicomplexan protozoans, such as Plasmodium.

Small numbers of gametocytes are ingested during one blood meal and large numbers of sporozoites are inoculated during the next meal.

Microfilariae from filarid nematodes are also transmitted by insect vectors.


One-host ticks such as Dermacentor albipictus are poor vectors because all feeding is done on the same animal. Parasites and pathogens that transmit through this kind of tick usually must opt for transovarian transmission, where they infect the tick egg.

Two- or three-host ticks, such as Rhipicephalus, are much more effective vectors.


Terrestrial life cycle cestodes, which produce a resistant egg, use an intermediate host to develop and transmit their metacestode stages, such as the cysticercoid in an arthropod intermediate host,

or cysticercus and hydatid in vertebrate intermediate hosts.


Acanthocephalans use terrestrial arthropods as intermediate hosts for development of their cystacanths.


Most nematodes have direct life cycles, but the lungworms use a terrestrial mollusc as an intermediate host. First stage larvae penetrate the foot of the mollusc and develop to an infective third stage larva.

Many of the terrestrial molluscs infected in nature are only a few mm in diameter, but they can be numerous. The mollusc also makes the parasite more accessible to the definitive host, which is usually a grazing or browsing animal, by ascending the vegetation that the host feeds on.