Nematode transmission patterns.

The transmission of nematode parasites of vertebrates is reviewed with special reference to the phenomena of monoxeny, heteroxeny, paratenesis, and precocity. Monoxeny is divided into 2 types. Primary monoxeny assumes that there was never an intermediate host in the transmission. Secondary monoxeny assumes the loss of an intermediate host during the course of evolution and its replacement by a tissue phase in the final host. Heteroxeny, or the use of intermediate hosts, is a common feature of many nematode groups. The Spirurida utilize arthropods, the Metastrongyloidea molluscs, and Ascaridida arthropods and vertebrates. Paratenesis, or the use of transport hosts, is a common feature of the transmission of nematode parasites of carnivores. It is postulated that in some instances paratenic hosts have become intermediate hosts and replaced the original intermediate host. Precocity in the development of nematodes in intermediate hosts (including what may have been paratenic hosts) is defined as growth and/or development beyond the expected. Its occurrence among the nematode parasites of vertebrates is reviewed. It is regarded as a transmission strategy which accelerates gamete production in the final host. Precocity could also provide the mechanism for the transfer of a parasite from a predator final host to a prey final host.