The Nematodes

Life cycle variations

Although the basic nematode life cycle described previously holds true for many nematode species, it is also true that other species show a number of variations and complications in their life cycle patterns. Most  of these variations are concerned with the infective stage and whether other hosts, in addition to the definitive host, may play a role in the life cycle.

There are two types of life cycle in nematodes infecting domestic animals - Direct and Indirect.

  • Direct life cycles - all preparasitic stages are found free-living in the environment and their development may take place either inside the egg or after hatching.

    • In nematodes where  first stage larvae hatch from their eggs subsequent development takes place in the environment and the third stage larva is the infective stage. Examples of this type of direct life cycle occur among members of the family Trichostrongylidae

    • In nematodes where eggs do not hatch, preparasitic larvae develop inside their eggs so that the infective stage is an egg containing an infective larva. Hatching will take place after these eggs are eaten by another host and the infective larva escapes. Example: Ascaris suum, the roundworm of pigs.

  • Indirect life cycles - larvae develop to the infective stage inside an appropriate intermediate host. In these life cycles there are two possible methods of transmission of infective larvae to the definitive host.

    • The intermediate host is ingested by the definitive host and infective larvae are released by digestion in the alimentary tract. Example: Parelaphostrongylus tenuis, the brainworm of white-tailed deer.

    • The intermediate host is a biting or sucking arthropod. In these cases transmission of infective nematode larvae occurs during feeding on the definitive host. Example: Dirofilaria immitis, the heartworm of dogs and cats.




Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000