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.
  
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