The Nematodes

Infective stages

Click here to view a table showing examples of nematode infective stages. In the majority of nematodes of veterinary importance, the third larval stage (L3) is the infective stage for the definitive host. In some species this L3 develops as a completely free-living stage from a hatched L1 while in others the L1 does not hatch and develops to an L3 inside the protective egg shell. Therefore, the infective stage may be either a free-living L3 or an egg containing an L3 (Egg+L3).

In nematodes where a larvated (embryonated) egg is is the infective stage, transmission to the definitive host is invariably via the oral route.

However, where a hatched L3 is the infective stage, transmission to the definitive host may be via the oral route or by skin penetration and in some nematode species, such as the hookworms,  either route is possible.

Whichever nematode stage is infective for the definitive host, it is usually quiescent, does not grow and does not feed because its mouth is sealed. In these respects, it is different from all other larval and adult stages.

Infective larvae, whether they are entirely free living or inside an egg, are usually also environmentally resistant and are often enclosed in a sheath - the loose-fitting cuticle of the preceding stage.

The infective stage is, in fact, the nematode stage that provides a link between a free living and a parasitic way of life. Even in nematodes where intermediate hosts play a critical role in the life cycle, the infective stage is still the link to the definitive host. In these cases the intermediate host serves as a "bridge" for the infective stage to link up with its definitive host.

A good example of this concept is found in the nematode Dirofilaria immitis ( the heartworm of dogs), belonging to the family onchocercidae. Blood-sucking mosquitos serve as intermediate hosts: they ingest circulating first stage larvae, called microfilariae. These grow through two molts, in the mosquito intermediate host, to the infective third stage and are transferred back to a definitive host when the mosquito takes a blood meal. Although this is still an example of transmission via skin penetration it is the intermediate host that bridges the skin barrier of the definitive host rather than the L3 directly as is seen in the hookworms.

Diagnostic stages

Click here to view a table showing examples of nematode diagnostic stages. The diagnostic stage of nematodes is the stage which leaves the definitive host in order to continue its development and which can be detected by sampling appropriate tissues in a live, infected animal. In gut dwelling nematodes such as the Trichostrongylidae of grazing ruminants, the diagnostic stage is an egg and is found in the host's feces. In Dirofilaria immitis, the dog heartworm, the diagnostic stage is a microfilaria (L1), circulating in the peripheral blood, where it is available for ingestion by an intermediate host.

Since the diagnostic stage is the life cycle stage leaving the definitive host, it is the stage that links the parasitic way of life with either the free-living phase of the life cycle or the phase of development that occurs in an intermediate host.

The diagnostic stages of most nematodes are found in the feces of the definitive host since this is the easiest route of exit from the host. This is obviously true for nematodes found in the alimentary tract but is also true for nematodes found in the lungs and other organs remote from the gut. For example, the adults of Parelaphostrongylus tenuis , a nematode of white-tailed deer are found in the venous sinuses and the subdural space of the cranium. (A) Eggs laid by female are deposited into the venous circulation, carried to the heart (B) then lungs (C) where they lodge in the capillaries. First stage larvae (L1s) break out into the alveoli, migrate up the bronchial tree (D) to the pharynx where they are coughed up,  swallowed and pass out with host feces (E).

In summary, the diagnostic stage is that stage of the life cycle that leaves the definitive host and thus links the parasitic phase in the definitive host with the preparasitic phase occurring either as free-living developing stages or as stages developing inside an intermediate host.

The infective stage is that stage of the life cycle that enters the definitive host and thus provides the link between the preparasitic and parasitic phases of the life cycle.

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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on February 17, 2000