community dynamics (Holmes and Price, 1986; Price, 1986) and phylo-
genetic constraints (Brooks and McLennan, 1993; Adamson and Caira,
1994; Sukhdeoet al., 1997). Space prevents detailed treatment of the
many ideas. Nevertheless, although the situation remains unresolved, it is
likely that all or several of these mechanisms shape the decision-making
strategies of parasites as they progress through their life cycles.Laboratory Models
Nematodes have similar developmental patterns; after hatching from the
egg, usually as first-stage larvae (L1), the worms undergo four moults
to adulthood. In almost every parasitic nematode, whether the L1 is
contained within an egg or a sheath, the infective stage is usually the L3
larva. Eggs offer good protection, but they must be passively dispersed to
their hosts. Ensheathment of the infective stage combines mobility with
protection against desiccation and other adverse environmental extremes.
There is usually a large active component in the host-finding strategies of
ensheathed species. In these worms, the unshed cuticle of the previous
moult covers the infective L3 like a protective sheath, but the worms
cannot feed and must use their stored energy for all activities during
finding and infecting the host (Medica and Sukhdeo, 1997).
Gastrointestinal nematodes have been recovered from every
vertebrate host examined (Anderson, 2000), but for most of these species
we know little beyond their descriptive taxonomy and some anecdotal
observations. Most of our detailed knowledge comes from a handful of
species that are amenable to experimentation because of their ease of
maintenance and infection in the laboratory. Much of the work discussed
in this chapter will focus on three of the most popular laboratory models.
Trichinella spiralisis the pig parasite that is responsible for the human
disease trichinellosis and is easily maintained in mice and rats. Infection
occurs through the ingestion of raw infected meat, and the worms
establish in the anterior small intestine, mature and produce larvae,
which migrate to skeletal muscles and encyst. Heligmosomoides
polygyrusis a luminal parasite of murine hosts and is used as a model for
nematode diseases of domestic animals. Infection is through the ingestion
of infective L3 larvae, and the adult worms establish in the lumen of the
small intestine. Eggs are passed in the faeces and develop into infective
L3 larvae.Nippostrongylus brasiliensisis a rodent parasite that is used
as a model for hookworm disease. Infection occurs when the larvae
penetrate the skin of the host and migrate through the body to the gut.
These worms are also lumen dwellers and, although generally restricted
to the anterior small intestine, their site fidelity is somewhat lower (Croll
and Smith, 1977). Eggs are passed in the faeces and develop into mobile
L3 larvae.Intestinal Nematode Parasites of Vertebrates 225