gut flow. Some nematodes, such as the human hookworms Necator
americanusand Ancylostoma duodenale, have armed buccal cavities,
which allow them to simultaneously attach and extract blood meals.
Several of the nematodes of cattle and sheep, such asOstertagia ostertagi
andHaemonchus contortus, similarly attach by embedding their anterior
ends in the tissue or in pits and crypts of the gut. Large ascarid worms
actively swim against the flow, as do some of the small nematodes, such
asN. brasiliensis. AdultH. polygyrusattach by wrapping their body coils
around villi in the duodenum.
Infecting L3 larvae tend to have the most difficulty with holdfasts,
because of their small size. A typical strategy during infection is for the
infecting larvae to penetrate into the tissue of the gut, where they are safe
from gut flow, develop to adulthood within the tissue and then emerge as
adults into the lumen. InH. polygyrusinfections, the larvae penetrate into
the small intestine within minutes of infection, develop to adults by 7–10
days and re-enter the lumen just before the specific immune response gets
going (Sukhdeoet al., 1984). A similar strategy is seen in several of the
nematodes infecting cattle and sheep. InO. ostertagiandH. contortus, the
infecting worms penetrate the gut wall and often arrest their development
within the tissue until an appropriate time. These arrested larvae can
survive for long periods, because they are metabolically inert and
therefore invisible to the host’s immune response. Hormonal responses
during lactation in the host triggers a ‘spring rise’, whereupon the worms
develop into adults, enter the lumen and produce their eggs just in time to
be available to the weanlings in the spring (Roberts and Janovy, 2000).Proximate Behaviours in Habitat Selection
Free-living nematodes and the free-living stages of parasitic nematodes
have a large repertoire (> 20 classes) of distinct behaviour patterns (e.g.
locomotion, swimming, nictation, jumping, oviposition) and complex
orientation responses to temperature, and a wide variety of simple and
complex molecules have been identified (Croll and Sukhdeo, 1981). In
contrast, while adult parasitic nematodes from freshly dead intestines are
always extremely active, with a lot of twisting and wriggling, it is not
clear how these worms behave in the living hosts. Intestinal nematodes
have extensive arrays of sensory organs and possess intricate nervous
circuitries, which are as complex as those of their free-living relatives
(McLaren, 1976; Sukhdeo and Sukhdeo, 1994; Halton et al., 1998;
Reuter et al., 1998). They may also possess an equally impressive
suite of behaviours, but we have been able to identify only two types
of behaviours that operate within the intestine: orientation responses
(attraction/repulsion) and releaser responses (fixed action patterns).
Intestinal nematodes orientate towards (are attracted to) members
of the opposite sex. This was long suspected. In hookworm infections,
when the sexes are unbalanced, with more males than females, there isIntestinal Nematode Parasites of Vertebrates 227