The population biology of the parasitic stages of Haemonchus contortus.
The population biology of the parasitic phase of the Haemonchus contortus life-cycle is discussed within the context of a mathematical model of those processes which regulate population density. Parasite death is assumed to be attributable to at least three processes: failure of the 3rd-stage larvae (L3) to exsheath following ingestion, failure to become established following exsheathment (immune exclusion), and mortality of 4th- and 5th-stage worms on the mucosal surface. The proportion of worms which avoid immune exclusion is a declining sigmoidal function of the duration of infection, at least under conditions of continual exposure to L3 larvae. Under the same conditions, the death rate of 4th- and 5th-stage worms can be modelled as an increasing linear function of the host's experience of 3rd-stage larvae. The model provides a good description of the worm burdens observed during the course of a long-term trickle infection experiment even though a priori arguments indicate that it would be an inadequate representation of events in the field.
Smith
G
G
School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square 19348.
D000818:Animals; D006188:Haemonchiasis / Q000276:immunology / Q000469:parasitology*; D006189:Haemonchus / Q000254:growth & development* / Q000276:immunology; D006790:Host-Parasite Interactions; D008433:Mathematics; D008954:Models, Biological; D014251:Trichostrongyloidea / Q000254:growth & development*; D014252:Trichostrongyloidiasis / Q000469:parasitology*