Nematode reproductive and ingestive responses to helminth and host chemical stimuli

Current knowledge of the involvement of chemical stimuli in the reproductive and ingestive physiology of zooparasitic nematodes is reviewed. The habitat of zooparasitic nematodes, coupled with their sensory reduction, indicates that chemical stimuli may modulate most aspects of their behavioral physiology. Nematodes respond to the feeding status of the host so that the helminth's pharyngeal pumping, site selection, and sexual activity may be altered. Biogenic amines from the host, such as histamine, dopamine, or serotonin, may synchronize host and helminth ingestion. Octopamine, which is released by nematodes, depresses ingestion by male, but not female, worms. Biogenic amines may also regulate ovipositioning by female helminths. Nematodes release pheromones that enable precopulatory location of a mate. The male helminth is activated by both components of the female's pheromone while movement by the female is probably arrested by male-produced chemicals. Continuation of spectral analyses may allow future identification of female pheromone. The male's pheromone receptor binds lectin that is specific for mannose residues. Host immunity also alters the behavior of nematodes. Ingestion by nematodes is depressed by serum, particularly IgG1, from infected animals. Similar results occur after immunization of noninfected hosts. Immune serum has no effect on pheromone communication between the sexes of helminth, but depresses ovipositional behavior of female worms. Exploitation of these chemical signals and the helminths' responses may allow novel techniques for disease control.