Strongyloides stercoralis: Amphidial neuron pair ASJ triggers significant resumption of development by infective larvae under host-mimicking in vitro conditions.

Resumption of development by infective larvae (L3i) of parasitic nematodes upon entering a host is a critical first step in establishing a parasitic relationship with a definitive host. It is also considered equivalent to exit from the dauer stage by the free-living nematode Caenorhabditis elegans. Initiation of feeding, an early event in this process, is induced in vitro in L3i of Strongyloides stercoralis, a parasite of humans, other primates and dogs, by culturing the larvae in DMEM with 10% canine serum and 5mM glutathione at 37 degrees C with 5% CO(2). Based on the developmental neurobiology of C. elegans, resumption of development by S. stercoralis L3i should be mediated, in part at least, by neurons homologous to the ASJ pair of C. elegans. To test this hypothesis, the ASJ neurons in S. stercoralis first-stage larvae (L1) were ablated with a laser microbeam. This resulted in a statistically significant (33%) reduction in the number of L3i that resumed feeding in culture. In a second expanded investigation, the thermosensitive ALD neurons, along with the ASJ neurons, were ablated, but there was no further decrease in the initiation of feeding by these worms compared to those in which only the ASJ pair was ablated.

New oral linguiform projections and their associated neurons in the third-stage infective larva of the parasitic nematode Oesophagostomum dentatum.

The infective larvae (L3i) of the nematode parasite of swine, Oesophagostomum dentatum, are passively ingested by their hosts. The L3i exhibit certain behaviors that are probably selected to increase the likelihood of ingestion, by strategic positioning in the environment. The larvae show positive geotactic behavior and respond to temperature variations in their environment, as shown by their behavior on a thermal gradient. To investigate neuronal control of this behavior, we initiated a study of the structure of the amphidial neurons of this parasite. The same number and types of neuronal dendritic processes are found in the amphids of the O. dentatum L3i as in those of its close relatives Haemonchus contortus and Ancylostoma caninum. Well-developed dendritic processes of wing cells are located in the amphidial sheath cells, these being similar to wing cells AWA in the free-living nematode Caenorhabditis elegans but actually more extensive. Similar to its close relatives just mentioned, and C. elegans as well, O. dentatum L3i has prominent finger cell processes, the finger cell neurons being the thermoreceptors in all 3 of the preceding species. However, unlike the arrangement seen in H. contortus and A. caninum, where the microvilli-like "fingers" of these neurons lie dorsal to the amphidial channel and occupy a very large portion (>50%) of the anterior end of the larva, the dendritic process of the finger cells in O. dentatum extends into unusual linguiform projections that, in turn, extend into the lumen of the mouth tube, a complex structural arrangement that has not been described for any other nematode.

Amphidial structure of ivermectin-resistant and susceptible laboratory and field strains of Haemonchus contortus.

The development of anthelmintic resistance by nematode parasites is a growing problem for veterinarians, pet owners, and producers. The intensive use of the macrocyclic lactones for the treatment of a variety of parasitic diseases has hastened the development of resistance to this family of parasiticides. As a result, resistance to ivermectin, moxidectin, nemadectin, and doramectin by Haemonchus contortus has been documented throughout the world. Sensory neurons located in the cephalic end of nematodes are in close contact with the external environment. Through these neurons, important chemical and thermal cues are gathered by the parasite. Examination of serial electron micrographs of ivermectin-susceptible and ivermectin-resistant H. contortus allows for comparison of neuronal structure, arrangement of neurons within the amphidial channel, and distance of the tip of the dendritic processes to the amphidial pore. The latter of these characteristics provides a useful means by which to compare the association between the neurons and the external environment of the worm. Comparison of parental laboratory strains of ivermectin-susceptible strains of H. contortus with related selected, ivermectin-resistant strains and with a wild-type ivermectin-susceptible field strain of H. contortus from Louisiana reveal that the ivermectin-resistant worms examined have markedly shorter sensory cilia than their ivermectin-susceptible parental counterparts. Additionally, the amphidial neurons of ivermectin-resistant worms are characterized by generalized degeneration and loss of detail, whereas other neurons outside of the channels, such as the labial and cephalic neurons, are normal in structure. These findings raise a number of questions regarding the relationship between amphidial structure and ivermectin resistance as well as the role of amphids as a means of entry for ivermectin. While shortened amphidial sensilla are associated with ivermectin resistance, it remains unclear if such a structural modification facilitates survival of nematodes exposed to macrocyclic lactones.

Response to carbon dioxide by the infective larvae of three species of parasitic nematodes.

The response of infective third-stage larvae (L3) of three species of parasitic nematodes, Ancylostoma caninum, Strongyloides stercoralis, and Haemonchus contortus to carbon dioxide (CO(2)) at physiological concentrations was investigated. L3 of the skin-penetrating species, A. caninum and S. stercoralis, were stimulated by CO(2) at the concentration found in human breath (3.3-4%); these larvae responded by crawling actively, but not directionally. Crawling was not stimulated by breath passed through a CO(2)-removing "scrubber" or by "bench air". Both A. caninum and S. stercoralis L3 stopped crawling when exposed to 5% CO(2) for 1 min. L3 of A. caninum became active 9-14 min after exposure to 5% CO(2) ended, but activity resumed more rapidly (10-15 s) if larvae were subsequently exposed to breath or breath through the scrubber. L3 of S. stercoralis resumed crawling 30-35 s after exposure to 5% CO(2), but resumed crawling within a very few seconds when exposed to breath or breath through the scrubber. Thus, while 5% CO(2) was inhibitory, lower concentrations of this gas stimulated L3 of both species. Apparently, exposing immobilized larvae to breath or breath through the scrubber causes the environmental CO(2) concentration to drop to a level that is stimulatory. The L3 of H. contortus ceased crawling and coiled when exposed to human breath or to 1% CO(2), but continued to move within the coil in both cases. The crawling response of the L3 of the two skin-penetrating species, A. caninum and S. stercoralis, to stimulation by CO(2) probably relates to their active host-finding behavior, while the cessation response elicited by CO(2) in H. contortus larvae may relate to the fact that they rely on passive ingestion by a ruminant host.