Temperature-dependent behaviors of parasitic helminths

Abstract

Parasitic helminth infections are the most common source of neglected tropical disease among impoverished global communities. Many helminths infect their hosts via an active, sensory-driven process in which environmentally motile infective larvae position themselves near potential hosts. For these helminths, host seeking and host invasion can be divided into several discrete behaviors that are regulated by both host-emitted and environmental sensory cues, including heat. Thermosensation is a critical sensory modality for helminths that infect warm-blooded hosts, driving multiple behaviors necessary for host seeking and host invasion. Furthermore, thermosensory cues influence the host-seeking behaviors of both helminths that parasitize endothermic hosts and helminths that parasitize insect hosts. Here, we discuss the role of thermosensation in guiding the host-seeking and host-infection behaviors of a diverse group of helminths, including mammalian-parasitic nematodes, entomopathogenic nematodes, and schistosomes. We also discuss the neural circuitry and molecular pathways that underlie thermosensory responses in these species.

Keywords: Host seeking; Parasitic helminth; Parasitic nematode; Schistosomes; Sensory behavior; Strongyloides; Thermosensation.

Fig. 1.. The life cycles of parasitic helminths.

A-C. Life cycles of mammalian-parasitic nematodes. Soil-dwelling developmentally arrested infective larvae (iL3s) seek out hosts using host-emitted sensory cues, including heat [19]. Across species, infection routes include skin penetration (A) and oral ingestion (C), or both in the case of certain hookworm species (B) [,,–39,89]. Following host infection, the nematodes resume development and migrate to the small intestine, where they take up residence as reproductively capable parasitic adults [2]. Larvae or eggs then exit hosts in feces. For Strongyloides stercoralis, larvae may develop into iL3s or free-living adults; the progeny of free-living adults exclusively become iL3s (A). For hookworms and passively ingested nematodes, the progeny of parasitic adults develop into iL3s (B-C). D. The life cycle of entomopathogenic nematodes (EPNs). Soil-dwelling infective juveniles (IJs), which are developmentally similar to the iL3s of mammalian-parasitic nematodes, invade and then rapidly kill insect hosts [50,51]. EPNs can develop and reproduce inside the host cadaver for multiple generations, until depleted resources within the cadaver trigger the formation of IJs that are released into the environment [50]. E. The life cycle of schistosomes. Unlike parasitic nematodes, the schistosome life cycle involves an intermediate and a definitive host animal [27,28]. Some schistosome species seek out both intermediate and/or definitive hosts using host-emitted sensory cues. Free-swimming miracidia infect aquatic snails (intermediate hosts). Following snail penetration the schistosomes develop into mother sporocysts and produce daughter sporocysts whose larval progeny become cercariae [27]. Water-transmitted cercariae emerge from snails and infect the definitive hosts. Inside the definitive host, cercariae transform into schistosomula, which develop and migrate through the host circulatory system. Depending on the schistosome species, parasitic adults will ultimately reside in the veins draining blood from the intestines, liver, or bladder. The eggs of parasitic adults are excreted in feces or urine, and subsequently develop into miracidia [27]. Diagrams are not drawn to scale.

Fig. 2.. Temperature-dependent navigation behaviors of parasitic nematodes

A. Schematic of a thermotaxis assay. A linear thermal gradient is established across a 22 × 22 cm agar surface, using a custom thermal stage [69]. iL3s are placed at a selected starting temperature (T_start) and allowed to disperse. Two cameras record worm movements, each camera monitoring approximately half of the thermal gradient. The final position of worms in the thermal gradient is calculated post hoc: images corresponding to the desired experimental time point are divided into 1°C temperature bins, and the number of worms in each bin is tallied [69]. Positive thermotaxis is defined as movement into a temperature bin warmer than T_start; negative thermotaxis is defined as movement into a temperature bin cooler than T_start. Worms are not drawn to scale. B. S. stercoralis iL3s engage in long-range positive and negative thermotaxis, and the switch point between these behaviors is set by the recently experienced cultivation temperature (T_C). Left: S. stercoralis iL3s cultivated at 23°C and then placed at 25°C in a ~22°C-34°C gradient engage in long-range positive thermotaxis toward mammalian body temperatures. Center: S. stercoralis iL3s cultivated at 23°C and then placed at 23°C in ~22°C33°C gradient display both positive and negative thermotaxis. Right: S. stercoralis iL3s that have been cultivated at 15°C for 7 days exhibit only positive thermotaxis when placed at 23°C in a ~22°C-33°C gradient. Assay duration: 15 minutes, n = 15 trials with >50 iL3s per trial. Gray shading indicates the starting temperature of the iL3s (T_start). All graphs show medians and interquartile ranges; in some cases, error bars are too small to be visible. Data are reproduced with permission from Bryant et al., 2018 [69]. C. Schematic of a chemotaxis assay. iL3s are placed in the center of a 10 cm agar plate containing a point source of an odorant on one side and a point source of a control (often paraffin oil) on the other side. The distribution of iL3s in the odorant gradient is then quantified after 3 hours by calculating a chemotaxis index using the formula shown. The chemotaxis index ranges from −1 to +1, with −1 indicating maximum repulsion and +1 indicating maximum attraction. Worms are not drawn to scale. Figure is adapted from Lee et al., 2016 [80]. D. Temperature-dependent changes in the chemosensory responses of the skin-penetrating nematode Strongyloides ratti. Left: S. ratti iL3s cultivated at 15°C for 7 days are repelled by the host-emitted odorant 3meythl-1-butanol, whereas S. ratti iL3s cultivated at 30°C for 7 days show significantly reduced repulsion. Right: S. ratti iL3s cultivate at 15°C for 7 days are neutral to isovaleric acid, whereas S. ratti iL3s cultivated at 30°C for 7 days are attracted to isovaleric acid. *, p<0.05; **, p<0.01; two-way ANOVA with Tukey’s post-test. n = 6–8 trials with >100 iL3s per trial. Lines and boxes show medians and interquartile ranges. Figure is adapted from Lee et al., 2016 [80]. E. Time course of temperature-dependent changes in chemosensory responses of the EPN Steinernema carpocapsae. Temperature-swapping IJs from 25°C to 15°C altered chemosensory responses over the course of days. Prior to the temperature swap, IJs cultivated at 25°C were strongly repelled by the insect-emitted odorant 2-propanone; over time at 15°C, the response gradually shifted to strong attraction. When IJs were swapped back to 25°C, their response to 2-propanone reverted to repulsion over the course of days. n = 6–22 trials for each time point. Graph depicts means and standard errors of the mean. Data are from Lee et al., 2016 [80].

Fig. 3.. Neuroanatomy of C. elegans, S. stercoralis and H. contortus thermosensory amphid neurons.

A-B. The cell body positions (A) and dendritic structures (B) of the thermosensory amphid neurons in a C. elegans L1 larva. The C. elegans AFD neurons are the primary thermosensory neurons in the amphids; they are characterized by highly elaborate “finger-like” endings [174]. The C. elegans AWC olfactory neurons also respond to thermosensory cues; their dendritic endings are characterized by large “wing-like” structures. A number of other amphid sensory neurons are also labeled. A is modified from Ashton et al., 1995 with permission [161]; B is reproduced from Altun and Hall, 2010 [234]. C-D. The cell body position (C) and dendritic ending (D) of the ALD thermosensory neuron pair in an S. stercoralis iL3. S. stercoralis lacks cells with “finger-like” or “wing-like” dendritic endings; the ALD neuron has a “lamellar” structure, has thermosensory function, and is the homolog of either C. elegans AFD or AWC [,–163]. A number of other amphid sensory neurons are also labeled. C-D are modified from Ashton et al., 1995 and Lopez et al., 2000 with permission [65,161]. E-F. The cell body positions (E) and dendritic ending (F) of the AFD and AWC neurons in an H. contortus L1 larva. The H. contortus AFD neurons are required for thermotaxis, whereas the H. contortus AWC neurons are not known to be required [95]. A number of other amphid sensory neurons are also labeled. E-F are adapted from Li et al., 2000a and Li at al., 2000b with permission [95,166]. For panels showing cell body positions (A, C, E), anterior is to the left. For panels showing dendritic endings (B, D, F), anterior is to the top.

Fig. 4.. Temperature-driven behaviors of skin-penetrating nematodes.

Thermal cues elicit a diverse set of behaviors in the soil-dwelling iL3s of skin-penetrating nematodes. These behaviors include: (A) arousal, characterized by non-directional movement in the presence of heat; (B) environmental navigation, characterized by positive and negative thermotaxis; (C) long-range host seeking, characterized by positive thermotaxis; (D) skin penetration; and (E) activation, in which the developmentally arrested iL3s resume development inside the host. Diagrams are not drawn to scale.