Relative importance of chemical attractiveness to parasites for susceptibility to trematode infection

Abstract

While the host immune system is often considered the most important physiological mechanism against parasites, precontact mechanisms determining exposure to parasites may also affect infection dynamics. For instance, chemical cues released by hosts can attract parasite transmission stages. We used the freshwater snail Lymnaea stagnalis and its trematode parasite Echinoparyphium aconiatum to examine the role of host chemical attractiveness, physiological condition, and immune function in determining its susceptibility to infection. We assessed host attractiveness through parasite chemo‐orientation behavior; physiological condition through host body size, food consumption, and respiration rate; and immune function through two immune parameters (phenoloxidase‐like and antibacterial activity of hemolymph) at an individual level. We found that, although snails showed high variation in chemical attractiveness to E. aconiatum cercariae, this did not determine their overall susceptibility to infection. This was because large body size increased attractiveness, but also increased metabolic activity that reduced overall susceptibility. High metabolic rate indicates fast physiological processes, including immune activity. The examined immune traits, however, showed no association with susceptibility to infection. Our results indicate that postcontact mechanisms were more likely to determine snail susceptibility to infection than variation in attractiveness to parasites. These may include localized immune responses in the target tissue of the parasite. The lack of a relationship between food consumption and attractiveness to parasites contradicts earlier findings that show food deprivation reducing snail attractiveness. This suggests that, although variation in resource level over space and time can alter infection dynamics, variation in chemical attractiveness may not contribute to parasite‐induced fitness variation within populations when individuals experience similar environmental conditions.

Chance or choice? Understanding parasite selection and infection in multi-host communities

“…Similarly, Langeloh and Seppälä (2018) reported a disconnect between hosts' attractiveness to cercariae and their suitability for infection. In experimental exposures, larger snails were both more attractive to echinostome cercariae but also exhibited lower susceptibility, likely due to higher metabolic activity and immune activity.…”

“…Previous investigations have found mixed evidence for host selection in relation to host competence. While some studies have reported a positive relationship between host susceptibility and parasite or parasitoid attraction (e.g., Sears et al, 2012;Han et al, 2013), many other parasites select hosts that are of low competence or completely unsuitable (e.g., Driessen et al, 1990;Kimura and Suwito, 2014;Langeloh and Seppälä, 2018) -an outcome with implications for biocontrol applications (e.g., Goldson et al, 1992). The avoidance of already infected individuals by parasites -presumably as a mechanism to reduce intra-host competition -has also been demonstrated across a range of systems including parasitoid wasps, entomopathogenic nematodes, and trematode miracidia (e.g., Grewal et al, 1997;Allan et al, 2009;Ruschioni et al, 2015).…”

New species, new story: The impact of invasive non‐host predators on host‐trematode interactions

“…In a search for a target host, trematode larvae also use the appropriate host chemical signals (Haberl et al, 2000). The chemo‐orientation of E. aconiatum cercariae towards the host's signals has been shown to be sufficiently specialised to even recognises its condition (Langeloh & Seppälä, 2018; Seppälä & Leicht, 2015), causing a change in the swimming behaviour of cercariae (Haas et al, 1995). Moreover, actively searching cercariae can regulate their swimming speed in the aquatic environment (Morley, 2020).…”

To the skies and underground : Ecological and behavioural aspects of dispersal and pupation of the small hive beetle (Aethina tumida Murray; Coleoptera: Nitidulidae) as an invasive species

“…It is likely that the attractiveness of colonies to SHBs depends on strength of host cues (Graham et al, 2011;Suazo et al, 2003;Torto et al, 2005), although the quantity of volatiles emitted by colonies, and the relative attractiveness to SHBs has not been explored in this study or other studies. Furthermore, other factors such as host size itself can play a role in the attractiveness of hosts to parasites (Langeloh & Seppälä, 2018;Lo et al, 1998;Valera et al, 2004), but for honey bee colonies this is likely an interactive effect as larger colonies release higher amounts of odours than smaller ones. In our experiment, we removed worker bees to create weakened colonies.…”