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What Determines the Risk of Trematode Infections in Amphipod Hosts?

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What Determines the Risk of Trematode Infections in Amphipod Hosts? Parasitology Research (2018) 117:1915–1923 https://doi.org/10.1007/s00436-018-5885-8 ORIGINAL PAPER Prior infections or defence priming: what determines the risk of trematode infections in amphipod hosts? Olivia G. McPherson1 & Olwyn C. Friesen1 & Christian Selbach1 & Robert Poulin1 Received: 25 February 2018 /Accepted: 18 April 2018 /Published online: 25 April 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Inducible defences against parasites that are only activated when needed can mitigate the cost of immune or behavioural evasion of parasites. Priming of the immune system and activation of behavioural defences can follow exposure to cues associated with imminent infection risk. In contrast, prior infection can cause immune depression or leave the host with less energy to defend itself against further infections. We investigate the priming of anti-parasite defences and the effect of prior infections in the amphipod Paracalliope fluviatilis, the second intermediate host of the trematode Coitocaecum parvum. During experimental infections, amphipods that had been primed by exposure to chemical cues (from first intermediate snail hosts infected by C. parvum) of infection risk were not better at avoiding further infection than control amphipods. All amphipods showed the same swimming behaviour, whether or not they had been primed by chemical cues from infected snails, or whether or not they were in the presence of live infective stages. In contrast, regardless of whether or not they had been exposed to control water or chemical cues from infected snails, amphipods harbouring prior infections acquired in nature were significantly more likely to acquire new parasites under controlled conditions. These results suggest that the induction of defences via external cues asso- ciated with the threat of infection do not play a role in the amphipod’s anti-parasite strategy. However, prior infections may pre- dispose a host to acquire further parasites, with consequences for the distribution of parasites among host individuals and the regulation of the host population. Keywords Coitocaecum parvum . Immune defence . Metacercariae . Paracalliope fluviatilis . Resistance . Swimming activity Introduction immune system compared to control individuals that are shown similar images not associated with infection threat To reduce the fitness costs associated with parasitic infections, (Schaller et al. 2010; Stevenson et al. 2011). Priming of im- animals have evolved a range of anti-parasite defences, rang- mune or behavioural defences by naïve individuals may occur ing from complex immune responses to simple behavioural through various kinds of external cues associated with an el- avoidance of parasites (Hart 1994). However, these defences evated risk of infection, either originating from conspecifics themselves are costly, whether energetically or, in the case of (Masri and Cremer 2014;Mothersilletal.2015)orfromthe behavioural defences, in terms of lost time (Sheldon and parasites themselves (Karvonen et al. 2004a;Rohretal.2009; Verhulst 1996; Lochmiller and Deerenberg 2000). As a con- Sharp et al. 2015). sequence, selection has favoured inducible defences, i.e. de- Here, we investigate the potential priming of anti-parasite fences that are only activated if and when needed (Harvell defences in the amphipod Paracalliope fluviatilis,which 1990; Shudo and Iwasa 2001). For instance, humans visually serves as the second intermediate host of the trematode perceiving signs of potential infection risk upregulate their Coitocaecum parvum (Opecoelidae). Trematodes have a com- plex life cycle, involving two or three transmission steps among different host species (Galaktionov and Dobrovolskij 2003). Typically, after multiplying asexually in a snail first * Robert Poulin intermediate host, infective stages known as cercariae leave [email protected] the snail to seek and penetrate a second intermediate host, in which they encyst as metacercariae and await transmission via 1 Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand predation to their vertebrate definitive host. Second 1916 Parasitol Res (2018) 117:1915–1923 intermediate hosts of trematodes can be primed for increased 2009b). Alternatively, individuals that already pay the cost defence upon perceiving cues indicative of imminent infection of infection by a parasite may have less energy to defend risk. For instance, fish and amphibians alter their behaviour to themselves against further parasite attacks. Indeed, inverte- avoid infection after detecting chemical cues they associate brates that have acquired many trematode metacercariae dur- with the presence of cercariae, or alarm cues released by con- ing a first exposure may accumulate more during a second specifics during infection by cercariae (Poulin et al. 1999; exposure than those that were more resistant to begin with Karvonen et al. 2004a; James et al. 2008;Rohretal.2009). (e.g. Leung et al. 2010). This is either due to intrinsic differ- In addition, adult female treefrogs can even avoid laying their ences in susceptibility among individuals, parasite-induced eggs in water bodies in which they detect chemical cues from immune depression, or prior infection predisposing a host to either cercariae or just the snail species from which cercariae acquire further parasites later. Given the lack of evidence for may emerge, thereby avoiding infection for their offspring acquired immunity against trematodes in our system, we feel (Kiesecker and Skelly 2000). However, it is unclear whether that the energetic cost of prior infection is likely to increase the priming of anti-parasite defences based on perceived chemical susceptibility of the host to further infections. cues of infection risk occurs in invertebrate intermediate hosts, The present study tests how both prior infections and prim- such as amphipods. ing via olfactory cues associated with infection risk affect the There are good a priori reasons to expect selection to have avoidance behaviour and overall ability to resist infection by favoured adaptive inducible defences based on perceived cues the trematode C. parvum in the amphipod P. fluviatilis.We in amphipods. First, amphipods often experience huge fitness hypothesise that (i) amphipods primed with chemical cues losses following infection by trematode metacercariae, includ- from the parasites will subsequently better resist cercarial at- ing intensity-dependent increases in mortality and reductions tacks than control individuals and (ii) amphipods already in reproductive output (Thomas et al. 1995; Fredensborg et al. harbouring metacercariae will be more susceptible to further 2004; Bates et al. 2010;Rauqueetal.2011). Second, some infections, regardless of whether they are primed or not. We defences against infection are costly in amphipods. test these in terms of both actual acquisition of parasites and Behavioural defences, such as increased activity and swim- induced behavioural defences manifested as altered swim- ming away from cercariae, can be very effective (O. ming patterns. McPherson and R. Poulin, personal observations) and are not very costly. However, immunological defences against parasites, involving encapsulation and melanisation, are costly Material and methods in arthropods (Moret and Schmid-Hempel 2000), including amphipods (Cornet et al. 2009a). The ability to mount such Animal collection and handling immune responses against trematodes is lost in amphipod populations never exposed to infection, presumably because Snails (Potamopyrgus antipodarum) and amphipods they are costly to maintain (Bryan-Walker et al. 2007). (Paracalliope fluviatilis) were collected in May–June 2017 Therefore, natural selection should favour efficient defences from among macrophytes in Lake Waihola, Otago, New against trematodes, but because of their cost, these should be Zealand (46° 01′ 14.1″ S, 170° 05′ 05.8″ E), using a dip net. inducible defences that are only induced via priming when In the laboratory, snails were separated into wells of culture needed. plates, with five individuals per well and approximately 2 ml Besides priming and upregulation of anti-parasite defences of filtered lake water. Infected snails were identified by being after perceiving external cues indicating a high infection risk, incubated for at least 3 h at 20 °C under a light source, a there is another way in which a host can have prior informa- procedure which induces cercarial emergence (Fredensborg tion about parasites, since it may already harbour parasites, et al. 2005). Snails were subsequently screened again individ- with consequences for its ability to resist further infections. ually to verify infection status. Infected and uninfected snails In vertebrates, acquired immunity often allows a host to better (N = 200 in each case) were maintained separately in aerated resist subsequent exposure to parasite species it already har- 10 L stock tanks filled with aged lake water and macrophytes bours (Litman et al. 2010). However, for some types of para- (Chara corallina). sites, acquisition of one or a few parasites can alter the behav- New amphipods were collected from the field prior to each iour or odour profile of the host and make it more susceptible series of trials. Only male amphipods were used to reduce to acquire further parasites in future exposures (e.g. Norval inter-individual variability in susceptibility to infection (N = et al. 1989;Poulinetal.1991). Some invertebrates display 600 for
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