A brain-infecting parasite impacts host metabolism both during exposure and after infection is established

Metabolic costs associated with parasites should not be limited to established infections. Even during initial exposure to questing and attacking parasites, hosts can enact behavioural and physiological responses that could also incur metabolic costs. However, few studies have measured these costs directly. Hence, little is known about metabolic costs arising from parasite exposure. Furthermore, no one has yet measured whether and how previous infection history modulates metabolic responses to parasite exposure. Here, using the California killifish Fundulus parvipinnis and its brain‐infecting parasite Euhaplorchis californiensis, we quantified how killifish metabolism, behaviour and osmoregulatory phenotype changed upon acute exposure to parasite infectious stages (i.e. cercariae), and with long‐term infection. Exposure to cercariae caused both naïve and long‐term infected killifish to acutely increase their metabolic rate and activity, indicating detection and response to parasite infectious stages. Additionally, these metabolic and behavioural effects were moderately stronger in long‐term infected hosts than naïve killifish, suggesting that hosts may develop learned behavioural responses, nociceptor sensitization and/or acute immune mechanisms to limit new infections. Although established infection altered the metabolic response to parasite exposure, established infection did not alter standard metabolic rate, routine metabolic rate, maximum metabolic rate, aerobic scope or citrate synthase enzyme activity. Unexpectedly, established infection reduced lactate dehydrogenase enzyme activity in killifish brains and relative Na+/K+‐ATPase abundance in gills, suggesting novel mechanisms by which E. californiensis may alter its hosts\u27 behaviour and osmoregulation. Thus, we provide empirical evidence that parasites can disrupt the metabolism of their host both during parasite exposure and after infection is established. This response may be modulated by previous infection history, with probable knock‐on effects for host performance, brain energy metabolism, osmoregulation and ecology. A free Plain Language Summary can be found within the Supporting Information of this article

Regional Distribution of a Brain-Encysting Parasite Provides Insight on Parasite-Induced Host Behavioral Manipulation

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Behavioural Differences in California Killifish Experimentally Infected With the Brain Parasite Euhaplorchis Californiensis

Parasitized animals often display altered behaviours in ways that promote parasite transmission to subsequent hosts in their life cycle. Euhaplorchis californiensis (Euha) is a trematode parasite with a three host life cycle; moving between the California horn snail, the California killifish, and a number of fish-eating bird species. Notably, Euha encycst on the brain of the killifish host and alters its behaviour, which make Euha infected killifish 10-30 times more likely to be eaten by a bird compared to uninfected fish. In order to get a more comprehensive understanding of the behavioural changes underlying this increased transmission rate, we exposed lab-reared killifish to one of four experimental treatments; negative control (seawater), positive control (a low dose of a cyathocotylid trematode), low and high Euha load. Experimentally infected fish and controls were then subjected to a stress regime lasting a total of 90 minutes. Our results show that the behavioural effects of stress are highly affected by infection intensity. Fish infected with a low Euha load spent significantly less time moving compared to the high infected groups. In addition, a low Euha load induced a significantly lower cortisol response to stress compared to the negative control. Conspicuous behaviours associated with increased predation risk were higher in fish exposed to a high load compared to negative controls. We conclude that behavioural patterns associated with increased predation risk are modulated by both presence and intensity of parasite infestation. The next step will be to study brain mechanisms behind this behavioural manipulation

Regional distribution of a brain-encysting parasite provides insight on parasite-induced host behavioral manipulation

Some parasite species alter the behavior of intermediate hosts to promote transmission to the nexthost in the parasite’s life cycle. This is the case for Euhaplorchis californiensis, a brain-encystingtrematode parasite that causes behavioral changes in the California killifish (Fundulus parvipinnis).These manipulations increase predation by the parasite’s final host, piscivorous marsh birds. Themechanisms by which E. californiensis achieves this manipulation remain poorly understood. As E.californiensis cysts reside on the surface of the killifish’s brain, discerning regional differences inparasite distribution could indicate mechanisms for host control. In this study, we developed amethod for repeated experimental infections. In addition, we measured brain-region specific density using a novel methodology to locate and quantify parasite infection. We show that E. californiensis cysts are non-randomly distributed on the fish brain, aggregating on the diencephalon/mesencephalon region (a brain area involved in controlling reproduction and stress coping) and the rhombencephalon (an area involved in controlling locomotion and basal physiology).Determining causal mechanisms behind this pattern of localization will guide future researchexamining the neurological mechanisms of parasite-induced host manipulation. These findingssuggest that parasites are likely targeting the reproductive, monoaminergic, and locomotor systemsto achieve host behavioral manipulation

How a Brain-Infecting Parasite Alters Energy Metabolism in a Shoaling Fish: Implications for Conditioned Fear Responses and Mechanisms of Behaviour Modification

Parasites are increasingly being recognised as important players affecting individuals, populations, communities, and even ecosystems. Some parasites change their host’s behaviour to aid transmission to the next host in their life cycle. Parasite-induced behaviour modification may both drive and be driven by changes in host energy metabolism. Here, we examined the gregarious estuarine fish species, the California killifish (Fundulus parvipinnis) and its brain-infecting, behaviour-manipulating trematode parasite (Euhaplorchis californiensis). Killifish were reared in shoals from hatching in either a control (uninfected) or twice-weekly infection (infected) treatment for 12 months. Standard metabolic rate (SMR), maximum metabolic rate (MMR), and acute metabolic response to infectious propagules (MRacute) were then measured using respirometry chambers that allowed these social fish to smell and see their shoal-mates to prevent isolation stress. Brain, gill, and white muscle samples were extracted to assess the way infection influences citrate synthase (CS) and lactate dehydrogenase (LDH) enzymatic activities as indicators of aerobic and anaerobic metabolism, respectively. While SMR and MMR were unaffected by infection status, both uninfected and infected fish exhibited spikes in MRacute. Intriguingly, infected fish mounted a stronger response to exposure than naïve (control) hosts, implying a learned ‘fear’ response to parasite exposure. Measures of enzyme activity suggested that only the brain (the site of the infection) was impacted by the parasites, with infected individuals exhibiting lower LDH activity than uninfected fish. Given the importance of lactate in brain function, these results could suggest a mechanism for parasiteinduced behavioural modification

A brain‐infecting parasite impacts host metabolism both during exposure and after infection is established

Metabolic costs associated with parasites should not be limited to established infections. Even during initial exposure to questing and attacking parasites, hosts can enact behavioural and physiological responses that could also incur metabolic costs. However, few studies have measured these costs directly. Hence, little is known about metabolic costs arising from parasite exposure.Furthermore, no one has yet measured whether and how previous infection history modulates metabolic responses to parasite exposure.Here, using the California killifish Fundulus parvipinnis and its brain-infecting parasite Euhaplorchis californiensis, we quantified how killifish metabolism, behaviour and osmoregulatory phenotype changed upon acute exposure to parasite infectious stages (i.e. cercariae), and with long-term infection.Exposure to cercariae caused both naïve and long-term infected killifish to acutely increase their metabolic rate and activity, indicating detection and response to parasite infectious stages. Additionally, these metabolic and behavioural effects were moderately stronger in long-term infected hosts than naïve killifish, suggesting that hosts may develop learned behavioural responses, nociceptor sensitization and/or acute immune mechanisms to limit new infections.Although established infection altered the metabolic response to parasite exposure, established infection did not alter standard metabolic rate, routine metabolic rate, maximum metabolic rate, aerobic scope or citrate synthase enzyme activity.Unexpectedly, established infection reduced lactate dehydrogenase enzyme activity in killifish brains and relative Na+/K+-ATPase abundance in gills, suggesting novel mechanisms by which E. californiensis may alter its hosts' behaviour and osmoregulation.Thus, we provide empirical evidence that parasites can disrupt the metabolism of their host both during parasite exposure and after infection is established. This response may be modulated by previous infection history, with probable knock-on effects for host performance, brain energy metabolism, osmoregulation and ecology

Experimental infections with <i>euhaplorchis californiensis </i>and a small cyathocotylid increase conspicuous behaviors in California killfish (<i>fundulus parvipinnis</i>)

Some parasites manipulate their host’s phenotype to enhance predation rates by the next host in the parasite’s life cycle. Our understanding of this parasite-increased trophic transmission is often stymied by study-design challenges. A recurring difficulty has been obtaining uninfected hosts with a coevolutionary history with the parasites, and conducting experimental infections that mimic natural processes. In 1996, Lafferty and Morris provided what has become a classic example of parasite-increased trophic transmission; they reported a positive association between the intensity of a brain-infecting trematode (Euhaplorchis californiensis) in naturally infected California killifish (Fundulus parvipinnis) and the frequency of conspicuous behaviors, which was thought to explain the documented 10–303 increase in predation by the final host birds. Here, we address the primary gap in that study by using experimental infections to assess the causality of E. californiensis infection for increased conspicuous behaviors in F. parvipinnis. We hatched and reared uninfected F. parvipinnis from a population co-occurring with E. californiensis, and infected them 1–2 times/week over half their life span with E. californiensis and a small cyathocotylid trematode (SMCY) that targets the host’s muscle tissue. At 3 time points throughout the hosts’ lives, we quantified several conspicuous behaviors: contorting, darting, scratching, surfacing, and vertical positioning relativeto the water’s surface. Euhaplorchis californiensis and SMCY infection caused 1.8- and 2.5-fold overall increases in conspicuous behaviors, respectively. Each parasite was also associated with increases in specific conspicuous behaviors, particularly 1.9- and 1.4-fold more darting. These experimental findings help solidify E. californiensis–F. parvipinnis as a classic example of behavioralmanipulation. Yet our findings for E. californiensis infection–induced behavioral change were less consistent and strong than those previously documented. We discuss potential explanations for this discrepancy, particularly the idea that behavioral manipulation may be most apparent when fish are actively attacked by predators. Our findings concerning the other studied trematode species,SMCY, highlight that trophically transmitted parasites infecting various host tissues are known to be associated with conspicuous behaviors, reinforcing calls for research examining how communities of trophically transmitted parasites influence host behavior