Differences in susceptibility and immune responses of three-spined sticklebacks (Gasterosteus aculeatus) from lake and river ecotypes to sequential infections with the eye fluke Diplostomum pseudospathaceum.

Jörn Peter Scharsack,Martin Kalbe

doi:10.1186/1756-3305-7-109

Abstract

BackgroundThe eye fluke Diplostomum pseudospathaceum is a frequent parasite of many fresh-water fish species, among those three-spined sticklebacks, particularly in lakes with lymnaeid snails, its first intermediate hosts. Cercariae released from host-snails, penetrate the skin of their fish hosts and within 24 h migrate to the immunologically inert eye lenses. Thus, individual D. pseudospathaceum are exposed to the fish immune system only for a short time, suggesting that only innate immunity can be active against the parasite. However, in nature sticklebacks are exposed to D. pseudospathaceum repeatedly since snails are shedding cercariae from late spring to autumn. Therefore, acquired immunity after initial infection would be advantageous against subsequent parasite encounters.MethodsWe investigated if sticklebacks originating from a lake with high and from a river with low prevalence of D. pseudospathaceum differ in susceptibility to repeated exposure to the parasite. We compared infection success and immune functions in laboratory-bred sticklebacks from both habitats in naïve fish with fish that had been pre-exposed to eye flukes. Head kidney leukocytes (HKL) from experimental sticklebacks were investigated for respiratory burst activity and the proliferation of lymphocytes and monocytes 1.5, 5 and 15 days after infection.ResultsLake sticklebacks were less susceptible than river sticklebacks, however, in both populations pre-exposure led to a similar relative reduction in infection success. The respiratory burst activity was higher with HKL from lake sticklebacks and was up-regulated in pre-exposed fish but dropped 1.5d after an additional exposure, suggesting that activation of phagocytic cells is crucial for the defense against D. pseudospathaceum. Changes in lymphocyte proliferation were only detectable 1.5d after the last exposure in lake sticklebacks, but not 5 and 15d post exposure, indicating that a lymphocyte mediated acquired immune response was not induced. Proliferation of monocytes was significantly increased 1.5d after the last exposure with HKL from both stickleback populations.ConclusionsIncreased resistance to D. pseudospathaceum in sticklebacks from both populations upon pre-exposure cannot be explained by a prominent adaptive immune response. Monocytic leukocytes were more responsive, suggesting that rather cells of the innate than the adaptive immune system are active in the defense of D. pseudospathaceum.

Highlights

The eye fluke Diplostomum pseudospathaceum is a frequent parasite of many fresh-water fish species, among those three-spined sticklebacks, in lakes with lymnaeid snails, its first intermediate hosts
Infection success In the eye lenses of sticklebacks exposed to D. pseudospathaceum, metacercariae from pre-exposures had larger body size (Figure 2) and could be discriminated from metacercariae from the single exposure 1.5 and 5 days after the last exposure (Figure 3), but not 15 days post last exposures
In the generalized linear model (GzLM) analysis of the infection data, origin of the stickleback hosts and stickleback family nested in origin had strong effects on the infection success of D. pseudospathaceum (Table 1)

Summary

Introduction

The eye fluke Diplostomum pseudospathaceum is a frequent parasite of many fresh-water fish species, among those three-spined sticklebacks, in lakes with lymnaeid snails, its first intermediate hosts. Cercariae released from host-snails, penetrate the skin of their fish hosts and within 24 h migrate to the immunologically inert eye lenses. Cercariae of D. pseudospathaceum, hatched from lymnaeid snails, penetrate through the skin of their fish hosts and within 24 h migrate to the immunologically inert eye lenses [1]. Selection pressure on host immunity, to defend the infection is high Eye fluke parasites such as D. pseudospathaceum, are exposed to the immune system of their fish hosts only for a relatively short time, before they hide in the immunologically privileged eye lens [7]. We test if three-spined sticklebacks (Gasterosteus aculeatus), a typical host of D. pseudospathaceum, are capable to acquire immunity against the eye fluke after repeated exposure

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