Impacts of the swimbladder nematode Anguillicola crassus on Anguilla anguilla: variations in liver and spleen masses

Abstract

Variations in the liver and spleen masses of the eel Anguilla anguilla were analysed in relation to the parasite load of Anguillicola crassus at autopsy (current infection by swimbladder lumen worms) and in relation to the severity of damage observed in the swimbladder (a way of assessing the intensity of past infections). None of these measures of parasite pressure were shown to account for variation in the relative liver mass, either when controlling for somatic mass or eel age. In marked contrast, a significant increase in spleen size was revealed in eels harbouring many lumen worms and also in eels with severe damage in the swimbladder. Splenic enlargement was nearly two-fold higher among severely affected eels (harbouring more than seven lumen parasites and showing severe damage in the swimbladder) than among infectionfree eels (no lumen parasites and no pathological signs in the swimbladder). Several possible hypotheses are reviewed before arguing for an adaptive host response involving the haematological and immunological functions of the spleen. Indeed, among eels with no pathological signs in the swimbladder, the relative spleen mass was positively associated with the mass of lumen parasites, which suggests a hyper-synthesis of blood cells by the spleen in response to the bloodsucking activity of lumen worms. Nevertheless, among eels with no lumen parasites at autopsy, there was still an increase in spleen size in relation to the severity of the swimbladder damage, which also suggests a hyper-synthesis of splenic immune cells (lymphocytes and macrophages) in reaction to damaged tissues and particularly to larvae in the swimbladder wall. # 2004 The Fisheries Society of the British Isles

Is the continental life of the European eel Anguilla anguilla affected by the parasitic invader Anguillicoloides crassus ?

“…In laboratory inoculation experiments [16][17][18], the data for most proxy parameters used (including haematocrit, glucose and cortisol) have suggested a physiological cost of infection. In a companion paper (using a subset of the same dataset), we revealed a significant enlargement of the spleen (up to a factor of two) in severely affected eels, which reflects either a defence or metabolic response to infection, or both [44]. The positive effect we revealed here is especially surprising, as we found no effect of infection on any of the silvering parameters investigated; in other words, the increased length and mass was not achieved at the expense of reproduction (contrary to the theory of resource allocation [45]).…”

Parasite communities in eels of the Island of Reunion (Indian Ocean): a lesson in parasite introduction

“…Conversely, in populations of European and the American eels, the prevalence and intensity are significantly higher which coincides with a substantially increased size of parasite individuals and conspicuous pathogenicity. This difference seems to result from a lack of adaptation between host and parasite in the novel Atlantic hosts, which do not develop a concomitant immunity like the well-adapted natural host (Würtz and Taraschewski 2000;Lefebvre et al 2002Lefebvre et al , 2004Knopf 2006;Münderle et al 2006;Taraschewski 2006).…”

Immunocompetence does not correlate with resistance to helminth parasites in house mouse subspecies and their hybrids

“…The use of spleen size as an indirect measure of immunocompetence is questioned by some authors who argued that variation of spleen size reflects only current infection (splenomegaly) or other environmental effects (Brown and Bomberger Brown 2002;Smith and Hunt 2004). One recent study on the eel, Anguilla anguilla, tested the relationship between spleen mass and parasite load of the nematode Anguillicola crassus, showing that splenic enlargement is the host response to infection (Lefebvre et al 2004). In our study, animals are bred under controlled conditions without any macroparasites, allowing us to test without bias the relationship between spleen mass and parasite resistance.…”