An evaluation of indices of gross pathology associated with the nematode Anguillicoloides crassus in eels

Abstract

This study compares two alternative indices for quantifying the gross pathology of the swimbladder of eels, Anguilla anguilla (L.), infected with the nematode Anguillicoloides crassus. Two observers recorded twice the scores obtained by the two indices on the same set of 71 wild caught eels (from elver to silver eels, French Mediterranean lagoons). The Length Ratio Index (LRI), performed better than the Swimbladder Degenerative Index (SDI), in three of four predefined criteria of decision. First, the LRI better correlated with an estimate of the swimbladder volume reduction, a functional consequence of the infection (representativeness). Also, the LRI was less prone to subjectivity (inter-observer variability) and more precise (intra-observer variability), although less easy to generate (time needed for measurement/assessment). Using a sub-sample of 32 unaffected eels (showing minor if any swimbladder damage and no living worms at autopsy), we ascertained a linear relationship between the swimbladder length and the total body length, a prerequisite of isometric growth, to definitively accept the new ratio index as a valid alternative to the SDI. Also, because the LRI can be recorded on live specimens with radio-imagery (non-invasive method), we recommend its use, and provide a graph of correspondence between the SDI scores, the LRI scores and the estimated proportion of gas loss in the swimbladder.

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

“…Moreover, we recently demonstrated a very strong association between the severity in the SDI scores and the functional loss in swimbladder gas volume, thereby definitively validating the biological significance of the index [25]. For statistical reasons (i.e.…”

“…In the case of anguillicolosis, we suggest that the only realistic way to resolve the observed paradox (i.e. greater size/better condition among severely affected eels, observed in this and other studies) is to combine mark -recapture sampling with echo/radio imagery techniques to non-invasively assess the swimbladder condition among tagged eels (for reliability and applicability; [25,64]). Therefore, we call for scientists and eel managers to consider integrating an epidemiological dimension into their long-term monitoring programmes.…”

Influence of introduced vs. native parasites on the body condition of migrant silver eels

“…Nevertheless, using the Swimbladder Degenerative Index, Lefebvre et al [44] recently demonstrated that the most affected eels had greater body length and mass (+11% and +41% respectively) than unaffected eels of the same age. Despite these surprising counterintuitive results, high virulence and severe impacts of A. crassus are expected because A. anguilla lacks an adaptive immune response, and various pathogenic potentially lethal effects (e.g., anemia, energy drain, swimming performance decrease) have been demonstrated, threatening the success of spawning migration in the Sargasso Sea [35, 43, 44, 53, 58]. …”

Contamination, parasitism and condition of Anguilla anguilla in three Italian stocks

“…Following Székely et al (2005), we computed the infection level as the ratio between the parasite total weight and the eel weight (Wp/ W). The relative swim bladder length or Length Ratio Index (LRI), computed as the ratio between the swim bladder length and the eel length (Ls/L), was used for establishing the degree of the mechanical swim bladder damage: the smaller LRI, the higher the damage (Lefebvre et al 2011). …”