Anguillicola crassus: monitor eel swim bladders

European eels undertake a long oceanic spawning migration and major daily vertical movements. Their swim bladder contributes to buoyancy control and reduces the energetic cost of these movements. The invasive nematode Anguillicola crassus lives in this organ, where it can cause inflammation, thickening and impaired function. For surveillance programmes, presence alone is insufficient: parasite burden, historical damage and eel life stage need to be recorded separately.

A life cycle maintained through food webs

A. crassus is a nematode native to East Asia that reached Europe in the early 1980s. It rapidly established in Anguilla anguilla. Adults reproduce in the swim bladder lumen. Eggs pass through the pneumatic duct, larvae enter the environment and are consumed by copepods or other intermediate hosts. Fish, molluscs, amphibians and insect larvae may also carry stages as paratenic hosts.

Eels acquire infection by eating these organisms. Larvae cross the digestive wall and migrate to the swim bladder, where development continues. This food-web pathway helps explain why handling visible eels alone does not necessarily interrupt transmission in a natural water body.

Salinity modifies transmission. The German thirty-year study found lower burdens in brackish coastal waters than in inland freshwater. Higher salinity reduces survival and infectivity of some stages, but it does not ensure that coastal eels are parasite-free.

Thirty years of data show change, not disappearance

Unger and colleagues examined 16,508 eels from 217 sampling events between 1991 and 2020 in inland and coastal waters of Mecklenburg-Western Pomerania. During the first decade, mean prevalence reached 79.5% in freshwater and 61.3% in coastal areas. It later declined, with a stronger reduction in inland waters and coastal prevalence stabilising near 45.6% in the final decade.

That decline does not mean the problem has resolved. The parasite remains widely established, and a more balanced host-parasite relationship across a time series does not imply absence of damage in individual eels. The authors discuss possible mechanisms including selective mortality of heavily affected individuals, host-population immunity and environmental constraints on the parasite cycle.

Prevalence is the proportion of examined eels carrying at least one parasite. It describes neither the number of nematodes in positive fish nor organ function. Useful comparison should combine prevalence, intensity, abundance and a swim-bladder damage score using consistent methods across sites and years.

Damage may remain after parasites have gone

Adults feed on blood inside the swim bladder. Larval and adult stages trigger inflammation, haemorrhage and wall thickening. Repeated infections can produce fibrosis, volume loss and lasting changes in mechanical properties. A bladder without visible nematodes at necropsy may therefore remain severely damaged.

Histopathology documents inflammation, fibrosis and tissue remodelling. At programme scale, a standard macroscopic score can compare a normal, opaque, thickened or severely deformed wall. It requires an explicit reading protocol and observer training.

Pelster’s physiological review explains why the organ matters during migration. Silver eels travel more than 5,000 kilometres and move between substantially different depths each day. A functioning bladder helps adjust buoyancy. Swim-tunnel studies and tissue observations support adverse effects from severe damage, but they cannot predict an individual eel’s migration success from one parasite count.

Build interpretable surveillance

Sampling plans should record catchment, habitat type, season, capture method, body size, yellow or silver stage and whether eels are wild or stocked. These variables influence exposure and interpretation. Trend series should use comparable periods and size classes.

At necropsy, the swim bladder is opened using a standard method. Staff count parasites, distinguish stages where possible and score the wall. Reports include the total number of eels examined: a percentage positive without its denominator is difficult to compare.

Non-lethal methods are desirable for conservation-sensitive populations, but no simple external observation currently replaces direct examination for precise description of parasites and previous damage. Ultrasound, radiography or biomarkers may answer focused questions after validation with explicit sensitivity and specificity.

Prevent avoidable transfers

Restocking, research and conservation operations can move parasite stages or hosts along with eels and water. Biosecurity procedures document origin, health status, transport water, equipment cleaning and effluent destination. Measures must fit the regulatory framework and objectives of the relevant eel management plan.

Individual treatment is not an ecological solution at catchment scale. Decisions must avoid unauthorised antiparasitic use and environmental discharge. Prevention relies mainly on movement controls and comparable long-term surveillance.

Limits and cautions

The German data do not represent every European river. Salinity, temperature, host communities and invasion history differ. Association between bladder damage and body condition cannot by itself establish an effect on spawning, because migration barriers, pollutants, fishing and food availability also influence eels.

Stable prevalence should not be interpreted as stable risk. Intensity, cumulative lesions, age composition and habitat quality can move in different directions. Health surveillance complements demographic and ecological indicators; it does not replace them.

Conclusion

Anguillicola crassus is firmly established in Europe and can damage an organ central to eel buoyancy. Effective surveillance separates presence, burden and lesion, then relates them to stage, site and season. A decline in one long German series describes local dynamics rather than disappearance of the hazard.

Vetofish can help define necropsy and scoring protocols, train field teams, interpret parasitological results and integrate biosecurity into eel operations.