What is Anguillicoloides crassus?

Anguillicoloides crassus (we’ll call it A. crassus) is a tiny worm that lives inside the swim bladder of European eels.
The swim bladder is like the eel’s built-in buoyancy aid — it helps them float, dive and swim properly.

This parasite isn’t originally from Europe. It arrived in the 1980s when eels were moved around for fish farming, and has now spread across many rivers and lakes in the UK. Sadly, it’s now one of the biggest health threats our eels face.

What does A. crassus do to eels?

Once inside an eel, these worms can:

• Damage the swim bladder, making the walls thicker and inflamed
• Make it harder for the eel to control its buoyancy
• Use up the eel’s energy
• Reduce swimming ability, especially on long journeys

For young anglers: imagine trying to swim with a heavy rucksack full of water — that’s what it’s like for an eel with a bad infection.

These problems make it much harder for eels to reach the Sargasso Sea, where they swim thousands of miles to spawn.

The Life Cycle — How the parasite spreads

It may be tiny, but A. crassus has quite a complicated life cycle.

Here’s the simple version:

1. Adult worms live in an eel’s swim bladder
2. They lay eggs, which leave the eel and enter the water
3. The eggs hatch into larvae
4. Tiny water creatures called copepods eat the larvae
5. Eels become infected when they eat those copepods
6. Larvae travel through the eel’s body and end up in the swim bladder, where the cycle begins again

Other small fish can also swallow infected copepods, and then eels get infected by eating those fish.

How does this affect our eels?

European eels are already critically endangered. They face lots of challenges — barriers in rivers, pollution, long migrations, and of course diseases like A. crassus.

Research has shown:

• Heavily infected eels often fail their long migration to the Sargasso Sea
• Infection can also affect how eels move in freshwater
• In controlled trials, infected eels were less willing to move past obstacles, which slows their journey and uses more energy

So, even though these worms are small, they have a massive impact on eel survival.

How do we protect eels?

In the UK and across Europe, lots of work is being done to help eels recover:

• Reducing overfishing
• Helping eels move past barriers with special eel passes
• Checking eel health before moving or restocking them
• Monitoring new diseases and stopping them from spreading
• Working with universities and organisations to study eel behaviour and health

Healthy eels have a much better chance of reaching the sea and keeping their species alive.