Skinny cod. Photo Marie Plambech Ryberg

Liver worms inflict fatal damage on the livers of cod

Wednesday 14 Oct 20
by Helle Falborg


Marie Plambech Ryberg
DTU Aqua
+45 93 51 14 14


Jane Behrens
Senior Researcher
DTU Aqua
+45 23 29 68 63


This research project on liver worm in cod has received funding from the European Maritime and Fisheries Fund and the Danish Fisheries Agency.

EU flag and Danish EMFF logo 

Additionally, the project has received funds from the private foundation, Direktør J.P.A. Espersen og hustru, Fru Dagny Espersens Fond.

Cod with high numbers of parasitic liver worm show symptoms similar to those of humans with chronic liver disease, such as cancer and cirrhosis.

A large number of the cod in the central and eastern parts of the Baltic Sea are infected with liver worm, a parasite that infects the liver of the cod and causes tissue damage. Cod with many liver worms are often very skinny.

New research from DTU Aqua shows that cod infected with a high number of worms in their liver also exhibit a range of symptoms similar to those of humans with diseases like liver cancer or cirrhosis.

"They have an increased number of gamma-globulins, which is an immune response showing that their immune system is activated. They have a significantly lower amount of albumin in their blood—one of the most important proteins in the blood for both fish and mammals. Altogether, this tells us that the liver does not function optimally, when it has a large number of worms”, says Marie Plambech Ryberg, DTU Aqua. She is in charge of the new studies that have been published by the scientific journal, Conservation Physiology.

4-5 worms per gram of liver is the critical limit

These results are part of Marie Plambech Ryberg’s PhD project, where she investigates the physiological effects of liver worm in cod, and the mechanism behind these effects. The work is a combination of experimental studies and data collected from field investigations and surveillance cruises, as well as bioenergetics and statistic modelling.

In the experiments, which she performed in the fish-holding facilities at DTU Aqua, she determined the metabolism of the fish and the composition of proteins in their blood, all in relation to liver worm load, and estimated how many worms it takes before things ‘start to turn bad’ for the infected cod.

"The fish infected with the largest number of liver worms are also the skinniest. These infected fish also have the lowest amount of protein, and have the lowest fat content in the liver. The fat content in the liver of the fish with many worms is only approximately one third of what is found in the healthy fish. And fat in the liver is most critical for the fish, since the cod is a very lean fish, who uses its fatty (and therefore rich in energy) liver as a 'lunchbox'. We can see that the cod start to become fatally thin, when they are infected with more than 4 to 5 worms per gram of their liver, which can increase their risk of dying", she says.

 Cod liver with many worms. Source: Ryberg et al., 2020, Conservation Physiology
Cod liver with many worms.  
Ryberg et al., 2020, Conservation Physiology
Thin cod have reduced swimming performance

The life cycle of the adult liver worm in the Baltic Sea starts in grey seals (Halichoerus grypus) – to crustaceans, then another species like the sprat (Sprattus sprattus) or herring (Clupea harengus) – then cod, where the parasite infects the liver, exclusively. Cod with many worms are often very thin, and often have a poor health status. This may be an advantage to the liver worm as it needs to return home to its final host—the stomach of a seal—to grow and reproduce.

"When a fish is very thin and generally has a poor health status, then its swimming performance suffers too. This means that catching prey will become more difficult, and it will presumably be an easier catch for seals. But we do not know if the cod that are thin already, because of insufficient nutrition due to limited prey availability, are more susceptible to liver worm", says Marie Plambech Ryberg. 

Clear negative effect on the physiological condition of the fish

The liver worms have been found especially in cod belonging to the Eastern Baltic stock, and during the last decade, the fish in this stock have become increasingly infected. The Eastern Baltic cod is generally in a bad condition, and the number and size of the fish, as well as the amount of offspring, has diminished so drastically that fishery of this stock has been prohibited entirely, since 2019. The regress of the stock is most likely due to the combination of a number of factors, such as low oxygen levels and less prey for the cod in the area, which coincide with the stress from the infection of liver worms.

Senior Researcher Jane Behrens, from the fish biology research group at DTU Aqua, is a supervisor and co-author of the new scientific paper. For many years, she has studied the occurrence and effects of liver worm in cod:

"The new results clearly show that there is a negative effect of the worm on the health status condition and potential growth of the fish. We can now see, that we cannot ignore the liver worm, when calculating and evaluating the impact of factors that push a stock in a completely wrong direction", she says.

Liver worm in cod must be monitored all over the Baltic Sea

Through the past few years, the liver worms in cod have been observed primarily in the central and eastern Baltic Sea. But the newest findings indicate that the cod in the more westerly areas are now starting also to become infected with liver worm. The monitoring of diseases is usually not part of the protocol on scientific cruises. But since the challenges with liver worms are becoming more common, it will henceforth be mandatory for the countries monitoring cod in the Baltic Sea also to collect information on the liver worms in this fish. This applies to both the eastern and western areas—including Kattegat and Oresund.

"We will then be able to track the development in space and time of the prevalence and intensity of infection of liver worm in cod, which is an important step, since there are signs that this parasite has started showing up in new areas as well. There are however indications that this specific species of parasite does not thrive too well in salty waters. It may be a natural barrier, but only time will tell", says Jane Behrens.

Read the scientific article

Marie Plambech Ryberg, Peter V. Skov, Niccolò Vendramin, Kurt Buchmann, Anders Nielsen and Jane W. Behrens: 
Physiological condition of Eastern Baltic cod, Gadus morhua, infected with the parasitic nematode Contracaecum osculatum
Conservation Physiology, 2020.
13 MAY 2021