Eels transmit a virus through biting: a discovery that changes our understanding of viral infections in fish

Contrary to what was previously believed, viruses infecting fish are not transmitted exclusively through water acting as an abiotic vector. A team of researchers from the University of Liège has demonstrated that a virus affecting European eels can spread very efficiently through biting during aggressive interactions between individuals. Published in the scientific journal Journal of Virology, this discovery challenges a widely accepted principle in aquatic virology. It also provides new insights into the diseases threatening the European eel, a species currently classified as critically endangered.

The European eel (Anguilla anguilla) is a threatened species. Its life cycle involves travelling thousands of kilometers between the Atlantic Ocean, where it reproduces, and European inland waters, where it spends several years growing. However, its populations have experienced a dramatic decline over recent decades: the number of young eels reaching European coasts today represents only around 1% of the levels estimated in the early 1980s.

Barriers to migration, pollution, overfishing and climate change all contribute to this decline. Infectious diseases pose an additional threat, particularly those caused by a herpesvirus known as AngHV-1. This virus can cause hemorrhagic lesions on the skin and gills, with mortality rates reaching up to 30% in intensive aquaculture facilities.

But how does this virus manage to spread, even among declining wild populations in which eels live relatively isolated from one another?

A virus that exploits aggressive behavior

This is the question investigated by scientists at the University of Liège, under the leadership of Professor Alain Vanderplasschen, from the Laboratory of Virology and Vaccinology at the Faculty of Veterinary Medicine and WEL Research Institute.

Until now, it was generally accepted that fish viruses were transmitted mainly through water acting as an abiotic vector, which carries infectious viral particles from infected individuals to healthy ones. However, the researchers have discovered another particularly effective route of transmission: biting.

Eels are indeed animals that can display aggressive behavior towards members of their own species. In particular, they may bite one another around the mouth or on the tail. The experiments carried out by the scientists demonstrate that a single such contact can be sufficient to transmit the virus.

To investigate this, the researchers experimentally reproduced several epidemiological scenarios involving biting between infected and uninfected eels. In all three tested scenarios examined, every exposed animal developed an infection.

This finding suggests that social interactions, even antagonistic ones, can play a decisive role in the spread of viruses in aquatic environments.

Making the virus glow to track its spread

To reach these conclusions, the team used a particularly innovative technique: in vivo bioluminescence imaging.

The researchers genetically modified the virus to express an enzyme from the firefly so that it would produce light as it replicated in infected tissues. This light, invisible to the naked eye but detectable using specialized equipment, makes it possible to track the progression of the infection within the animal's body.

This technique revealed a second important discovery: the susceptibility of eels to the virus depends strongly on their developmental stage.

Glass eels, the still-translucent young eels arriving on European coasts, were found to be resistant to infection when exposed to the virus through immersion in water. By contrast, the "yellow eels", which live and grow in inland waters, become particularly susceptible. The gills and the mucous membranes of the mouth acts as the main entry points for the virus.

The scientists also observed that infected eels kept in groups developed more sites of infection on their skin than those kept in isolation. This difference helped identify the role of biting in viral transmission.

An important advance for eel conservation

Why is this mode of transmission so important? Experiments show that the virus rapidly loses its infectivity in water, with a reduction of approximately 90% after just twelve hours under the conditions studied. The mucus that naturally coats the skin of eels also has properties capable of neutralizing the virus.

Transmission through biting could therefore allow the virus to bypass these obstacles by passing directly from one fish to another, without having to survive for long periods in the aquatic environment.

This discovery helps understanding how a viral infection can continue to circulate among low-density eel populations, despite limited stability of virus infectivity  in the environment.

The study is the result of a collaboration between several teams at the University of Liège, bringing together expertise in veterinary virology, ecology and species conservation, as well as aquatic resource management and aquaculture. It notably involved the work of Natacha Delrez, first author of the publication, alongside Mathieu Denoël, Michael Ovidio and Billy Nzau Matondo, with contributions from scientists at the University of Mons and the University of Glasgow.

Led by Professor Alain Vanderplasschen, this research illustrates the value of combining molecular biology, epidemiology and behavioral ecology to gain a better understanding of animal diseases. The development of this type of interdisciplinary research is at the very heart of ULiège’s new aquaculture research centre.

Beyond the European eel, these findings call for a reassessment of the mechanisms by which viruses spread among fish. They could help improve disease prevention in aquaculture and ensure that infectious risks are better taken into account in conservation and wild population restoration programs.