Bird flu outbreak caused mass mortality but also provided long-term protection against the virus

Highly pathogenic avian influenza has increased dramatically in Europe in the last years. The virus is believed to circulate among certain wild bird species that can carry it without developing severe symptoms. When the virus jumps over to more vulnerable species, it can lead to deadly outbreaks. One such event happened in Kalmar in 2023, when the infection spread among black-headed gulls.

High mortality

A new study from Linnaeus University provides a closer look at the outbreak that caused mass mortality. Between 17 June and 11 July 2023, more than 700 dead gulls were reported in Kalmar, Oskarshamn and Mörbylånga municipalities. The virus hit the chicks hardest, with a mortality rate over 90 per cent, according to the new study published in Proceedings of The Royal Society B.

"At every affected colony, young chicks showing signs of the disease could be seen at the water's edge, while dead juveniles were floating in the water. Adult birds showed neurological sympyoms such as inability to use their legs properly and spasmic movements of the head", says Jonas Waldenström, professor of Disease ecology.

Valuable tracking data

"We were capturing and tagging birds as late as May 2023, a month later the outbreak started," says Alessia Ostolani, doctoral student.

A year before the outbreak, the researchers had tagged around 30 black-headed gulls in Kalmar with GPS trackers for a different project, examining the birds' use of sources for antimicrobially resistant bacteria in the environment. These birds became an unexpected and valuable source of data when the virus struck Kalmar. Additionally, some tagged birds lived in an Öland colony, where the virus never reached and could serve as a comparison.

"The outbreak became a natural experiment for us to study how avian influenza effects the behaviour and survival of black-headed gulls. Thanks to the GPS tagging we could follow the movement of a number of birds. This is very useful, as our current understanding of the virus' effects on wild bird populations remains largely based on mortality counts and analyses of dead birds" says Alessia Ostolani, doctoral student of ecology.

Changed behaviour

The tracking data showed that gulls at the affected colonies moved considerably less than usual, an indication that something was wrong.

"Black-headed gulls show a quite predictable behaviour during the breeding season. Both males and females move back and forth to the colony to gather food and feed their chicks, basically doing the same thing over and over again. If that changes, you know something has happened" says Alessia Ostolani.

While the drop in movement could be due to a lack of chicks to feed, researchers think the infection is a more likely cause.

"We know from other studies that when birds lose their chicks they tend to move more because they are no longer constrained by having to return to the nest. We therefore believe the reduced movement was caused by the infection" says Alessia Ostolani.

Earlier migration

Another behaviour change in affected colonies was that gulls began migrating almost two weeks earlier in autumn. This surprised the researchers, as the migration timing of the gulls is often very precise. But with their chicks dead, adults did not have to stay around for as long as usual.

This is valuable information for researchers working on early warning systems that predict avian influenza outbreaks before they occur. Many of such models rely on birds following regular life cycles. If, however, the timing of important events such as migration changes in response to the virus, models must account for it. Earlier migration may also bring birds into contact with new species, enabling new transmission pathways.

"Knowing that birds might migrate substantially earlier if they are struck by bird flu during the breeding season means that we can incorporate this information into future models of migration behaviour." says Mariëlle van Toor, researcher of Ecology at Linnaeus University.

GPS data shows the birds' migration from Kalmar. ”A gull can move across Europe in a couple of days” says Mariëlle van Toor.

Long-lasting immunity

With avian influenza threatening wild birds around the globe, mass mortalities in wild bird populations like the one in Kalmar is expected to be a recurring phenomenon. One key question is how long-lasting immunity wild birds develop after surviving an infection.

Following the outbreak, the researchers have recaptured black-headed gulls to measure their antibody levels against the specific virus strain. A year after the outbreak, the antibody count was very high in the surviving individuals that were tested, and after another year the percentage of birds still having protection was the same, although the levels were lower.

"These results indicate that Black-headed gulls that do survive the infection can be protected against the virus for at least two years, if not longer. Should the virus return to Kalmar, this would hopefully result in outbreaks of much less dramatic dimensions" says Alessia Ostolani.

More information:

Link to the study: "Effects of highly pathogenic avian influenza on the behaviour and survival of a colonial breeding seabird" in the journal Proceedings of The Royal Society B.

The research is part of the Horizon Europe project Kappa-Flu, the Zoonotic Ecology and Epidemiology research group and Linnaeus University Centre for Ecology and Evolution in Microbial model Systems (EEMiS).