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Avian flu spreads rapidly among domesticated birds with weaker immunity and can mutate to become especially deadly. If these strains return from livestock to the wild, birds such as this year’s bird of the year, the Great cormorant (Phalacrocorax carbo), can be among the casualties.
Wild birds can be a “reservoir” for avian flu. Typically, infection does not cause serious illness or visible symptoms, since they are immune or experience milder strains. Avian flu is still contagious, a bird gets infected through contact with feces-contaminated water or food, writes bird ecologist Marko Mägi in the Linnuvaatleja blog.
Domestic birds’ weaker immunity allows the virus to mutate quickly. This has led to especially contagious and deadly strains. Once back in nature, they spread rapidly. During migration, the virus can travel hundreds of kilometers in hours. The global bird trade also spreads disease.
The infamous H5 strain, first identified in China in 1996, spread among poultry and domesticated birds in East Asia. Once it returned to the wild, it caused mass deaths among wild fowl. Since 2005, the strain has spread globally. Great cormorants and other species have not been spared.
In 2006, the virus killed five birds in Ukraine. Between 2021–2022, it killed up to 15,900 Cape cormorants (Phalacrocorax capensis) in South Africa. In neighboring Namibia, 6,500 cormorants died. In North America, at least 2,779 Double-crested cormorants (Phalacrocorax auritus) perished.
The H5 strain reached Europe in fall/winter 2006, and again in 2014, 2016, and 2020. Up to 2017, the strain receded annually. But in 2021, it persisted through summer, spreading among wild and domestic birds.
At least 1,700 cormorants died of H5N1 in the Baltic Sea region in summer 2021 and 2022. Dead birds were found in Denmark, Germany, Sweden, and Latvia. Estonia’s first major case occurred in 2021 on Kuralaid islet in Pärnu Bay, a nature protection zone hosting 1,367 cormorant nests when the outbreak began.
On May 25, 71 dead cormorants were found there. Several others showed coordination disorders – a neurological symptom of the virus.
Thirteen herring gulls, two oystercatchers, two mute swans, one great black-backed gull, and four grey seals were also found dead. Samples from six birds that died on June 2 confirmed avian flu. That same year, unusually high mortality hit cormorants and gulls. Two white-tailed eagle chicks found dead in the Matsalu nature protection area also had avian flu.
The following year, cormorants from other Baltic Sea colonies also succumbed, along with other seabirds. Herring gulls may get infected by raiding cormorant nests or “stealing” regurgitated fish. In one German colony, only 20 chicks from 424 nests fledged. In previous years, just 0.4–1.3 percent of German cormorants were infected. In 2022, the rate rose to 12 percent.
Avian flu spreads more easily in colonies with ground-level nests, as they are closer together and contact between birds is more frequent than in treetop nests. Nesting cormorants often visit several colonies during breeding season, helping spread the virus.
However, after the highly pathogenic avian influenza (HPAI) outbreaks of 2021 and 2022, no major new outbreaks were observed. Colonies have begun to grow again. This may suggest developing immunity, but confirmation would require detecting antibodies.
The full HPAI study in English is here.
This article is written by Marko Mägi. It was originally published on the “Birdwatcher blog”.
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