Wild ducks contaminated with mercury are more likely to be affected by bird flu, a study revealed on Wednesday, once again pointing to the impact of environmental pollution linked to human activities on the spread of viruses.
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Wild ducks are reputed to be major vectors of avian flu, in particular because of their migrations: during their long journeys, they are likely to contaminate many farmed birds (ducks, chickens, geese, etc.), in which this viral disease is particularly deadly.
For this study, published in the Royal Society’s biological research journal, “Proceedings B”, scientists shot nearly 750 wild ducks from 11 different species in San Francisco Bay (United States), located on a migration corridor from Alaska to Patagonia.
Knowing the harmful effects of certain heavy metals on immunity, they measured in the laboratory the level of mercury contained in the blood of individuals and, in parallel, tested an infection with avian influenza – or the presence of antibodies against the virus.
Result: ducks contaminated with mercury – mainly via the food chain – had up to 3.5 times more risk of contracting the disease during the year. And the higher the mercury concentration, the higher the prevalence of antibodies.
The study states that the ducks tested negative for the highly pathogenic strain of the H5N1 virus, detected in numerous outbreaks around the world.
Avian flu, generally asymptomatic in wild birds, can become very contagious and deadly by being transmitted to their congeners in farms.
resurgence fear
The accumulation of mercury in the body can anyway “suppress the immune reactions of the body, exposing it more to all infections, including avian influenza”, explains to AFP Claire Teitelbaum, ecologist at the American scientific institute US Geological Survey, a branch of which is devoted to the conservation of wildlife.
The San Francisco Bay is also a “hot spot of mercury contamination in North America, due to the historical activity of gold mines whose extraction used mercury”, adds the researcher, main author of the ‘study.
In the United States, the epizootic slowed down during the summer, because “a lot of wild birds returned to their nests”, further north. But “when they start to come back down, we’re probably going to see a resurgence,” she predicts.
This year, Europe also faced an episode of avian flu on an unprecedented scale, leading to the slaughter of tens of millions of poultry, particularly in France and Italy.
The study on these “super-contaminating” ducks appears as experts continue to sound the alarm on the impact of climate change, deforestation and pollution linked to human activities on wildlife, favoring zoonoses , diseases transmitted to humans by animals.
While most avian viruses do not infect humans, some subtypes manage to cross the species barrier, such as the H5N1 virus, pathogenic for humans and present in Asia.
Studying how pollution increases the risk of spreading viruses is “adding a brick to a more global vision of what is happening in the world”, argues Claire Teitelbaum.
“Surprisingly, few works look at the links between contamination of wild life and viral infections”, comments Daniel Becker, biologist at the American University of Oklahoma, who welcomes the results of this study to which he did not took part.