USUV exposure, determinants and immunity after the 2018 epizootic among captive birds in French zoos

In the quiet corners of Europe, a tiny virus travels on the wings of mosquitoes, moving from bird to bird with a speed that often goes unnoticed until it strikes. This is the Usutu virus, a pathogen that has been present in Africa for decades but only recently began to cause significant trouble in Europe. It belongs to a family of viruses that includes West Nile virus, and like its cousin, it relies on mosquitoes to jump between hosts. While humans can catch it, they usually remain healthy or experience only mild symptoms; the real danger lies with birds. For many species, especially owls and songbirds, the virus is deadly. In 2018, a major outbreak swept across France, killing large numbers of wild birds and raising alarms for those kept in zoos. Scientists knew the virus was there, but they did not fully understand which birds were most at risk, what factors made an infection more likely, or whether the survivors had developed a shield against future attacks.

To answer these questions, a team of researchers from French zoos and veterinary laboratories launched a three-year investigation starting in 2018. They focused on the birds living in thirteen different zoological gardens across the country. Instead of waiting for birds to get sick, the team took a proactive approach, collecting blood samples from hundreds of healthy-looking birds during routine health checks. They looked for signs of past infection, specifically searching for antibodies, which are proteins the immune system creates to fight off a virus and remember it for the future. By testing birds from 70 different species, ranging from tiny songbirds to large owls and ostriches, the researchers could map out exactly who had been exposed and who had remained safe.

The results revealed a clear picture of vulnerability. Overall, about one-third of the birds tested had been exposed to the virus. However, this risk was not shared equally. The researchers found that owls and songbirds were significantly more likely to have been infected than birds of prey like hawks and falcons. Some specific species, such as the common magpie and the starling, showed very high rates of infection, while others, like the great grey owl, showed very low rates, likely because many of their kind had already died during the initial outbreak. The location of the zoo also mattered greatly. Birds in zoos located in western France were much more likely to be infected than those in the central part of the country. This suggests that the virus may have entered the country through different routes or that local environmental conditions in the west were more favorable for the mosquitoes that carry it.

Beyond where the birds lived, the researchers discovered that the birds themselves played a role in their own fate. Older birds, those born before 2015, were more likely to be infected than younger ones, possibly because they had simply lived longer and had more opportunities to encounter the virus. Heavier birds also faced higher risks, perhaps because their larger bodies offered more surface area for mosquito bites. Crucially, the study showed that human management could make a real difference. Zoos that used mosquito nets to cover their enclosures saw far fewer infections. This simple physical barrier proved to be a powerful tool, suggesting that keeping mosquitoes out is more effective than trying to treat the birds after they have been bitten.

The study also tracked how long the birds' immunity lasted. The researchers followed a group of survivors for three years, checking their blood levels repeatedly. They found that for most birds, the protective antibodies remained strong and stable throughout the entire period. This means that once a bird survived the virus, it likely retained the ability to fight it off for years. However, this protection was not universal; some smaller birds, including certain types of owls and vultures, saw their antibody levels drop over time, leaving them potentially vulnerable again. The findings confirm that while the virus is a serious threat, the birds' own immune systems are capable of mounting a strong defense, and that simple changes in how zoos house their animals can significantly reduce the risk of infection.

This work provides a vital roadmap for protecting captive birds in the future. By identifying which species are most at risk and which management practices work best, zookeepers can take targeted steps to prevent future outbreaks. The study suggests that keeping susceptible birds indoors during peak mosquito seasons and using nets to block vectors are highly effective strategies. While the virus may continue to circulate in the wild, these insights offer a way to shield the birds in our care, ensuring that the tragic losses of 2018 do not become a repeating pattern. The research underscores a simple truth: in the battle against mosquito-borne diseases, understanding the environment and the specific needs of each species is just as important as the biology of the virus itself.