Highly pathogenic avian influenza became one of the most significant threats to wild bird populations globally. Since the emergence of an H5N1 variant with a particularly high virulence in terns and gulls we have lost several thousands of wild birds in the Dutch Delta alone. Deltamilieu Projecten plays an active role in the impact assessment of the outbreak in wild bird populations. Unfortunately, this is not enough to combat avian flu. The carcasses must be urgently removed from natural and public areas to mitigate the risk of further transmission. During the peaks of outbreaks, the team tirelessly cleans the impacted areas often in difficult circumstances: in the heat of the summer, fully gowned, carrying bags filled with 100s of carcasses, working on remote islands covered with high vegetation. In addition, the ringers of Deltamilieu Projecten participate in a surveillance program to monitor the presence of avian influenza in wild birds. They also collect blood samples with the hope of detecting antibodies against avian flu in the serum of surviving individuals.
Since the arrival of the highly pathogenic H5N1 strain in Europe in 2005, outbreaks showed a seasonal pattern associated with the presence of wintering geese and duck flocks. However, in 2022 this pattern changed. A variant of H5N1 clade 188.8.131.52b emerged around the autumn of 2021 and entered colonial bird populations leading to mass mortality throughout the summer of 2022. Sandwich and common terns were hit the hardest (Report – Vogelsterfte in het Deltagebied in 2022). Although the outbreak slowed down after the breeding season by the dispersion and migration of the most impacted species, the virus did not disappear. It persisted throughout the winter, causing sporadic mortality in gulls and was ready to strike again in the breeding season, killing thousands of gulls during the summer of 2023. Common and sandwich terns were no exceptions either. However, the lives of adult sandwich terns were spared this time, 75% of the chicks succumbed to the disease just before fledging on Bliek in the Haringvliet.
In poultry, avian influenza outbreaks are controlled through measures such as culling animals and implementing transport bans. In wild birds these measures obviously cannot be applied. At present, mass vaccination in a wildlife setting is not a feasible option either. During the onset of an outbreak, it is crucial – and pretty much the only immediate action one can take – to remove all affected birds from nature whenever and wherever possible. Predators (large gulls, foxes, martens) by consuming them, expose themselves to a large dose of the virus. Additionally, healthy birds may interact with the diseased ones (attacking them due to their abnormal behavior, thereby increasing the risk of transmission. Birds often die on beaches or washed ashore. Given the popularity of beach tourism in the Delta, there is a significant risk of people coming into contact with sick or deceased birds. Therefore, it is very important to remove bird carcasses from recreational areas to mitigate this risk.
Vogeltrekstation and Erasmus MC initiated the Zoonosis Program several years ago focusing on the surveillance of zoonotic diseases carried by wild birds with the potential to spill over to humans. The birds, when captured for ringing purposes, are also being sampled in the frame of this program. The ringers of Deltamilieu Projecten joined this initiative in 2022 and by now has four ringers with a special permit to sample wild birds during ringing. Swab samples are taken from the throat and the cloaca to detect birds with active infection. Positive birds – even if they are not sick themselves – can spread the virus. Blood samples are taken from the vein under the wing. From the blood serum antibodies raised against avian flu can be detected if the bird encountered the virus before and survived the infection. Each bird is color-ringed, enabling the tracking of their long-term survival. It is of utmost importance to monitor the presence of avian influenza circulating in wild bird populations, even in “peacetime”. This way we can see what strains are circulating in wild birds enabling us to assess the potential for a new outbreak.