For the third year in a row common blackbirds (Turdus merula) in the Netherlands are affected by usutu virus. Since the end of July 2018 there has been an increase in the number of reports of sick and deceased common blackbirds to the Dutch Wildlife Health Centre (DWHC) and the Dutch Center for Field Ornithology (Sovon). These reports are shared from across the Netherlands (see map). Like previous years, there are also mortalities among great grey owls in captivity.
Diagnostics by the DWHC and Erasmus MC show that mortalities are due to usutu virus. In last week of July and the first week of August, 14 common blackbirds were examined and tested for usutu virus. Of the 14 birds, 13 died due to usutu virus infection and the cause of death of one bird was determined to be avian malaria. Avian malaria, like usutu virus, is transmitted by mosquitoes. Usutu virus was also detected in Zeeland (most south western province) and the northern part of Groningen, thereby spreading throughout nearly all of the Netherlands. Due to our experiences in 2016 and 2017, combined with the information on the symptoms of the ill common blackbirds, we believe another usutu virus outbreak is currently ongoing.
In the second week of August usutu virus was detected in six common blackbird carcasses, putting the total usutu virus diagnosis in common blackbirds at 19 for the year. See updated map.
It is hard to assess the impact the disease will have on the common blackbird population. It is also unknown how long it will take for the population to build sufficient immunity to reduce mortality of the disease. In the backyard bird count of 2018 the common blackbird ranked fifth. In 2016 and 2017, it held the third spot. Preliminary data from Sovon showed that common blackbirds were observed about 15% less in cities and villages compared to 2017.
Several institutions are conducting research on the usutu virus. Sovon monitors the abundance and range of the birds by counts and observations. The DWHC collects reports of dead birds. DWHC focuses on the examination of diseases in wildlife. DWHC collaborates with the Erasmus MC for diseases caused by viruses, like usutu virus. The bird trekking station by the Netherlands institute of ecology (NIOO-KNAW) follows the spread and survival of birds in nature through banding birds and monitoring them; the banded birds are tested for usutu virus by the Erasmus MC.
The studies on the common blackbird mortalities are continued, but it is impossible to pick up all birds reported to us. The reports of sick and deceased birds are very important, because it is the only way to get an insight in the diseases and mortality of wild animals in the Netherlands. You can report the discovery of a dead bird to Sovon at sovon.nl/dodevogels or on the website of DWHC by clicking ‘Report a dead animal’.
More information and previous posts on usutu virus can be found at: https://www.dwhc.nl/en/diseases/usutu-virus/
More information and previous reports on avian malaria can be found at: https://www.dwhc.nl/en/diseases/avian-malaria/