A unique long-term study, in which biological samples were collected from the same population of blue tits over a 30-year period, shows that rising spring temperatures have doubled the incidence of avian malaria in southern Sweden.
Researchers from Lund University in Sweden have collected samples every year from hundreds of blue tits in a single population at a local breeding area outside Lund. This has provided a detailed picture of the trend over time.
"The study examines birds infected by parasites, which are closely related to the malaria parasites that affect humans. And the pattern is clear: when the temperature rises during the birds' breeding period, the percentage of infected individuals increases considerably," says Olof Hellgren, a biology researcher at Lund University.
Specific time of year identified
In the mid-1990s about 45 per cent of all the birds carried an infection, but in recent years the annual incidence has risen to between 85 and 90 per cent. The researchers have also been able to link this increase to a specific period in the spring - from 9 May to 24 June - when temperatures have been higher than previously.
"This means we can focus on these periods in order to fully understand the biological link between climate change and disease transmission," says Olof Hellgren.
It is the first time that a Swedish study using collected data can show that an increase in vector-transmitted diseases - infections spread by living organisms such as blood feeding insects - is a direct result of climate change.
No climate focus in the 90s
In addition, the results show that it is not a matter of a general link to higher temperatures in general - but a very specific period of the year during which the chicks hatch and are in the nest.
Interestingly enough, there was no intention to illuminate climate change when the study began.
"There were other more important issues at that time, Also, it was not planned to be a long-term study. However, long-term studies often contain a lot of data that could be put to good use in future research," says Jan-Åke Nilsson, professor of biology at Lund University.
The next step for the researchers will be to find out exactly why heat promotes the spread of parasites. Preliminary data indicates that blood-sucking insects seek out nests during warmer years, but it may also be the case that the parasites thrive better when its warm. Either way, the results are not just important for Swedish nature conservation.
"As parasites and vectors occur globally, it's likely that similar patterns can also be found in other parts of the world. Our results could therefore be relevant for both international research and disease monitoring," concludes Olof Hellgren.
