The mosquito-borne Ockelbo disease is caused by a Sindbis virus which can lead to symptoms of fever and joint pains for several months. What was surprising was that those infected lived as far north as Piteå because, according to the current state of knowledge, Ockelbo disease did not occur further north than Hälsingland. The suspicions of virologists at Norrland’s University Hospital in Umeå were eventually confirmed.
At that time FOI and the University Hospital were working on the development of new methods for the detection and identification of mosquito-borne viruses in Sweden.
“Since there are so few entomologists in Sweden, we had created a new method for the identification of mosquitos in which we use DNA analysis to differentiate between different species of mosquito. We were able to identify almost 50 different species in Sweden of which 16 were to be found in Västerbotten,” says Göran Bucht, a laboratory scientist at FOI.
When the alarm about Ockelbo disease was sounded, medical student Oscar Forsman was sent out to the forest areas round Lövånger armed with mosquito traps. He returned with 1600 mosquitos which were then analysed using FOI’s new screening method. One of the mosquitos was found to be carrying a large amount of the Sindbis virus. The mosquito in question was a culiseta morsitans, a species thought to spread the virus between birds but which can, perhaps, also infect humans. Analysis showed that the mosquito was carrying a new variant of the Sindbis virus unlike any virus previously found in Sweden.
But there it stopped. Because the doctors, who were unfamiliar with this disease, had sent the patients home without taking the necessary samples. And the virus that causes Ockelbo disease disappears from the body after about a week. Antibodies against the virus were, however, detected in around 60 patients which made it highly likely that there is an infection chain between mosquito and human.
Since there is great scientific interest in mosquito-borne diseases, and not least in the Sindbis virus which is found in all parts of the world, knowledge of this work has spread widely in scientific circles worldwide.
An article about FOI’s screening method for differentiating between mosquito species has been published in the highly regarded scientific journal Molecular Ecology Resources (Dec 2013) and an article about the work of seeking and characterising the Sindbis virus in mosquitos has been submitted to the journal Emerging Infectious Disease for publication.
Further work is now in progress at FOI.
“Our aim is to find the same virus in birds, mosquitos and humans, so that we have a secure chain of evidence in which we see which species of bird and which species of mosquito it is that is spreading the infection to humans,” says Göran Bucht.