At the same time, the European Union’s veterinary experts have advised anyone in areas where the virus has been detected to keep cats indoors and dogs on a leash.
Seven dead birds found in Västervik, just north of Oskarshamn, have been sent to the National Veterinary Institute for analysis.
Rather than being found together, the birds were collected one by one.
EU experts said in a statement that “contact between tame carnivores, especially cats, and wild birds ought to be prevented”. A cat in Germany has been found to be carrying the virus, and the possibility that dogs could be infected has not been ruled out.
In Sweden, the test results from dead birds collected on the island of Simpevarpshalvön, north of Oskarshamn, will be available on Friday, according to Ulla Carlsson at the National Veterinary Institute.
On Thursday a team of vets from the Swedish Board of Agriculture hunting for dead birds will be strengthened by 15 members of the Swedish National Guard, said Håkan Henriksson, head of operations in the observation zone.
A seriously ill mink which was found in the protected area and which later died will be tested for the virus, despite the fact that it is considered highly unlikely that it was infected.
Another dead bird, this time a cormorant, found on Friday on Simpevarpshalvön is to be sent for analysis said Henriksson.
If there are no further major finds on Friday, the search efforts outside Oskarshamn will be scaled back on Friday. The protection zone, which covers a radius of three kilometres from the spot where the first two tufted ducks were discovered, will remain in place for 28 days after the last infected bird is found.
The EU is currently processing an application from the Swedish Board of Agriculture to vaccinate all birds in the country’s wildlife parks against bird flu.
“[The application] was made as a preventative measure so that if there is a dangerous outbreak of the virus we can protect certain special species,” said Ulla Carlsson.