species that normally can't carry bluetongue become able to carry it, while in species that can carry it, more individual midges acquire the ability.
This is because the midge genes that block or limit infection are temperature-sensitive. One effect is that warmer flies grow faster and are smaller, with thinner gut walls that allow virus swallowed with a blood meal to break through into the body fluids, move into the saliva, then infect the next bitten animal.
The real mystery is how the northern outbreak over-wintered. The last case in 2006 was in December; it reappeared in July. The virus doesn't persist in larvae and adults don't live that long.
Either some infected midges did last out, or a cow carried the virus through. One theory is that certain T cells, a type of immune cell in the blood, pick up the virus when they flock to inflamed areas around a midge bite. The virus then becomes quiescent. When the midges bite again, the T cells again move in, and the virus reactivates and escapes, just where it is likely to be picked up by a biting midge. Much of this has been observed in cows.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.