NEW YORK, Jul 06 (Reuters Health) - An increase in tick-borne encephalitis--a viral infection that causes brain inflammation--in Sweden appears to have occurred after a series of warm winters, researchers reported on Thursday.

The findings suggest that populations of disease-carrying ticks may increase in northern climates around the globe if predictions of global warming pan out, according to Dr. Elisabet Lindgren of Stockholm University and Dr. Rolf Gustafson of Huddinge University Hospital in Stockholm.

Tick-borne encephalitis is caused by a virus that is transmitted to humans through the bite of a tick. The illness occurs in scattered areas of Europe, including Sweden, Poland, Czechoslovakia, Austria and Hungary. The first symptoms are flu-like, followed by tremor and dizziness due to brain inflammation. The infection can be fatal in rare cases, but a vaccine is available.

In the study, the team of researchers compared temperature data from 1960 to 1998 with the number of cases of tick-borne encephalitis in Stockholm County.

"The findings indicate that the increase in tick-borne encephalitis incidence since the mid-1980s is related to the period's change towards milder winters and early arrival of spring," according to the report in the July 7 issue of The Lancet.

Specifically the study notes that "the spring vegetation season has advanced by 12 days on average...between 1960 and the mid-1990s."

The researchers stress in their report that "other factors may have influenced" rates of infection, such as more people traveling to areas where encephalitis-carrying ticks live. They also note that the increase in ticks may be a result of rise in populations of host animals like deer and small mammals.

Nonetheless, the authors write that "there has been a global warming trend during the last two decades of the 20th century, with the years 1990,1995 and 1997 having the highest mean temperatures ever registered in the northern hemisphere."

"In Sweden, the change towards a milder climate has been suggested to have contributed to latitude shifts in the northern distribution limit of the Ixodes ricinus tick and a general increase in tick population density between the early and mid-1990s," they add.

　　From Reutershealth.com