By Ben Hirschler LONDON (Reuters) - Around 390 million people are infected each year with dengue fever - the world's fastest-spreading tropical disease - more than triple the current estimate by the World Health Organization, experts said on Sunday. The new finding, based on several years of analysis, underscores the growing burden of the mosquito-borne viral disease, which is also called "breakbone fever" because of the severe pain it can cause. There is as yet no approved vaccine or specific drug to treat dengue, which is not normally fatal but lands many victims in hospital. ...