This work received financial support from the Franco-Thai collaborative research programme
(2009/2010), the MATECLID project (APR GICC 2002), and the EPIDENGUE project (ANR 00119 05).
D.G.L. has a PhD grant from the French Ministere de la `Recherche. We also thank a referee for useful suggestions,and we owe a lot to Michael A. Allen who pointed out a major blunder