Network amplification of dengue declines as endemicity rises: climate-adjusted directional spread across Costa Rican cantons, 1993-2012

\textbf{Background:} In Costa Rica, dengue is reported and controlled at the canton level, and outbreaks in one canton are often followed by outbreaks in others. Climate models describe where conditions favor transmission but not how dengue moves \emph{between} places, the directional, between-place spread that shapes where an outbreak travels next.
\noindent \textbf{Methods:} From weekly case counts for all 81 cantons (1993--2012; \num{246524} cases) we reconstructed a canton-to-canton spread map using the roughly three-week dengue generation interval, removed the shared seasonal and climatic signal so that only direction-specific spread remained, and summarized it by the receiving and source cantons, an amplification factor, and a directionality index, tracked over five-year windows.
\noindent \textbf{Results:} Climate-adjusted spread is strongly directional and concentrates in the lowland Caribbean and Pacific cantons (Limón, Matina, Guácimo, Garabito, Orotina). A local outbreak is amplified about three- to fourfold across the network even though overall transmission is not growing. This amplification was greatest during the emergence phase of the 1990s and declined markedly as annual reported cases increased, while the \emph{direction} of spread remained fixed; the decline persists after controlling for the broadening of surveillance coverage.
\noindent \textbf{Conclusions:} Routine surveillance alone can map which cantons tend to experience dengue and the pathways through which it appears to spread, providing a potential input for prioritizing surveillance and vector control, particularly when a serotype or the disease itself is newly establishing. As a historical description of average behavior over multi-year windows, it is a planning input whose prospective value remains to be tested.