Due to a number of high profile outbreaks, the need for understanding the spatial spread of infectious diseases has become self-evident. Human infectious diseases present an excellent opportunity to study spatial dynamics given the abundance of spatially explicit long-term data sets. Grenfell et al. (2001)'s seminal work documented travelling waves of measles transmission in England and Wales, with infectives from large "core" cities providing the epidemic spark to smaller "satellite" towns. In this case, travelling waves are interpreted as the consequence of repeated and rapid invasion waves, extending from endemic core areas (above the critical community size, CCS) into epidemic satellite regions (below the CCS). A similar phenomenon has been reported in the incidence of dengue haemorrhagic fever in Thailand (Cummings et al. 2004), and epidemics of influenza in the USA (Viboud et al. 2006).

Results/Conclusions

Here, we examine the spatial dynamics of whooping cough. We used monthly pertussis notification data from the 48 contiguous states. Wavelet decomposition shows a clear 4-year periodicity in the dynamics of most of the states. The examination of the phase of the 4-year filtered dynamics of these states suggest a disease progression westward from the east coast and eastward from the west coast, with the two coasts being roughly synchronous. Furthermore the wave speed westwards is about three times higher than in the easterly direction. These results are interpreted in light of the historic demographic and human mobility patterns in the USA.