Ecological association between scarlet fever and asthma

One plausible explanation for the worldwide epidemic increase of asthma prevalence is the hygiene hypothesis, which suggests that better control of infections shifts the immune response toward an allergic phenotype. However, studies demonstrating an inverse association between asthma and infectious diseases are scarce and possess conflicting results. To explore the relationship between asthma and scarlet fever, an ecological analysis of their national trends was carried out. Association of both diseases in their annual (1996-2003), seasonal (by month) and geographic (by state) trends was evaluated using the Spearman's correlation coefficient (r(S)). Results showed a strong inverse association between asthma and scarlet fever in all settings. Thus, annual incidence rates of both diseases showed an r(S)=-0.93 (P=0.0009). Seasonal patterns showed a higher proportion of new asthma cases from September to January, while the number of scarlet fever cases increased from March to June (r(S)=-0.84, P=0.0006, 1-month lag). Among the 32 Mexican states, the higher the incidence of scarlet fever the lower the incidence of asthma (r(S)=-0.47, P=0.007). These results suggest that Streptococcus pyogenes, the causative agent of scarlet fever, might be one of the major protagonists of the hygiene hypothesis, a possibility deserving of further investigation.