The Drivers of Acute Seasonal Infectious Diseases.

Abstract

Seasonality is a feature of all ecological systems. Earth's terrestrial and pelagic life has evolved in a highly seasonal abiotic environment with intra-annual variation in photoperiod, temperature, and precipitation, among many other abiotic and biotic factors. Seasonal aspects of mammals and birds include seasonally varying birth rates, seasonal changes in endocrine hormones, and seasonal variation in immunity. One area where seasonal biology is particularly salient is disease ecology. The mechanisms underlying the seasonality of communicable diseases are poorly understood. I propose that much of the unexplained seasonality observed in infectious disease dynamics could be attributed to seasonal biology, including (1) birth seasonality, (2) seasonal variation in immunity, and (3) seasonal cycles in parasite traits and parasite population parameters. In my dissertation, I present work on various aspects of seasonality. In Chapter II, I explored the seasonality of births in human populations and quantified the effects of birth seasonality on measles epidemics. In Chapter III, I reviewed circadian and circannual rhythms in host and parasite populations, and proposed both ecological and evolutionary models for integrating biological rhythms into the study of infectious diseases. In Chapters IV--V, I presented my in-depth ecological studies of poliovirus, a notoriously seasonal summertime infection. I explored geographical variation in polio's seasonality and tested whether human birth seasonality or transmission seasonality drove epidemics of this disease. In addition to studying polio seasonality, I revealed the connection between (i) polio's emergence and human demography, (ii) the geographical distribution of poliovirus and its persistence, and (iii) polio symptomatology and silent chains of transmission. Lastly, I highlighted the public health implications of seasonal transmission by measuring the efficacy of the two polio vaccines and discussing how seasonality can be utilized for vaccine interventions.