Authors: Patrick D. Shaw Stewart
Comments: 33 Pages. Key index phrases Respiratory tract infections, viral infections, temperature changes, temperature sensitivity, seasonality of the common cold, influenza seasonality, epidemiology of viruses.
Influenza A and B, and many unrelated viruses including rhinovirus, RSV, adenovirus, metapneumovirus and coronavirus share the same seasonality, since these viral acute respiratory tract infections (vARIs) are much more common in winter than summer. The lack of a viable explanation is a major problem for microbiology. Unfortunately, early investigations that used recycled “pedigree” virus strains seem to have led microbiologists to dismiss the common folk belief that vARIs often follow chilling, together with the abundant scientific evidence that supports this idea. Today, incontrovertible evidence from polar, tropical, and island-based studies, PCR-based surveys, as well as studies of the effects of outdoor dress and activities, shows that ambient temperature dips and host chilling increase the incidence and severity of vARIs. This review considers four possible mechanisms, M1 – 4, that can explain this link; M1: increased crowding in winter may enhance viral transmission; M2: lower temperatures may increase the stability of virions outside the body; M3: chilling may increase host susceptibility; M4: lower temperatures or host chilling may activate dormant virions. There is little evidence for M1 or M2, the second of which is incompatible with tropical observations. M3 is supported by a recent study that found that the immune response of chilled mouse airway cells was diminished. However, tropical observations and epidemiological anomalies such as the repeated simultaneous arrival of vARIs over wide geographical areas, the rapid cessation of influenza epidemics in midwinter, and the low attack rate of influenza within families are compatible with M4, but not M3 (at least not in its simple form). M4 is also compatible the natural temperature sensitivity of many wild and laboratory virus strains, and the frequent recovery of ts mutants from persistent infections, because seasonality may be a consequence of natural temperature sensitivity, which is (presumably) essential to viral tropism. The evidence suggests that M4 is the main driver of seasonality, but M3 may also play an important role.