Authors: Patrick D. Shaw Stewart
Current explanations of the seasonality of colds and influenza are incompatible with observations of the incidence of these diseases in the tropics. Many or most wild respiratory viruses possess temperature sensitivity (with less activity at higher temperatures) and it has been suggested that this prevents them from moving down the respiratory tract and infecting the lungs and internal organs of birds and mammals. This temperature sensitivity seems to be finely balanced, and to be continuously adjusted by natural selection, but it may be lost very rapidly in laboratory cultures. Nevertheless, many biochemical studies show decreased viral activity at elevated temperatures. Overdue weight seems to have been given to early volunteer investigations into viral respiratory tract infections (VRTIs) that often used recycled viral strains. [These “pedigree” strains were established by collecting nasal secretions from volunteers with colds, and inoculating subsequent batches of volunteers with the secretions.] Clear-cut evidence that outbreaks of VRTIs are closely (and inversely) correlated with ambient temperature, and that individuals are more likely to develop VRTIs after chilling may therefore have been overlooked. In the laboratory, the following unexpected observations need to be explained: (1) persistent viral infections of cell cultures often yield spontaneously-generated temperature-sensitive (ts) viral strains, and, (2) on at least two occasions, temperature sensitivity was lost when ts influenza A strains were incubated at a low temperature (33°C) in conditions that allowed rapid replication. In this review I note that diverse viral species cause very similar VRTIs, that the incubation periods of VRTIs have frequently been underestimated, that influenza A and B may be shed by asymptomatic patients who have not seroconverted, and that colds and influenza often infect only a subset of the susceptible individuals who are exposed to them. Mechanisms where temperature fluctuations can increase viral replication and transmission are considered, and explanations of VRTI seasonality in both temperate and tropical regions are discussed.
Comments: 41 Pages. Key index phrases Respiratory tract infections, viral infections, temperature changes, influenza seasonality, viral epidemiology.
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