Is ultra-violet radiation the main force shaping molecular evolution of varicella-zoster virus?

Abstract

BackgroundVaricella (chickenpox) exhibits a characteristic epidemiological pattern which is associated with climate. In general, primary infections in tropical regions are comparatively less frequent among children than in temperate regions. This peculiarity regarding varicella-zoster virus (VZV) infection among certain age groups in tropical regions results in increased susceptibility during adulthood in these regions. Moreover, this disease shows a cyclic behavior in which the number of cases increases significantly during winter and spring. This observation further supports the participation of environmental factors in global epidemiology of chickenpox. However, the underlying mechanisms responsible for this distinctive disease behavior are not understood completely. In a recent publication, Philip S. Rice has put forward an interesting hypothesis suggesting that ultra-violet (UV) radiation is the major environmental factor driving the molecular evolution of VZV.DiscussionWhile we welcomed the attempt to explain the mechanisms controlling VZV transmission and distribution, we argue that Rice's hypothesis takes lightly the circulation of the so called "temperate VZV genotypes" in tropical regions and, to certain degree, overlooks the predominance of such lineages in certain non-temperate areas. Here, we further discuss and present new information about the overwhelming dominance of temperate VZV genotypes in Mexico regardless of geographical location and climate.SummaryUV radiation does not satisfactorily explain the distribution of VZV genotypes in different tropical and temperate regions of Mexico. Additionally, the cyclic behavior of varicella does not shown significant differences between regions with different climates in the country. More studies should be conducted to identify the factors directly involved in viral spreading. A better understanding of the modes of transmissions exploited by VZV and their effect on viral fitness is likely to facilitate the implementation of preventive measures for disease control.

Epidemiology of varicella in Mexico

“…The incidence and age distribution significantly depart from the typical behavior observed in other tropical regions. 11 Strikingly, the temperature profile and the incidence of varicella in Mexico displayed remarkable similarities. Unexpectedly, higher temperatures were associated with an increase in the number of varicella cases.…”

“…11 To this pattern in more detail, the varicella incidence from 2002 to 2010 was calculated per month in each state. The analysis showed that the number of cases generally increased in February, reaching a maximum from March to May, and declined rapidly around June (Fig.…”

Solar radiation and water vapor pressure to forecast chickenpox epidemics

“…Therefore, varicella seasonality in Spain seems to be slightly different to the winter-spring pattern usually reported from other temperate regions, although the pattern of infection affecting preschool and school-aged children remains identical [16]. In tropical areas, the winter-spring pattern is also frequent, but peaks in August, for instance, have also been reported in tropical areas of Mexico [11]. These seasonal diversities increase the interest in determining the role of meteorologic factors in chickenpox epidemics.…”

“…In general, the factors involved in the seasonality and different epidemiology of varicella infections are not fully understood, but VZV genotypes, climatic factors, and population density have been considered to play a role in the dynamics of disease [7,10,11]. In addition, ultraviolet (UV) radiation has been hypothesized to be involved in the worldwide distribution of VZV genotypes [10], but there is no experimental or epidemiologic confirmation of this hypothesis [11].…”

“…Garnett et al in 1993 proposed that the combination of high ambient temperatures and humidity in tropical regions might adversely affect VZV transmission [8], but reduction of social interaction and other factors may also intervene [7,9]. In general, the factors involved in the seasonality and different epidemiology of varicella infections are not fully understood, but VZV genotypes, climatic factors, and population density have been considered to play a role in the dynamics of disease [7,10,11]. In addition, ultraviolet (UV) radiation has been hypothesized to be involved in the worldwide distribution of VZV genotypes [10], but there is no experimental or epidemiologic confirmation of this hypothesis [11].…”