Tuesday December 20 2011
Could sunlight really halt the spread of chickenpox?
“Exposure to sunlight may help impede the spread of chickenpox,” BBC News has reported. The broadcaster reported that the UV rays in sunlight could in theory influence the patterns of chickenpox cases seen worldwide, particularly as equatorial countries tended to have lower rates of chicken pox, which is a viral infection.
The news is based on an article that suggests that transmission of the chickenpox virus may be reduced by ultraviolet (UV) radiation. However, this is just a hypothesis, based on previously published observations. Although experiments to test this hypothesis further are proposed, they were not performed. It should be noted that this hypothesis has not been universally supported by chickenpox experts, and some have written to explain why they do not think the hypothesis stands up.
Scientific understanding progresses through the development of hypotheses that are then tested to see whether they can explain observations in the laboratory and in real life. This interesting observation about chickenpox and UV radiation needs further testing before it can be said that sunlight has an effect.
Chickenpox is usually a minor illness. Although it produces unpleasant itching and blistering, it rarely causes complications.
Where did the story come from?
The article was written by Philip Rice from St George’s Hospital. No source of funding was reported. The study was published in the peer-reviewed Virology Journal.
Although the BBC generally covered this study accurately, the headlines and tone of the story suggest that this is new experimental research, or that there is experimental evidence that the sun stops chickenpox spreading. However, the article only presents a new hypothesis to explain previous observations.
What kind of research was this?
This was a "hypothesis" article. A hypothesis is a proposed explanation for an observation or set of results. This study presented a novel explanation for why there are global variations in the patterns of chickenpox infection and virus types circulating across the world. It was based on the results of previously published studies. It's important to note that, although the researcher did propose ways that his explanation could be tested, these experiments have not yet been performed.
In an accompanying article, other experts explained why they disagreed with the hypothesis, suggesting that UV is unlikely to be the main factor driving the behaviour of the chickenpox virus. They used results they have obtained in Mexico to support their argument. Temperate versions of the virus are more common in both temperate and tropical areas of Mexico, and in both areas the number of chickenpox cases varies with season.
What did the research involve?
The researcher searched scientific literature for studies that had looked at the distribution of antibodies to chickenpox virus in the population. Possessing antibodies means a person has previously been exposed to chickenpox, and specific antibodies will denote exposure to specific versions of the virus. From the study it was not clear whether this search was performed using a systematic approach (that is, looking for all relevant research, regardless of its findings), or whether there were any criteria that the studies had to fulfil to be included.
The researcher combined the results of some studies and discussed the results of other studies in the article to provide reasons supporting his hypothesis. For example, he plotted the proportion of people with antibodies to chickenpox against various factors including latitude, temperature, rainfall, population density and sunshine hours. It was not clear how the results of the studies were combined.
What were the basic results?
The researcher presented the hypothesis that transmission of chickenpox virus is affected by UV radiation, and that this is responsible for the different patterns of chickenpox infection in countries worldwide.
The researcher presented the following points to support this:
- Childhood infection with chickenpox is less common in the tropics than in temperate areas (like the UK). Those children in the tropics who did get infected tended to do so later in childhood.
- There are different types of chickenpox virus, and they are separated into ones that are found in temperate areas and ones found in tropical areas. He proposes that these viruses are likely to have different resistances to ultra-violet radiation.
- There is good correlation between the proportion of the population with antibodies to chickenpox and increasing distance from the equator. Fewer people nearer to the equator (and therefore exposed to more UV) tend to have antibodies to chickenpox, suggesting that fewer people in these regions have been exposed to the virus. There are no consistent correlations between the proportion of the population with antibodies and the other factors investigated.
- In temperate countries, chickenpox occurs more frequently in winter and spring, when UV radiation is weakest.
- Although chickenpox cases peak in hot, dry and sunny periods in tropical countries, the researcher explains this finding by the fact that at these points, atmospheric pollution levels are likely to be high. This pollution would reduce the levels of UV radiation.
- One study from the 1940s found that artificial UV radiation successfully reduced the spread of chickenpox in schools.
How did the researcher interpret the results?
The researcher suggests ways that his hypothesis could be tested:
- The spread of different types of chicken pox virus could be compared with UV radiation levels.
- The different types of virus could be exposed to UV radiation in a laboratory setting, to see if temperate viruses are more sensitive than tropical ones.
The researcher then discusses the implications of his hypothesis. He reasons that there must be an advantage of reduced UV radiation resistance for the temperate viruses, such as the varicella-zoster virus, which causes chickenpox, and suggests that it might be more likely to re-infect people as shingles.
In this research article, one researcher presents his explanation for the patterns of chickenpox cases worldwide and for the distribution of different types of the virus. He suggests that ultraviolet radiation could reduce the spread of the virus.
He also proposes a number of ways that this hypothesis could be tested. However, these experiments have not been performed yet, and positive results would be required to support his explanation. In addition, this hypothesis does not have support from all experts on chickenpox, and in an accompanying article, other experts explain why they disagree with it.