Dengue infects tens of millions of people annually, resulting in more than 10,000 deaths. The disease is caused by an RNA virus, which is transmitted principally by the mosquito Aedes aegypti. Previous work in Drosophila has shown that the bacterium Wolbachia an endosymbiont living in the cytoplasm of the insect’s cells, largely confers immunity from RNA viruses. Wolbachia also affects Drosophila sexual reproduction. By biasing its transmission to offspring, the bacterium spreads through Drosophila populations over multiple generations. Might it do the same to mosquitoes, and in the process rid the world of dengue? 18 Wolbachia does not occur naturally in A. aegypti, so microinjection was used to create a laboratory strain of the mosquito (called WB1) that harbors the endosymbiont. To examine the potential effects on transmission of dengue, Bian et al. (2010) infected mosquitoes from both the WB1 strain and the original wild strain with dengue. Fourteen days later, the mosquitoes were allowed to feed on an artificial food solution for 90 min. Viral titers in the food solution were then measured (in plaque feeding units [pfu] per ml). The results are given below. Mosquitoes were tested in groups, and so each data value is an average for the group, measured in pfu/ml.

a. Plot the data with a strip chart. What trend is suggested? What features of the data might lead you to consider using a log transformation?

b. Why do you think we should use log(Y+1) rather than log(Y)

when transforming the data?

c. Using log(Y+1) transformed data, calculate a 95% confidence interval for the difference in mean viral titers. Let a difference between strain means of 1 or less on the log scale represent a small effect and a difference greater than 1 a large effect. Based on the confidence interval, can we conclude that the difference between strains is most plausibly small, large, or uncertain?

d. Convert the lower and upper limits of this confidence interval back to the original scale. What difference does this confidence interval estimate?

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WB1: 3102,3102, 364,364, 83,83, 83,83, 10,10, 0,0,0,0, 00 Wild: 81633,81633, 51533,51533, 11730,11730, 632,632, 538,538, 364. 364, 107,107, 6464