It is well established that earth’s climate is changing, and it is likely that this change will result not only in changes in global temperatures but also in changing weather patterns, including an increased incidence of extreme weather like droughts. Drought conditions create a range of problems, including the potential to alter disease transmission dynamics. Despite this risk, the impact of drought on disease transmission has not been widely explored; those studies that have examined drought’s effect on disease-causing organisms have largely focused on changes in parasite prevalence but have overlooked other potential effects, such as changes in parasite genetic diversity. Our study explores the effects of the severe drought that took place in California starting in 2012/2013 on the prevalence and genetic diversity of the lizard malaria parasite Plasmodium mexicanum. We utilized yearly samples collected between 2010 and 2016 to evaluate whether changes in prevalence or genetic diversity (number of strains per infection, allele frequencies; based on microsatellite markers) occurred during this period that may be associated with the drought. We also have data annual sampling going back to 1978 that will help determine whether any changes seen in this period are out of the ordinary.
Results/Conclusions
Analysis of the full data set is still underway, but preliminary results comparing prevalence in the three years prior to the drought to the three years of drought conditions indicate that P. mexicanum has become more common, increasing from 9% to 14% infected (P = 0.02). If this pattern is observed when considering the full data set, it would be consistent with trends seen in mosquito-borne parasites: prevalence often increases immediately after a drought due to mortality of mosquito larvae’s aquatic predators and the resulting increased potential for transmission. Interestingly, P. mexicanum is transmitted by sand flies, not mosquitoes. Sand fly larvae are not aquatic and feed on decaying organic matter (e.g. ground squirrel feces for the vector of P. mexicanum). Less is known about how drought impacts sand flies, but this result may suggest that sand fly populations also gain a short-term benefit from drought, though possibly due to different mechanisms. Genetic data has been collected but not yet analyzed; based on the preliminary prevalence results, we anticipate that the number of strains per infection may increase but major shifts in allele frequency are unlikely unless drought conditions place a strong selective pressure on P. mexicanum.