Warmer temperatures increase disease transmission and outbreak intensity in a host-pathogen system

Background/Question/Methods

While rising global temperatures are increasingly affecting species and their interactions relatively little is known about how climate change will affect disease transmission. Using a tractable and easily manipulated insect host-pathogen system, we examined how increased  temperatures affect disease transmission using a crop-defoliating pest, the fall armyworm (Spodoptera frugiperda) and its species-specific and fatal baculovirus. The fall armyworm, like other lepidopteran pests, experiences boom and bust cycles whereby populations rise in numbers and, after a peak, drop precipitously often due to baculovirus-driven epizootics. In each of our experimental field plots, we manipulated baculovirus density and temperature and repeated the experiment three times over the course of two years. 

Results/Conclusions

As  temperatures rose, disease transmission and outbreak intensity increased. However, this was not due to an increase in the disease’s mean transmission rate, which is often the focus  of research. Instead an increase in outbreak intensity was due to a decrease in the coefficient of variation (CV) associated with the transmission rate. As the CV decreased,  heterogeneity in disease risk across individuals declined, which resulted in an increase in outbreak intensity. Increases in outbreak intensity or epizootic severity, as the climate warms, can have important ecological and economic impacts and this work shows that the usual assumptions governing these effects, via changes in the mean transmission rate, may not be correct.