Temperature impacts on parasite epidemiology - case study of a contact-transmitted insect parasite

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Epidemiology is the study of how the frequency of disease changes over time, and the drivers of this change. For infectious disease (diseases that pass from individual to individual), the passage of disease through a community is intimately related to the rate of contact between individuals. For human diseases, the density and movement of people is an important element in determining how many people get the disease. Indeed, urbanization through human history has been followed by the emergence and persistence of new diseases. For humans, and warm-blooded animals, day-to-day activity levels are little affected by variation in the climate. However, for cold-blooded creatures, the pattern and rate of movement are intimately tied into the weather. On a cold, cloudy day, there will be little activity compared to a warm sunny day. In this study, we will examine the effect that thermal environment has on disease epidemiology: in essence, the degree to which the rate of disease spread through a population of cold blooded animals is affected by climate. The hypothesis is that warm weather will raise activity levels, which will increase contact rate between hosts. Thus, climate will be an important driver of disease epidemics in cold blooded animals. Beyond daily activity, climate also affects the timing of the yearly cycle of reproduction, birth and death - phenology. For insects in countries like the UK, adults commonly emerge from overwinter stasis, become active and start to reproduce in spring. Many of the parental generation will die before their progeny reach maturity and emerge in summer, such that there is little 'overlap' between the generations. The level of overlap varies geographically (generally, more overlap in Southern areas), and may be very important in affecting disease transmission- the disease has to pass between generations to persist. The hypothesis is that longer, warmer spring climates may speed the development of the next generation, increasing overlap with the parental generation, and increasing disease transmission to it. One of the problems in conducting research of this kind is in defining a 'contact' that exposes an animal to disease. We will circumvent this by examining a sexually transmitted infection, a mite that lives under the wing cases of ladybirds, and passes between partners during sex. Because mating is durable, this can be measured easily, and temperature-sensitivity assessed. This parasite also has a very simple epidemiology-sexual contact is by far the most common means of exposure, individuals can be easily and non-invasively scored for infection and infectivity, and there is no recovery from infection. There are annual epidemics each year associated with reproductive activity of the host in spring. Whilst we will study one particular system, the expectation is that the results will give a general insight into how climate affects the pattern of disease in insects from temperate regions. It should predict how current disease patterns will vary over space, and also how disease patterns of insects may change in the UK following climate change.