Climate change and infectious disease remains a hot but controversial topic. I (also) aim to predict what will happen to ectotherms, and to the diseases they vector, in a future warmer world. The largest part of my work, however, focuses on the present: We need to get a better grip on the basic biology and ecology of vectors and the diseases they transmit, as our current understanding is far from complete.
At the moment I work with malaria mosquitoes and malaria parasites. Keywords are (i) temperature, (ii) thermal performance, and (iii) life-history traits. The aim of my studies is to better inform the current climate-disease debate by exploring the following two topics:
(1) Revisiting the vectorial capacity equation (or basic reproductive number). Are some components truly temperature-independent? Is the rate of others really increasing linearly with temperature?
(2) Studying the relevant microclimate that ectotherms (and thus pathogens) experience. Where are vectors resting? To what temperatures are they exposed? How does this affect disease risk?
Part of the game is to use these new insights to inform disease control strategies. But to be honest: I find the underlying mechanisms fascinating, whether it’s a mosquito, an aphid, a bacteria or a fish… I am a biologist after all!
(1) Revisiting the vectorial capacity of arthropod disease vectors
Is vector competence temperature-independent?
Predicting the extent of possible changes in disease patterns requires detailed understanding of how a suite of vector and parasite traits respond to temperature. However, in many cases the nature of the temperature-dependent relationships remains poorly defined. For example, vector competence, which describes the ability of a vector to acquire, maintain and transmit a parasite/pathogen, is widely assumed to be temperature-insensitive. We showed that vector competence (the maximum proportion of infectious mosquitoes, which implicitly includes parasite survival across the incubation period) tails off at higher temperatures, even though parasite development rate increases. These findings have significant implications for the various strategic modeling frameworks informing current disease control and eradication efforts, as they suggest that control at higher temperatures might be more feasible than currently predicted. The results also add complexity to studies investigating the possible effects of climate warming, as increases in temperature need not simply lead to increases in transmission.
See paper in Biology Letters [pdf], highlighted in Nature News [link]
What are the physiological constraints of malaria risk?
Existing malaria risk models that factor in effects of climate frequently use monotonically increasing relationships between temperature and vital rates such as parasite development and immature development. Other variables are assumed to be temperature-insensitive. But what happens to malaria risk when we combine the thermal performance curves of all components that combined shape the basic reproductive number? Mordecai et al. is going to cause a stir. In review, more to come!
(2) The relevant microclimate in arthropod and pathogen biology
The meaningless mean temperature
There are six essential parasite and mosquito life-history traits (i.e. all the entomological parameters) that combine to determine malaria risk. Many studies link malaria dynamics to coarse measures of environmental temperature, such as mean monthly temperatures. Yet mosquitoes experience temperatures that vary from hour to hour and do not live under ‘average monthly conditions’. We have shown that in addition to mean temperatures, daily fluctuations in temperature affect parasite infection, the rate of parasite development, and the essential elements of mosquito biology that combine to determine malaria transmission intensity. In general, we find that, compared with rates at equivalent constant mean temperatures, temperature fluctuation around low mean temperatures acts to speed up rate processes, whereas fluctuation around high mean temperatures acts to slow processes down.
See papers in PNAS [malaria: 1, 2; dengue: 3], and commentary by Pascual et al. [pdf]
Mosquito larvae don’t live in the sky
The relationship between mosquito development and temperature is one of the keys to understanding the current and future dynamics and distribution of vector-borne diseases such as malaria. Many process-based models use mean air temperature to estimate larval development times, and hence adult vector densities and/or malaria risk. However, water temperatures in typical mosquito breeding sites are in general higher than the temperature of the adjacent air, resulting in larval development rates, and hence population growth rates, that are much higher than predicted based on air temperature. Existing models will tend to underestimate mosquito population growth under current conditions, and may overestimate relative increases in population growth under future climate change.
See paper in Malaria Journal [pdf]
Mosquitoes do rest inside houses
Evidence suggests that certain malaria vectors can spend large parts of their adult life resting indoors. If significant proportions of mosquitoes are resting indoors and indoor conditions differ markedly from ambient conditions, simple use of outdoor temperatures will not provide reliable estimates of malaria transmission intensity. To date, few studies have quantified the differential effects of indoor vs. outdoor temperatures explicitly, reflecting a lack of proper understanding of mosquito resting behaviour and associated microclimate. Published records from 8 village sites in East Africa revealed temperatures to be warmer indoors than outdoors and to generally show less daily variation. These differences lead to large differences in the limits and the intensity of malaria transmission. This finding highlights a need to better understand mosquito resting behaviour and the associated microclimate, and to broaden assessments of transmission ecology and risk to consider the potentially important role of endophily.
See paper in Malaria Journal [pdf]
These studies were supported by a Netherlands Organisation for Scientific Research grant and/or a National Science Foundation EID program grant.