My work mainly focuses on malaria mosquitoes and the malaria parasites they transmit. Keywords include (i) microclimate, (ii) ectotherm fitness, and (iii) disease risk.
The aim of my studies is to better inform current malaria elimination and eradication programmes and to help developing appropriate adaptation or mitigation strategies for future climates. I am exploring two topics of interest:
(1) Revisiting vectorial capacity (or R0)
1. Are some of its components truly temperature-independent? (short answer: No)
2. Do others really respond linearly to temperature? (short answer: No)
3. Are parasite development and the gonotrophic cycle synchronous? (short answer: No)
(2) Studying the relevant microclimate that vectors (and thus pathogens) experience
4. Do weather stations accurately reflect climatic conditions that vectors and pathogens experience? (short answer: No)
5. Do temperature fluctuations affect ectotherms differentially than constant temperatures? (short answer: Yes)
(1) Revisiting the vectorial capacity of arthropod disease vectors
Is vector competence temperature-independent?
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 show in a mosquito-rodent malaria system that vector competence tails off at higher temperatures, even though parasite development rate increases. These findings 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.
Paaijmans et al. Biology Letters 2012 [pdf]
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 mosquito 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? A new model, which includes empirically derived nonlinear thermal responses, predicts optimal malaria transmission at 25°C (6°C lower than previous models). Moreover, the model predicts that transmission decreases dramatically at temperatures >28°C, altering predictions about how climate change will affect malaria.
Mordecai et al. Ecology Letters 2013 [pdf]
What are the consequences of temperature-driven asynchrony between parasite development and mosquito biting?
A mosquito needs to bite at least twice for malaria transmission to occur: once to acquire parasites and, after these parasites complete their development in their mosquito host, once to transmit the parasites to the next vertebrate host. We show that the pre-bloodmeal period (the time lag between mosquito emergence and first bloodmeal) increases at lower temperatures. In addition, parasite development time and feeding exhibit different thermal sensitivities such that mosquitoes might not be ready to feed at the point at which the parasite is ready to be transmitted. Exploring these effects using a simple theoretical model of human malaria shows that delays in infection and transmission can reduce the vectorial capacity of malaria mosquitoes by 20 to over 60%, depending on temperature.
Paaijmans et al. PLoS ONE 2013 [pdf]
(2) The relevant microclimate in arthropod and pathogen biology
Local weather stations vs local mosquito resting sites – India
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. Tracking daily temperature profiles within various urban transmission sites in India revealed that the mean daily temperatures were generally warmer than those recorded at the local weather station. Mean temperatures and temperature variation also differed between specific resting sites within the transmission environments. Most differences were of the order of 1-3°C but were sufficient to lead to important variation in predicted parasite development times and hence, create variation in estimates of transmission intensity. Standard estimates of environmental temperature derived from local weather stations do not necessarily provide realistic measures of temperatures within actual transmission environments. Greater effort should be directed at quantifying adult mosquito resting behavior and determining the temperatures actually experienced by mosquitoes and parasites in local transmission environments.
Cator et al. Malaria Journal 2013 [pdf].
Indoor vs outdoor climate – Africa
Evidence suggests that certain African 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. 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 predicted limits and intensity of malaria transmission. This finding highlights again 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.
1. Paaijmans & Thomas. Malaria Journal 2011 [pdf]
2. Paaijmans & Thomas. In: Ecology of vector-parasite interactions. 2012: p. 103-122
3. Blanford et al. Scientific Reports 2013 [pdf]
Water vs air temperature
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.
1. Paaijmans et al. Malaria Journal 2010 [pdf]
2. Paaijmans & Thomas. In: Ecology of vector-parasite interactions. 2012: p. 103-122
Mean vs fluctuating temperature
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.
1. Paaijmans et al. PNAS 2009 [pdf]
2. Paaijmans et al. PNAS 2010 [pdf]
3. Lambrechts et al. PNAS 2011 [pdf]
4. Paaijmans & Thomas. In: Ecology of vector-parasite interactions. 2012: p. 103-122
5. Blanford et al. Scientific Reports 2013 [pdf]
6. Paaijmans et al. Global Change Biology [accepted]