Publications (14)149.39 Total impact
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Article: Malaria control under unstable dynamics: reactive vs. climate-based strategies.
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ABSTRACT: In areas of the world where malaria prevails under unstable conditions, attacking the adult vector population through insecticide-based Indoor Residual Spraying (IRS) is the most common method for controlling epidemics. Defined in policy guidance, the use of Annual Parasitic Incidence (API) is an important tool for assessing the effectiveness of control and for planning new interventions. To investigate the consequences that a policy based on API in previous seasons might have on the population dynamics of the disease and on control itself in regions of low and seasonal transmission, we formulate a mathematical malaria model that couples epidemiologic and vector dynamics with IRS intervention. This model is parameterized for a low transmission and semi-arid region in northwest India, where epidemics are driven by high rainfall variability. We show that this type of feedback mechanism in control strategies can generate transient cycles in malaria even in the absence of environmental variability, and that this tendency to cycle can in turn limit the effectiveness of control in the presence of such variability. Specifically, for realistic rainfall conditions and over a range of control intensities, the effectiveness of such 'reactive' intervention is compared to that of an alternative strategy based on rainfall and therefore vector variability. Results show that the efficacy of intervention is strongly influenced by rainfall variability and the type of policy implemented. In particular, under an API 'reactive' policy, high vector populations can coincide more frequently with low control coverage, and in so doing generate large unexpected epidemics and decrease the likelihood of elimination. These results highlight the importance of incorporating information on climate variability, rather than previous incidence, in planning IRS interventions in regions of unstable malaria. These findings are discussed in the more general context of elimination and other low transmission regions such as highlands.Acta tropica 04/2013; · 2.22 Impact Factor -
Article: The Potential Elimination of Plasmodium vivax Malaria by Relapse Treatment: Insights from a Transmission Model and Surveillance Data from NW India.
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ABSTRACT: With over a hundred million annual infections and rising morbidity and mortality, Plasmodium vivax malaria remains largely a neglected disease. In particular, the dependence of this malaria species on relapses and the potential significance of the dormant stage as a therapeutic target, are poorly understood. To quantify relapse parameters and assess the population-wide consequences of anti-relapse treatment, we formulated a transmission model for P. vivax suitable for parameter inference with a recently developed statistical method based on routine surveillance data. A low-endemic region in NW India, whose strong seasonality demarcates the transmission season, provides an opportunity to apply this modeling approach. Our model gives maximum likelihood estimates of 7.1 months for the mean latency and 31% for the relapse rate, in close agreement with regression estimates and clinical evaluation studies in the area. With a baseline of prevailing treatment practices, the model predicts that an effective anti-relapse treatment of 65% of those infected would result in elimination within a decade, and that periodic mass treatment would dramatically reduce the burden of the disease in a few years. The striking dependence of P. vivax on relapses for survival reinforces the urgency to develop more effective anti-relapse treatments to replace Primaquine (PQ), the only available drug for the last fifty years. Our methods can provide alternative and simple means to estimate latency times and relapse frequency using routine epidemiological data, and to evaluate the population-wide impact of relapse treatment in areas similar to our study area.PLoS Neglected Tropical Diseases 01/2013; 7(1):e1979. · 4.69 Impact Factor -
Article: Global malaria maps and climate change: a focus on East African highlands.
Trends in Parasitology 08/2011; 27(10):421-2. · 5.14 Impact Factor -
Article: Climate forcing and desert malaria: the effect of irrigation.
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ABSTRACT: Rainfall variability and associated remote sensing indices for vegetation are central to the development of early warning systems for epidemic malaria in arid regions. The considerable change in land-use practices resulting from increasing irrigation in recent decades raises important questions on concomitant change in malaria dynamics and its coupling to climate forcing. Here, the consequences of irrigation level for malaria epidemics are addressed with extensive time series data for confirmed Plasmodium falciparum monthly cases, spanning over two decades for five districts in north-west India. The work specifically focuses on the response of malaria epidemics to rainfall forcing and how this response is affected by increasing irrigation. Remote sensing data for the Normalized Difference Vegetation Index (NDVI) are used as an integrated measure of rainfall to examine correlation maps within the districts and at regional scales. The analyses specifically address whether irrigation has decreased the coupling between malaria incidence and climate variability, and whether this reflects (1) a breakdown of NDVI as a useful indicator of risk, (2) a weakening of rainfall forcing and a concomitant decrease in epidemic risk, or (3) an increase in the control of malaria transmission. The predictive power of NDVI is compared against that of rainfall, using simple linear models and wavelet analysis to study the association of NDVI and malaria variability in the time and in the frequency domain respectively. The results show that irrigation dampens the influence of climate forcing on the magnitude and frequency of malaria epidemics and, therefore, reduces their predictability. At low irrigation levels, this decoupling reflects a breakdown of local but not regional NDVI as an indicator of rainfall forcing. At higher levels of irrigation, the weakened role of climate variability may be compounded by increased levels of control; nevertheless this leads to no significant decrease in the actual risk of disease. This implies that irrigation can lead to more endemic conditions for malaria, creating the potential for unexpectedly large epidemics in response to excess rainfall if these climatic events coincide with a relaxation of control over time. The implications of our findings for control policies of epidemic malaria in arid regions are discussed.Malaria Journal 01/2011; 10:190. · 3.19 Impact Factor -
Article: Epidemic malaria and warmer temperatures in recent decades in an East African highland.
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ABSTRACT: Climate change impacts on malaria are typically assessed with scenarios for the long-term future. Here we focus instead on the recent past (1970-2003) to address whether warmer temperatures have already increased the incidence of malaria in a highland region of East Africa. Our analyses rely on a new coupled mosquito-human model of malaria, which we use to compare projected disease levels with and without the observed temperature trend. Predicted malaria cases exhibit a highly nonlinear response to warming, with a significant increase from the 1970s to the 1990s, although typical epidemic sizes are below those observed. These findings suggest that climate change has already played an important role in the exacerbation of malaria in this region. As the observed changes in malaria are even larger than those predicted by our model, other factors previously suggested to explain all of the increase in malaria may be enhancing the impact of climate change.Proceedings of the Royal Society B: Biological Sciences 11/2010; 278(1712):1661-9. · 5.41 Impact Factor -
Article: Underestimating malaria risk under variable temperatures.
Proceedings of the National Academy of Sciences 09/2009; 106(33):13645-6. · 9.68 Impact Factor -
Article: Do rising temperatures matter?
Ecology 05/2009; 90(4):906-12. · 4.85 Impact Factor -
Article: Assessing the burden of pregnancy-associated malaria under changing transmission settings.
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ABSTRACT: The clinical presentation of pregnancy-associated malaria, or PAM, depends crucially on the particular epidemiological settings. This can potentially lead to an underestimation of its overall burden on the female population, especially in regions prone to epidemic outbreaks and where malaria transmission is generally low. Here, by re-examining historical data, it is demonstrated how excess female mortality can be used to evaluate the burden of PAM. A simple mathematical model is then developed to highlight the contrasting signatures of PAM within the endemicity spectrum and to show how PAM is influenced by the intensity and stability of transmission. Both the data and the model show that maternal malaria has a huge impact on the female population. This is particularly pronounced in low-transmission settings during epidemic outbreaks where excess female mortality/morbidity can by far exceed that of a similar endemic setting. The results presented here call for active intervention measures not only in highly endemic regions but also, or in particular, in areas where malaria transmission is low and seasonal.Malaria Journal 01/2009; 8:245. · 3.19 Impact Factor -
Article: Inapparent infections and cholera dynamics.
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ABSTRACT: In many infectious diseases, an unknown fraction of infections produce symptoms mild enough to go unrecorded, a fact that can seriously compromise the interpretation of epidemiological records. This is true for cholera, a pandemic bacterial disease, where estimates of the ratio of asymptomatic to symptomatic infections have ranged from 3 to 100 (refs 1-5). In the absence of direct evidence, understanding of fundamental aspects of cholera transmission, immunology and control has been based on assumptions about this ratio and about the immunological consequences of inapparent infections. Here we show that a model incorporating high asymptomatic ratio and rapidly waning immunity, with infection both from human and environmental sources, explains 50 yr of mortality data from 26 districts of Bengal, the pathogen's endemic home. We find that the asymptomatic ratio in cholera is far higher than had been previously supposed and that the immunity derived from mild infections wanes much more rapidly than earlier analyses have indicated. We find, too, that the environmental reservoir (free-living pathogen) is directly responsible for relatively few infections but that it may be critical to the disease's endemicity. Our results demonstrate that inapparent infections can hold the key to interpreting the patterns of disease outbreaks. New statistical methods, which allow rigorous maximum likelihood inference based on dynamical models incorporating multiple sources and outcomes of infection, seasonality, process noise, hidden variables and measurement error, make it possible to test more precise hypotheses and obtain unexpected results. Our experience suggests that the confrontation of time-series data with mechanistic models is likely to revise our understanding of the ecology of many infectious diseases.Nature 09/2008; 454(7206):877-80. · 36.28 Impact Factor -
Article: Susceptibility to infection by a haemogregarine parasite and the impact of infection in the Australian sleepy lizard Tiliqua rugosa.
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ABSTRACT: The Hamilton and Zuk hypothesis on haemoparasite-mediated sexual selection and certain studies of fitness are based on the assumption that blood parasite infections are detrimental to their hosts. However, there are few reports that have demonstrated harmful effects of endemic blood parasites on fitness in wild populations, and it has even been suggested that they may be non-pathogenic. In this paper, we show that individuals of the Australian sleepy lizard (Tiliqua rugosa) have smaller home ranges when they are infected with the haemogregarine blood parasite Hemolivia mariae than when no infection can be detected. An apparently contradictory result was that lizards with larger home ranges were more susceptible to infection under experimental exposure to Hemolivia. We propose that lizards sacrifice defence against pathogens by increased activity, perhaps associated with maintaining home ranges and mating opportunities. As a consequence, they gain higher parasite loads, which in turn inhibit their activity. In this case, the parasite-host interaction may act as a buffer of lizard activity.Parasitology Research 05/2007; 100(5):949-54. · 2.15 Impact Factor -
Article: El Niño and health.
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ABSTRACT: El Niño Southern Oscillation (ENSO) is a climate event that originates in the Pacific Ocean but has wide-ranging consequences for weather around the world, and is especially associated with droughts and floods. The irregular occurrence of El Niño and La Niña events has implications for public health. On a global scale, the human effect of natural disasters increases during El Niño. The effect of ENSO on cholera risk in Bangladesh, and malaria epidemics in parts of South Asia and South America has been well established. The strongest evidence for an association between ENSO and disease is provided by time-series analysis with data series that include more than one event. Evidence for ENSO's effect on other mosquito-borne and rodent-borne diseases is weaker than that for malaria and cholera. Health planners are used to dealing with spatial risk concepts but have little experience with temporal risk management. ENSO and seasonal climate forecasts might offer the opportunity to target scarce resources for epidemic control and disaster preparedness.The Lancet 12/2003; 362(9394):1481-9. · 38.28 Impact Factor -
Article: Cholera and climate: revisiting the quantitative evidence.
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ABSTRACT: Cholera dynamics in endemic regions display regular seasonal cycles and pronounced interannual variability. We review here the current quantitative evidence for the influence of climate on cholera dynamics with reference to the early literature on the subject. We also briefly review the incipient status of mathematical models for cholera and argue that these models are important for understanding climatic influences in the context of the population dynamics of the disease. A better understanding of disease risk related to the environment should further underscore the need for changing the socioeconomic conditions conducive to cholera.Microbes and Infection 03/2002; 4(2):237-45. · 3.10 Impact Factor -
Article: Cholera Dynamics and El Niño-Southern Oscillation
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ABSTRACT: Analysis of a monthly 18-year cholera time series from Bangladesh shows that the temporal variability of cholera exhibits an interannual component at the dominant frequency of El Niño–Southern Oscillation (ENSO). Results from nonlinear time series analysis support a role for both ENSO and previous disease levels in the dynamics of cholera. Cholera patterns are linked to the previously described changes in the atmospheric circulation of south Asia and, consistent with these changes, to regional temperature anomalies.Science 09/2000; 289(5485):1766-1769. · 31.20 Impact Factor -
Article: Susceptibility to infection by a haemogregarine parasite and the impact of infection in the Australian sleepy lizard
[show abstract] [hide abstract]
ABSTRACT: The Hamilton and Zuk hypothesis on haemoparasitemediated sexual selection and certain studies of fitness are based on the assumption that blood parasite infections are detrimental to their hosts. However, there are few reports that have demonstrated harmful effects of endemic blood parasites on fitness in wild populations, and it has even been suggested that they may be non-pathogenic. In this paper, we show that individuals of the Australian sleepy lizard (Tiliqua rugosa) have smaller home ranges when they are infected with the haemogregarine blood parasite Hemolivia mariae than when no infection can be detected. An apparently contradictory result was that lizards with larger home ranges were more susceptible to infection under experimental exposure to Hemolivia. We propose that lizards sacrifice defence against pathogens by increased activity, perhaps associated with maintaining home ranges and mating opportunities. As a consequence, they gain higher parasite loads, which in turn inhibit their activity. In this case, the parasite–host interaction may act as a buffer of lizard activity.
Top co-authors
Top Journals
Institutions
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2009–2013
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Howard Hughes Medical Institute
Chevy Chase, MD, USA
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2008–2013
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University of Michigan
- Department of Ecology and Evolutionary Biology
Ann Arbor, MI, USA
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2000–2011
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London School of Hygiene and Tropical Medicine
London, ENG, United Kingdom
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2007–2010
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University of Groningen
- Centre for Ecological and Evolutionary Studies (CEES)
Groningen, Province of Groningen, Netherlands
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