[SANTIAGO] More people may contract malaria in the tropical highlands of Africa, Asia and South America as global warming makes the climate in these areas more suitable for the disease’s transmission, according to a study.. (11), who analyzed long-term meteorological records for four high-altitude locations in East Africa where malaria incidence has dramatically increased in the last two decades. At five of the seven sites (Kericho, Kabale, Gikonko, Muhanga and … By contrast, red noise, which is known to be more relevant to environmental data, is dominated by cycles of low frequency (long period) in its power spectrum and exhibits significant autocorrelations that decay over time (see, e.g., refs. We wish to thank Charles Godfray, Dave L. Smith, and several anonymous referees for their comments on the manuscript. We focus on these two parameters, because they exhibited the strongest effects on RD in the random exploration of parameter space and the associated sensitivity test (see Methods). 38) was used to distinguish signals from red noise, in its narrow sense. Yes 52 Update TRENDS in Parasitology Vol.21 No.2 February 2005 Climate variability and malaria epidemics in the highlands of East Africa Simon I. Hay1, G. Dennis Shanks1, David I. Stern2, Robert W. Snow3, Sarah E. Randolph1 and David J. Rogers1 1 Department of Zoology, University of Oxford, South Parks Road, Oxford, UK, OX1 3PS 2 Department of Economics, Rensselaer … No, Is the Subject Area "Malarial parasites" applicable to this article? The role of climate change in the exacerbation of the disease has been controversial, and the specific influence of rising temperature (warming) has been highly debated following a previous study reporting no evidence to support a trend in temperature. The maximum number of monthly cases seen in this period was 269 in August 1994 with an average of 23 per month. [19] across which CRU TS 2.1 and 3.0 data were tested: 4°S, 4°N, 28°E, 38°E. Data are stratified into low and high altitude regions and summarized by the median (dots) and interquartile range (bars) of surveys available in each year. the population densities ranged between 158 persons/km2 in Ethiopia and 410 persons/km2 in … Based on principal component analysis, the method generates a set of eigenvalues and eigenvectors from a symmetric covariance matrix obtained by considering the data for a given number of lags (the “order” of the analysis). This question is for testing whether or not you are a human visitor and to prevent automated spam submissions. The first and third models are nested within the second one. They found no evidence of a significant increase in temperature at any of the locations and some evidence of an increase in rainfall at some locations. Between 1949 and 1962, malaria was "eradicated" on the Highlands by joint house spraying and chemoprophylaxis measures. 26 for averages between 30°S and 30°N and figure 2.2 in Annex A of ref. They suggest that the differing results of Hay et al. This change is larger than would be expected under either a monotonically increasing trend or no difference in temperature before that time. However, statistical significance alone does not address the biological relevance of such warming. The average number of cases over the period was 29 per month. At a more regional level on land, evidence has also been reported for increasing monthly minimum (nighttime) surface temperatures and decreasing diurnal temperature ranges over Eastern Africa (e.g., ref. 2 • auGust 2011 eDitorial review Panel Dr. Berhanu M. Abegaz Professor of Chemistry and Executive Director, African Malaria in highlands was initially governed by low ambient temperature, trend of malaria transmission was therefore increased and several epidemics were observed in late 1980s and early 2000s. Whether the incidence of malaria will be (or has been) affected by the warming climate is poorly resolved. AN is supported by the Wellcome Trust as a Research Training Fellow (# 081829). performed research and analyzed data; and M.P., J.A.A., L.F.C., and X.R. 30 and refs. The malaria inpatient data (1995-2006) were kindly supplied by the chief physician Dr Walter Odonde, Unilever Tea Kenya Ltd and updated to 2010 by Mr. Geoffrey Kores, the records officer. PLOS ONE promises fair, rigorous peer review, Because the latitude of Muhanga is almost on the boundary between two grid points in the CRU global grid, both the grid points above and those below the latitude of 3.00 S were considered. In Kericho (a), this order is reversed, and the trend corresponds to the subdominant eigenvalue. Based on the results of this analysis, two parameters (the slope of larval development and larval survival) were selected for their higher influence on RD and examined systematically for their effect on this quantity. With both nonparametric and parametric statistical analyses, we find evidence for a significant warming trend at all sites. Citation: Stern DI, Gething PW, Kabaria CW, Temperley WH, Noor AM, Okiro EA, et al. Climate variability and malaria epidemics in the highlands of East Africa Simon I. Hay1, G. Dennis Shanks1, David I. Stern2, Robert W. Snow3, Sarah E. Randolph1 and David J. Rogers1 1Department of Zoology, University of Oxford, South Parks Road, Oxford, UK, OX1 3PS 2Department of Economics, Rensselaer Polytechnic Institute, Troy, NY 12180, USA … Reported malaria incidence shows significant increases at these sites in the last decades of the 20th century (ref. In western Kenya, for example, epidemics have spread from 3 to 15 districts. A total of 999 randomizations were used for the computation of MC-SSA. Records from tea estates in the Kericho district in Kenya show that malaria reemerged in the 1980s. In other words, this temperature pattern indicates a period of lower temperatures than expected without the nonlinear trend. Affiliation The only process that could depress mosquito population size at higher temperatures is increased desiccation of breeding cavities but, given the magnitude of the temperature signal increase (0.5°C over 20 years), we do not expect this process to have a major impact in vector population dynamics. This timing coincides with that of the increase in incidence reported in the literature (5). Malaria is a major public health burden around the tropics (6, 8) with the potential to significantly increase in response to climate change due to the role of temperature and rainfall in the population dynamics of its mosquito vector (5, 9). Competing interests: The authors have declared that no competing interests exist. Yes Pascual et al. [4] for the period 1950 to 2002, finding positive and significant trend components. The maximum number of monthly cases of 376 was recorded in August 1997 soon after the end of the earlier sample. Malaria in Kenya’s Western Highlands. Epidemics of malaria in the East African highlands in the last 2 decades have often been associated with climate variability, particularly the El Niño-Southern Oscillation (ENSO). For comparison, the model was also run for this site, Kericho, with both local and simulated rainfall. Our results on (CRU) temperature data at four high-altitude sites support the occurrence of a significant warming trend since the end of the 1970s of ≈0.5°C. Mosquito populations decay exponentially for parameter combinations lying in the white area at the bottom left of each graph. There has been considerable debate on the existence of trends in climate in the highlands of East Africa and hypotheses about their potential effect on the trends in malaria in the region. The test statistics are also highly significant when we consider the CRU TS 2.1 data for the period 1950-2002 used by Pascual et al [8]. This paper was submitted directly (Track II) to the PNAS office. Summary time series plots for community parasite rate surveys in Kenya, Tanzania, Burundi, Rwanda, and Uganda are shown in Figure 5. Area averages of surface temperature over tropical land regions have trended upwards in the last 50 years (see figure 1 in ref. PfPR2-10: Plasmodium falciparum parasite rate in the age-standardized two-up-to-ten year old age range (Smith et al. A small increase in temperature could have led to the recent dramatic rise in malaria in the highlands of East Africa, say researchers. By contrast, in endemic areas with generally high mosquito abundance, malaria incidence can exhibit a plateau and decline as a function of this variable with multiple possible explanations related to human behavior and parasite dilution (e.g., ref. The main vector An. [9]. No, Is the Subject Area "Uganda" applicable to this article? ↵ Global Warming and Health: The Issue of Malaria in Eastern Africa’s Highlands. The question arises whether the increase of malaria incidence in the East African highlands since the end of the 1970s is already a manifestation of climate change. funestus disappeared. By contrast, Omumbo et al. Model, error structure, and maximum number of seasonal lags, p, were chosen by using the Akaike Information Criterion. The filled circles correspond to the combination of parameters used in Fig. We will show in this paper that the differences between their results and our earlier work are due to differences in data rather than differences in methods. Yes In Kenya, the moving average lines for both low and high altitude regions display an overall decreasing trend across the period 1985-2008, although in low altitude regions this decline slows or reverses slightly during the period 1995-2000. For purposes of comparison with previous results, we also considered the period of 1970–1995 analyzed in Hay et al. We also test the CRU TS 2.1 and 3.0 data for an area of East Africa defined by Patz et al [17] (Figure 1) to investigate whether similar results are also manifest across the region. We have re-examined temperature time series from locations in highland East Africa using the tDAN robust test for trends [11], [14]. The 1979-1995 sample does not have a significant time trend. Ultimately, a better understanding of the respective roles of climate, drug resistance, and land-use change and the interplay of these concomitant underlying trends will require a more creative and complex set of mathematical models explicitly developed to examine their concomitant impact on malaria dynamics. prepared quality-controlled series for temperature and rainfall at Kericho that takes into account the shift in the location of the weather station in 1986. However, the new CRU TS 2.1 and CRU TS 3.0 datasets shows a highly significant temperature increase in Kericho in both the 1966-1996 period examined by Chavez and Koenraadt [6], in the shorter 1970-1995 period examined by Hay et al. Malaria Public Health and Epidemiology Group, Centre for Geographic Medicine, Kenya Medical Research Institute – University of Oxford - Wellcome Trust Collaborative Programme, Kenyatta National Hospital Grounds, Nairobi, Kenya, Affiliation We test this data for the following periods: (a) 1966:1-1995:1, the sample period covered by Hay et al. where We thank the participants of this group for their input and constructive discussion. [4] failed to find a significant temperature trend for Kericho in the 1970 to 1995 period. Online ISSN 1091-6490. The spatial scale of relevance to the vector itself and to the movement of the human population concerned must be determined to evaluate whether particular local stations, or even a single local station, are appropriate for addressing patterns of change in meteorological drivers. MC-SSA estimates the parameters of the AR(1) model from the time series itself by using a maximum-likelihood criterion (38). broad scope, and wide readership – a perfect fit for your research every time. 2, and initial condition According to the United Nations Population Fund, the region has the world's highest population growth rate. Finally, the development rate of larvae and survivorship of adults as a function of temperature were calculated from functional forms reported elsewhere (42), in particular S(τ) = exp((4.4 − 1.3τ + 0.03τ2)−1). The resulting change in relative abundance (RD) for the simulated mosquitoes is similar to those obtained with the simulated rainfall values based on CRU monthly data for the number of wet days (Fig. The overall increase in malaria cases is reduced to 23 but in the last four years only July 2009 saw that many cases and the average number of cases over the last four years has been just eight. To specify a realistic autocorrelation structure, we used daily rainfall at a local station in Kenya (Hail Research Station, Kericho; latitude, 0.37 S; longitude, 35.27 E) (see Supporting Text and Fig. We did not include adult survival in this sensitivity analysis, because for the range of temperatures considered, the function S(τ) is fairly flat. A nonparametric analysis that decomposes the variability in the data into different components reveals that the dominant signal in three of the sites and the subdominant signal in the fourth one correspond to a warming trend. Australian Army Malaria Institute, Enoggera, Queensland, Australia. An interesting consequence of this pattern is that mosquito abundance in the model exhibits a larger jump between the end of the 1970s and beginning of the 1980s. To examine the existence of a warming signal, we first applied a nonparametric method, Singular Spectrum Analysis (SSA www.atmos.ucla.edu/fcd/ssa), which decomposes the variability in the data into orthogonal components whose form is not specified a priori (refs. Increasing the start date to 1970 results in similar but slightly steeper slope coefficients. This stage-structured model is a simplified version of a discrete-time system originally built to simulate the dynamics of Southern House mosquitoes under varying temperature and rainfall regimes in Hawaii (18). Scientists should pursue a strategic approach to research, focusing on the accumulation of evidence via designed sequences of studies. As an additional indication of wider temporal patterns in malaria endemicity across East Africa, data for the region were extracted from the MAP database of community parasite rate surveys spanning the years 1985 to 2010 [18]. 2; see Methods). However, significant spatial variability could exist at finer spatial scales, and future work should examine these patterns. is the size of the mosquito population under the detrended temperature time series, both at time i. The increases in temperature over the period are much higher than we found for the 1970-1995 period using the CRU TS 1.0 series. Sensitivity analysis shows that the above amplification is not restricted to the mosquito parameters of these particular runs but applies to a large region of parameter space. For more information about PLOS Subject Areas, click is the temperature at time t, α is the intercept, φ1 Specifically, the development probability G from larvae to adults is nonlinear with a threshold at low temperature values (below 15°C) that are too cold for development to proceed. J can also be used in a left-tailed test of the null hypothesis that the errors in (1) contain a unit root autoregressive process or random walk. Although based on an opportunistic assembly of available community parasite rate surveys, and despite substantial within- and between- year variation not explained by the smoothed trends, these simple summary plots point to a consistent and substantial decline in P. falciparum prevalence since 2002 or earlier across both low and high altitude regions of East Africa. The incidence of malaria in the East African highlands has increased since the end of the 1970s. The null hypothesis of white noise was not considered, because it was inconsistent with the shape of the SSA spectrum (Fig. Projection of the data onto a set of EOFs allows its reconstruction for selected components, such as those above the noise floor accounting for most of the significant signal. Image credit: Mathieu Caffin on behalf of Bailey Ferguson/Schmidt Ocean Institute. 1998). 4). (7) argued that the use of a global climate data set was inappropriate given its coarse resolution (0.5 × 0.5°) and the large altitudinal variation within these regions. 's data for 1979:1-2009:12. [4], the actual temperature changes estimated by the trend component vary from 0.21 K for Kericho to 0.27 K for Gikongoro and Muhanga. Although, malaria is decreasing through intensified interventions since mid 2000s onwards, these environmental changes might expose population in the highlands of east Africa to an increase risk of malaria and its epidemic particularly if the current interventions are not sustained. Again, the deseasonalized results are very similar. Smoothed lines show the LOWESS-smoothed moving average. The map extent exactly matches the area defined by Patz et al. Crawford School of Economics and Government, Australian National University, Canberra, Australian Capital Territory, Australia, Affiliation The trends in the newer data are even more significant when post-1996 data are added to the samples. 1 (those for the pair of subdominant eigenvalues in b–d and the dominant pair in a are plotted in Fig. A response by Patz et al. The resurgence of malaria caused by P. falciparum in the East African highlands has been reported widely (see Supplementary Information). This decomposition allows the separation of the significant signal from the noise in the data and the identification of (nonlinear) trends and periodic components including those with unharmonic shapes (22). Fig. Hence, while high quality data is of value, use of the data from a single met station vs. use of the interpolated CRU database is not the reason that Hay et al. All results remain the same for these two time series, and we report here only the results for the grid point centered at 2.75 S (a, Kericho; b, Kabale; c, Gikongoro; d, Muhanga). The tDAN [11] or Dan-J test [12] is based on a modified t-test on the slope parameter of the simple linear trend regression model:(1)where t is a linear time trend, u, is a stochastic process that may or may not be stationary and and are regression parameters to be estimated. Isotope ratios could offer a new way to closely track animal movements. For the combined Burundi, Rwanda, and Uganda region, parasite rates in both high and low altitude regions are seen to generally increase until the late 1990s/early 2000s after which a marked decrease is displayed. The relative difference (RD) in the output of the model for the two temperature regimes shows that the mosquito dynamics significantly amplify the temperature increase. All of the J statistics used to test for stochastic trends in the data are highly significant at the 1% level allowing us to reject the null hypothesis of a unit root in the residuals of the trend regression. During the 1970s, the cold season temperatures tend to be lower in the new series, while in the 1980s the warm season temperatures tend to be higher. (11). Our examination of the mean temperature series for Kericho prepared by Omumbo et al. No, Is the Subject Area "Kenya" applicable to this article? L Because of the importance of this debate [10], we apply a uniform methodology to test for trends in the various datasets used to date and time periods previously considered in the literature to determine whether differences in data or in methods are responsible for the variant findings. Edited by Burton H. Singer, Princeton University, Princeton, NJ, and approved February 10, 2006 (received for review October 12, 2005). Malaria in the highlands of Kenya is traditionally regarded as unstable and limited by low temperature. No, Is the Subject Area "Test statistics" applicable to this article? a and c). The resulting temporal dynamics of mosquito adult abundance show differences between the runs. Finally, through the data collection initiatives of the Malaria Atlas Project (MAP, http://www.map.ox.ac.uk) [16] we are able to present information on more than 5000 geo-positioned, post-1985, malaria prevalence surveys that allow insights into the changing endemicity of malaria in several East African countries during the last 25 years and if these show any difference by altitude. Both white and red noise can be considered. 6, which is published as supporting information on the PNAS web site). t wrote the paper. These factors are likely behind the high rates of poverty among the populations. The observed parasite rate in each survey was first standardized to the epidemiologically informative two-up-to-ten year old age group using a previously described age-adjustment algorithm [19], but no adjustments were made for sample size or whether the surveys refer to urban or rural areas. The following three models were considered Several factors account for these epidemics and this range from drug resistance, land The dominant eigenvalue in three of the four sites (c, Gikongoro; d, Muhanga; and b, Kabale) corresponds to the trend and is followed by a pair of eigenvalues associated with the seasonal cycle.
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