Scientists have long predicted large-scale responses of infectious diseases to climate change, giving rise to a polarizing debate, especially concerning human pathogens for which socioeconomic drivers and control measures can limit the detection of climate-mediated changes. Climate change has already increased the occurrence of diseases in some natural and agricultural systems, but in many cases, outcomes depend on the form of climate change and details of the host-pathogen system. In this review, we highlight research progress and gaps that have emerged during the past decade and develop a predictive framework that integrates knowledge from ecophysiology and community ecology with modeling approaches. Future work must continue to anticipate and monitor pathogen biodiversity and disease trends in natural ecosystems and identify opportunities to mitigate the impacts of climate-driven disease emergence.

http://science.sciencemag.org/content/sci/341/6145/514.full.pdf

Climate Change and Infectious Diseases: From Evidence to a Predictive Framework

Ectotherms are considered to be particularly vulnerable to climate warming. Descriptions of habitat temperatures and predicted changes in climate usually consider mean monthly, seasonal or annual conditions. Ectotherms, however, do not simply experience mean conditions, but are exposed to daily fluctuations in habitat temperatures. Here, we highlight how temperature fluctuation can generate ‘realized’ thermal reaction (fitness) norms that differ from the ‘fundamental’ norms derived under standard constant temperatures. Using a mosquito as a model organism, we find that temperature fluctuation reduces rate processes such as development under warm conditions, increases processes under cool conditions, and reduces both the optimum and the critical maximum temperature. Generalizing these effects for a range of terrestrial insects reveals that prevailing daily fluctuations in temperature should alter the sensitivity of species to climate warming by reducing ‘thermal safety margins’. Such effects of daily temperature dynamics have generally been ignored in the climate change literature.

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Temperature variation makes ectotherms more sensitive to climate change.

Forecasting the impacts of climate change on Aedes-borne viruses—especially dengue, chikungunya, and Zika—is a key component of public health preparedness. We apply an empirically parameterized model of viral transmission by the vectors Aedes aegypti and Ae. albopictus, as a function of temperature, to predict cumulative monthly global transmission risk in current climates, and compare them with projected risk in 2050 and 2080 based on general circulation models (GCMs). Our results show that if mosquito range shifts track optimal temperature ranges for transmission (21.3–34.0°C for Ae. aegypti; 19.9–29.4°C for Ae. albopictus), we can expect poleward shifts in Aedes-borne virus distributions. However, the differing thermal niches of the two vectors produce different patterns of shifts under climate change. More severe climate change scenarios produce larger population exposures to transmission by Ae. aegypti, but not by Ae. albopictus in the most extreme cases. Climate-driven risk of transmission from both mosquitoes will increase substantially, even in the short term, for most of Europe. In contrast, significant reductions in climate suitability are expected for Ae. albopictus, most noticeably in southeast Asia and west Africa. Within the next century, nearly a billion people are threatened with new exposure to virus transmission by both Aedes spp. in the worst-case scenario. As major net losses in year-round transmission risk are predicted for Ae. albopictus, we project a global shift towards more seasonal risk across regions. Many other complicating factors (like mosquito range limits and viral evolution) exist, but overall our results indicate that while climate change will lead to increased net and new exposures to Aedes-borne viruses, the most extreme increases in Ae. albopictus transmission are predicted to occur at intermediate climate change scenarios.

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Global expansion and redistribution of Aedes-borne virus transmission risk with climate change.

Recent epidemics of Zika, dengue, and chikungunya have heightened the need to understand the seasonal and geographic range of transmission by Aedes aegypti and Ae. albopictus mosquitoes. We use mechanistic transmission models to derive predictions for how the probability and magnitude of transmission for Zika, chikungunya, and dengue change with mean temperature, and we show that these predictions are well matched by human case data. Across all three viruses, models and human case data both show that transmission occurs between 18-34°C with maximal transmission occurring in a range from 26-29°C. Controlling for population size and two socioeconomic factors, temperature-dependent transmission based on our mechanistic model is an important predictor of human transmission occurrence and incidence. Risk maps indicate that tropical and subtropical regions are suitable for extended seasonal or year-round transmission, but transmission in temperate areas is limited to at most three months per year even if vectors are present. Such brief transmission windows limit the likelihood of major epidemics following disease introduction in temperate zones.

/pdf/detecting-the-impact-of-temperature-on-transmission-of-zika-3g17u21e99.pdf

Detecting the impact of temperature on transmission of Zika, dengue, and chikungunya using mechanistic models.

Understanding HIV disclosure: a review and application of the Disclosure Processes Model.

The ecology of mosquito vectors and malaria parasites affect the incidence, seasonal transmission and geographical range of malaria. Most malaria models to date assume constant or linear responses of mosquito and parasite life-history traits to temperature, predicting optimal transmission at 31 °C. These models are at odds with field observations of transmission dating back nearly a century. We build a model with more realistic ecological assumptions about the thermal physiology of insects. Our 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. A large data set on malaria transmission risk in Africa validates both the 25 °C optimum and the decline above 28 °C. Using these more accurate nonlinear thermal-response models will aid in understanding the effects of current and future temperature regimes on disease transmission.

/pdf/optimal-temperature-for-malaria-transmission-is-dramatically-47ra14blcf.pdf

Optimal temperature for malaria transmission is dramatically lower than previously predicted.

More than 60% of the world’s HIV-infected population lives in sub-Saharan Africa,1 and South Africa, with a national antenatal HIV prevalence of 30.2%,2 is host to one of the world’s worst HIV epidemics. The predominant mode of HIV infection throughout southern Africa is unprotected heterosexual intercourse.3 Prevention programs that target secondary transmission between those known to be HIV-positive and their sexual partners have recently gained favor4 but are still underdeveloped in generalized epidemic settings.3,5,6 Better understanding patterns of sexual behavior among individuals who know their HIV status remains central to informing these efforts.

Studies from industrialized countries suggest that some HIV-infected individuals, even after learning of their positive status, continue to engage in risky sexual behavior.7–12 In developing countries, experience is limited, although similar patterns are emerging. Research from West Africa and Uganda suggests that patterns of high-risk activity are common among those with known and unknown HIV status.13–16 In KwaZulu/Natal, South Africa, nearly 50% of HIV-infected patients in a single clinical setting were sexually active and 30% of those reported having unprotected sex in the previous 3 months.17

Increasing access to antiretroviral therapy (ART) has magnified the importance of addressing risk-taking behavior among HIV-infected adults. Few people in low- and middle-income countries were on ART before 2001; however, by the end of 2005, an estimated 1.3 million were on treatment, with 190,000 individuals on treatment in South Africa alone.1 Although the clinical benefits of ART are well established,18 the impact of treatment on preventing secondary transmission is less well understood. Although less infectious, people on treatment live considerably longer, thereby increasing the duration of potential exposure. Mathematic models predict that although high levels of ART coverage could potentially reduce HIV incidence, its benefits could be overshadowed by simultaneous increases in risky sexual behavior.6,19,20 In industrialized countries, concerns have emerged regarding increased risk behavior possibly attributable to ART treatment optimism21–23 and improving health; such data on the impact of care and support programs on patterns of sexual behavior are limited in developing countries.24 Furthermore, there are few available data on how these behaviors may differ in rural versus urban settings.

This research assessed sexual behavior among HIV-positive South Africa patients attending a rural clinic and an urban clinic. We explored the determinants of and factors associated with safe sexual behavior, highlighting implications for prevention programs to reduce secondary transmission that target those living with HIV.

Sexual behavior and reproductive health among HIV-infected patients in urban and rural South Africa.

Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using 15N additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream. These three streams were within several km of each other and on similar soils. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO3
 −) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH4
 + uptake length, higher uptake rates into organic matter components and a shorter 15NH4
 + residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added 15NH4
 +) but not in the second-order pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added 15N in organic matter compartments and exported 53% (15NH4
 + = 34%; 15NO3
 − = 19%). In contrast, the second-order pasture stream retained 75% of added 15N, predominantly in grasses (69%) and exported only 4% as 15NH4
 +. The fate of tracer 15N in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported (15NH4
 + = 9%; 15NO3
 − = 6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first- and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.

/pdf/amazon-deforestation-alters-small-stream-structure-nitrogen-4l62vp22ys.pdf

Emily de Moor

Papers

Optimal temperature for malaria transmission is dramatically lower than previously predicted.

Sexual behavior and reproductive health among HIV-infected patients in urban and rural South Africa.

Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers