The Effects of Climate Change on Vector-Borne Diseases Lori Hollidge GG 612 Ranga Myneni 1 November, 1999

Transcription

1 The Effects of Climate Change on Vector-Borne Diseases Lori Hollidge GG 612 Ranga Myneni 1 November, 1999

2 INTRODUCTION Through both direct and indirect means, climate change is having, and will continue to have, significant impacts on human health. While a broad range of health effects will be felt, this paper will focus on the response of infectious, vector-borne diseases, to climate change. With increasing temperatures in the latter part of this century, we have seen a resurgence of old infectious diseases (McMichael, 1996). While this is due to many factors, one of the primary factors is regional climatic disturbances and climate change. It is therefore imperative that we create an interdisciplinary understanding of climate change and human health interactions so that we can develop appropriate policies aimed at curbing any negative relations between the disciplines. While it is currently difficult to develop clear and precise patterns for how and where changes will take place, available evidence and climate change models indicate that climate change will alter the pattern of our world s infectious diseases (ACSH, 1997). In addition, available predictive models agree in forecasting increases in disease range and disease sensitivity to climate. Future effects are forecasted to be felt, most intensely, by those regions that currently lie just outside endemic areas. Figure 1 illustrates areas that have seen emerging and re-emerging disease outbreaks since 1995: - 1 -

3 Figure 1: Selected Emerging and Re-emerging Disease Outbreaks in 1995 HOW VECTORS WORK To understand the relationship between climate change and disease incidence, it is imperative to have detailed knowledge of how vectors enable disease transmission. Vectors are typically cold-blooded organisms, such as insects, ticks, snails, and crustaceans. A vector incubates an infective agent, (such as a virus, single-celled, or multi-celled organisms), and then transmits that agent or parasite to humans, thereby spreading infection. The two primary determinants of how vectors function are - 2 -

4 geographical distribution of the vector and its vectoral capacity. Abundance is particularly important for vectors with long life spans, (such as the tsetse fly, bugs, and ticks), whereas geographic distribution is key to vectors with short life spans (such as mosquitoes, sandflies, and blackflies) (McMichael, 1996). Box 1 illustrates the primary factors to consider when studying the effects of climate change on disease distribution: Distribution Box and 1: Climate abundance Change of and vectors Vector-Borne and their Disease intermediate Distribution host species are The following factors in particular must be considered when attempting to predict the effect of global climate change on the distribution of vector-borne diseases: influenced I. by o mean The current climate geographic variables, distribution climate of variability, the disease; and extreme events. Sustained II. o The range of non-human hosts and reservoirs (i.e. insect or mammal); and III. widespread o Temperature-related disease transmission vector and depends parasite development, upon vector and population adaptive processes and favorable pertaining to reservoir and parasite interactions; environmental IV. o conditions Capacity for migration for the vector, of vectors its and parasite, parasites; and andany necessary intermediate host V. o The current seasonality of transmission. Source: Shope, 1991 species. The three primary climatic variables that affect vector biology are temperature, humidity, and precipitation (Patz, 1997). Temperature Based on fossil records from 10,000-15,000 years ago, most shifts in distribution of insects have been associated with temperature change, particularly with minimum temperature fluctuations, and these shifts occur rapidly. Insects are particularly temperature sensitive ants even run faster in warmer weather (Epstein, 1997)! Temperature affects vectors in several important ways. First, increases in temperature accelerate a vector s metabolism, which alters nutritional requirements and therefore feeding patterns (McMichael, 1996). For example, blood-feeding vectors would need to feed more often, which means that their biting rates would increase. This would result in increased egg production and disease transmission. In terms of human behavior, under increasing temperatures humans would likely wear fewer clothes, thereby increasing their - 3 -

5 vulnerability (Patz, 1997). Another example is the distribution of arthropod vectors, as minimum and maximum temperatures geographically limit these vectors. In addition, the majority of physiological functions of arthropod vectors require optimal temperatures, and even slight changes in minimum temperatures can significantly change arthropod survival. For example, in recent decades we have seen several years in which warm winters have precluded frosts, thereby increasing mosquito, cockroach, and termite populations. Humidity High relative humidity normally favors most metabolic processes in vector organisms. With warmer temperatures, high humidity extends arthropod survival, although susceptibility to fungal and bacterial infections could increase. There is also the exception that under relatively low humidity some vectors feed more frequently to avoid dehydration. Precipitation Precipitation is particularly important with respect to insects such as mosquitoes and blackflies, as rainfall determines the presence or absence of their breeding sites. This impact depends upon local evaporation rates, soil percolation rates, terrain slope, and proximity of large water bodies and rivers. A number of species breed in the standing water that remains after the floods in the rainy season. Some vectors depend upon fresh, well-oxygenated waters, while others have adapted to urban environments and are able to use any type of available water as a breeding site. Other Factors in Vector Functioning - 4 -

6 In addition to these effects of climate on vector function, biotic factors, such as vegetation and populations of host species, predators, competitors, and parasites, and human involvement are relevant. For example, shifts in environmental conditions, such as vegetative cover, could cause a vector species in one area to be replaced by another species that was formally absent in that local area. The vectoral capacities of the original and displacing species may differ significantly. This brings the possibility of introduction of a new disease that the local population is not equipped to handle. In addition, ticks, mites, and other species that rely upon flora and fauna to provide habitat and a favorable microclimate for host species and feeding, will be affected by changes in vegetation shifts resulting from climate change. This could benefit the areas that are vacated by the vector, but will likely harm the region that becomes a new home for the species. By shifting species and host patterns, climate change may also result in a new mixture of species, which could have many unknown negative affects on human health (McMichael, 1996). Climate change may also result in a shift in human habitation by rendering previously undesirable or uninhabitable areas inviting. Human population shifts may also be forced by climate change, if, for example, sea level rise impinges upon seaside residents and pushes them to higher land. This Homo sapiens migration could place people in areas where diseases are currently only transferred in wildlife cycles, and would place them at risk of new infection. Table 1 lists ten major tropical vector-borne diseases and the likelihood that their distributional patterns will be altered by climate change: Table 1: Major Tropical Vector-borne Diseases and the Likelihood of Change in Their Distribution as a Result of Climate Change - 5 -

7 Source: McMichael et al., MALARIA While all diseases are transmitted by different means and are affected differently by different variables, this study focuses on malaria as a representative disease because it is globally the most prevalent vector-borne disease (Patz, 1997). Malaria, once nearly eliminated in many regions, has made a global comeback, and is surpassing previous recorded infection levels in some areas. This is due both to the fact that the primary malaria-transmitting mosquitoes are becoming increasingly resistant to treatments and drugs, and that climate change is providing a greater range of livable space for - 6 -

8 mosquitoes. In addition, mosquitoes easily adapt and establish themselves in distant new environments. Due to the uncertainty of climate change predictions, it is difficult to determine the actual effects that climate change will have on malaria transmission, but some forecasts can be made. As discussed above, temperature sensitivity is often the limiting factor in disease transmission. Figure 2 illustrates the critical temperatures for mosquito and malaria parasite development, and Figure 3 shows the correlation between mean November temperatures and annual falciparum malaria rate in Pakistan. (P. falciparum is the most clinically dangerous malaria parasite.) (McMichael, 1996): Figure 2: Critical Temperatures in Malaria Epidemiology ( C) Source: McMichael et al., Figure 3: Variations in November temperatures and Annual Falciparum Malaria Rate in Northeast Pakistan Between 1981 and

9 Based on current available information, warming should increase survival rates in currently temperate areas, which indicates that malaria transmissions in temperature regions would likely increase. These newly exposed areas could initially have a high case-fatality rate due to lack of knowledge about the disease and how to treat it, but acquired immunities would likely control long-term catastrophe. In addition to the effects on temperate nations, climate change will cause malaria to extend its latitudinal and altitudinal range in tropical countries. Transmission may also start occurring throughout the year if breeding sites become available year round. However, other factors must also be taken into account. For example, we must consider the abilities of temperate regions to handle new disease introductions (Lowenhaupt, 1999). Additionally, warming without increased precipitation may actually decrease mosquito longevity because of the temperature-humidity relationship and its influence on mosquito survival. We must consider the fact that mosquitoes will not necessarily move only where it will be warmer. They will also take into account other ecosystem characteristics such as precipitation and the availability of adequate breeding - 8 -

10 sites. In terms of breeding sites, if the effect of climate change is to create more distinct seasonal patterns, then breeding sites may become more limited throughout the year. Through understanding mosquito preferences and future climate scenarios, we can concentrate efforts to protect human health in the proper areas. This calls for a multidisciplinary look at how climate change, disease, and health care options are interlinked on a region-by-region basis. A simple model that has been used in India could serve as a starting point for linking these various disciplines. Based on historical data from selected Indonesian provinces, modelers looked at the relationship between annual average temperatures, total precipitation, and malaria incidence. The model forecasted that the 1994 level of malaria incidence, (2705 per 10,000 persons), would, under mid-range climate change scenarios, increase marginally by 2010, and by approximately 25% by 2070 (McMichael, 1996). While this is a step in the right direction in terms of modeling, the estimates assume that efforts in Indonesia to prevent or control malaria will stay constant. This is unlikely because as development progresses in the area, it is likely that health institutions will be enhanced and will have better capabilities to deal with malaria. Health institution capabilities will play a major role in vector-borne diseases, and research should be done so that modelers can incorporate their contributions into the equation for a more realistic picture. Despite their shortcomings, global malaria predictions, and their link to climate change, should be granted merit. We must study these predictions, enhance them, and then act accordingly. Modeling studies performed by Martens et al. in 1995 have estimated that an increase in global mean temperature of several degrees by 2100 would - 9 -

11 increase the vectoral capacity of mosquito populations in tropical countries two-fold, and more than 100-fold in temperate countries. The models show that by the latter half of the next century the percentage of the world population living within the potential malaria transmission zone will have risen from 42% to approximately 60% (Epstein, 1997). Again, this is a valuable tool to start with, but disease surveillance and other preventative measures were not considered. Overall, while current models are valuable, they must be improved with more details and a more interdisciplinary approach. Models should take into account differences in vector species and local ecological conditions, and they must be validated against historical data sets. PEST SPECIES, DISEASE, AND CLIMATE CHANGE In studying disease and climate change, it is important to consider the effects that climate changes will have on disease-carrying pests such as rodents, bats, flies, and cockroaches (ACSH, 1997). Rodents, as well as insects and microorganisms, are opportunists that reproduce very rapidly and can easily colonize over-stressed areas or areas that have been impacted by events such as climate change or extreme weather events (Epstein, 1997). Rodent populations are known to fluctuate with local and global climate change. Warmer temperatures would result in increasing rodent populations in temperate regions, which would increase human-rodent interaction and lead to a higher risk of disease transmission. A primary example is the emergence of hantavirus pulmonary syndrome in south-western U.S. in 1993 (McMichael, 1996). Rodents are the primary reservoir for hantaviruses, and transmit the viruses through their saliva, urine, and feces. It is believed that climate and land use changes reduced natural rodent

12 predator populations, while also increasing local precipitation and food availability, thereby causing a ten-fold increase in the deer mice that transmitted the virus. Climate change could lead to higher metabolic rates in bats and a subsequent need for more blood meals. Temperature change impacts would also alter disease transmission by cockroaches. In temperate countries higher temperatures could cause roaches to venture into the sewers. In addition, infestation control would be much more difficult if homes were kept open for a greater majority of the year (McMichael, 1996). DISEASES RELATED TO WATER SUPPLY AND SANITATION Water quality, availability, and sanitation are all affected by climate change and are important pathways for the spread of disease. There are many factors to consider, and the relationship between climate change, water, and disease is difficult to quantify, but there are observations that can be made. In areas predicted to be drier in the future, decreased water availability will lead to lower sewer efficiency and an increase in the concentration of pathogenic organisms in raw water supplies. Decreased water availability would also mean reliance on poorer quality freshwater supplies, such as contaminated rivers, which could elevate levels of diarrhoeal diseases. This has been the case in southern Africa, where severe drought conditions have been felt for several years, and diarrhea has increased because local people have had to rely on contaminated water supplies. These impacts will be felt primarily by developing urban areas that have inadequate drinking supplies and sanitary systems. In addition, climate change could warm aboveground piped-water supplies, which would provide a better feeding and breeding ground for disease carriers. Areas that are predicted to become wetter may not

13 see availability-related disease, but could likely see an increase in diseases spread by water-related vectors such as malaria, dengue, and yellow fever. ENSO, CLIMATE CHANGE, AND EXTREME EVENTS Climate models predict that the El Nino/Southern Oscillation (ENSO) may interact with climate change to increase anomalous weather pattern events. This could affect the frequency of climatic extremes, such as drought and precipitation, which will have significant impacts on human health. As discussed above, the incidence of vectorborne diseases is often linked to the occurrence of climatic extremes. Epidemic prediction may be enhanced using the Southern Oscillation Index (SOI), which is a component of ENSO that relates to pressure deviations from the norm. Observations of strong associations between disease outbreaks and El Nino years include eastern equine encephalitis in the northeast U.S., the Ross River virus in Australia, and Murray Valley encephalitis in southeast Australia (McMichael, 1996). The National Oceanic and Atmospheric Administration (NOAA) has initiated efforts to look at the strong ENSO that was seen in and to study its effects on vector-borne diseases. They have concluded that ENSO-related changes in precipitation, temperature, and other environmental variables have both direct and indirect effects on human health (Colwell et al., 1998). Although there is little information or evidence to describe the occurrence of vector-borne disease outbreaks after extreme events, we do know that these events can either eliminate entire vector populations, or they can aid in their transmission. For example, in southeastern Australia epidemics of Ross River virus infection often arise after heavy rains in the Murray-Darling basin. There are also more tangible examples of

14 how extreme events can lead to disease. Breakdowns in sanitation, a lack of clean fresh water, over-crowding, and damage to local health care infrastructure, resulting from extreme events, can facilitate disease transmission (McMichael, 1996). SEA LEVEL RISE, CLIMATE CHANGE, AND HEALTH While sea level rise will not be one of the primary means through which climate change affects disease transmission throughout the world, it will be extremely critical to coastal populations. Sea level rise will result in salt-water intrusion, decreasing freshwater availability, and will lead to increased disease transmission by means already discussed (such as reliance on contaminated waters). In addition, sea level rise could directly contaminate water supplies through disrupting sanitation supplies or submerging previously aboveground waste dumps. Sea level rise could also influence the distribution of disease vectors such as Anopheles sundaicus, which is a saltwater vector of malaria (McMichael, 1996). RESEARCH AND MONITORING Research and monitoring efforts that focus on the effects that climate change has on disease transmission are impeded by a number of factors. These factors include the spatial scales of the issue; the large number of both biological and physical-systems based processes involved; the inevitable uncertainty inherent to predictions of the impacts of a previously unknown phenomenon; and the broad timeframe on which it is occurring. Climate change itself is a global phenomenon, yet because its health impacts on any given area will be site specific, the impacts will need to be understood on a local scale as well. Efforts are made even more difficult by considerable variation in vulnerability of populations around the world. This results in the need to determine local

15 vulnerabilities across the globe, which will be both time and resource demanding. This leads to the need to understand these complex relations on multiple scales, which will require both extreme research efforts and funding. The large number of biological and physical-systems based processes that interact to form the climate change-disease link make it extraordinarily difficult to understand and quantify. Figure 4 diagrams the interaction of these systems: Figure 4: Interactive Pathways by Which Climate Change Influences Health Source: McMichael et al., 1996 Due to the diverse, non-linear patterns of biological response to climate change, simple quantitative models will not usually be sufficient for forecasting its potential health impacts. This multidisciplinary subject will require the cooperation and unity of a wide range of specialists, including climate experts, biologists, modeling experts, health care professionals, and policy makers. Attention and commitment, by all of these people, will be required if we are to successfully avoid many of the dire consequences that climate change could have

16 This issue is plagued by the uncertainties in climate predictions and scenarios, as well as by the fact that the global community has never realized the need to be concerned over climate change and disease. As a result, we do not have any substantially analyzed data that links the two disciplines or gives us a framework from which to work. While this is primarily a hindrance, it also allows room for new, innovative, and creative ways of looking at the issue. There are some cases where extrapolation will prove helpful. For example, as was discussed earlier, there is some information on recent increases in infectious disease following regional climate change. While this type of data is valuable, the complexity of the task at hand will require the use of integrated mathematical models, knowledge input from various scientific disciplines, and cooperation amongst numerous groups of people. The use of models to understand climate change and human health is still at an early stage, and the current models lack regional and local capability, neglect out critical variables, and have not been fully validated. Finally, climate change is not an immediately obvious phenomenon, and its effects on human health and disease transmission may only be easily noticed once it is too late and lives have been lost. Due to this non-tangible characteristic, and many people s need for instant gratification of their work, it may be difficult to generate support for research and funding. The efforts taken now may only be observed by future generations that look back upon our historical records and are able to see that disease transmission and distribution patterns have been positively affected. CONCLUSIONS

17 It would be beneficial to begin monitoring changes in health status across the globe and to study in detail the climatological and ecological precursor events of disease outbreaks. This will involve integrated monitoring, which should be supplemented with Geographic Information Systems and remote sensing techniques. By organizing and storing data on a geographical basis, we could superimpose disease incidence, vector population, demographic, and climate images and link them to specific locations around the globe (Patz, 1997). Understanding when and why diseases spread will enable us to implement a framework for advance decision making by giving us early evidence of potential disease outbreaks. In addition to new programs and monitoring techniques, we should incorporate modeling data into health monitoring programs that are already in existence. In areas where this is possible, it would be a much more cost-effective option than establishing entirely new systems. The World Health Organization and other affiliated UN agencies should emphasize the need for climate change and disease prevention research. These agencies should make it a priority to involve developing nations in these efforts, and should support nations that will be most affected and that have the least capacity to handle future changes. These agencies should also perform periodic evaluation of progress being made in research, monitoring, and capacity building. The Climate Agenda, which was adopted by the World Meteorological Organization in 1995, is a step in the right direction (McMichael, 1996). This Agenda was adopted to ensure that various international climate programs are united in their efforts and to make certain that human health issues

18 are fully addressed. This is imperative, as we are dealing with an issue that has major international implications (Patz, 1997). While modeling will play a key role in the task at hand, other actions can also be taken to help reduce disease transmission. For example, vaccines, pesticides, and other control tools could be used more efficiently and could be stockpiled in areas that show a higher likelihood of disease outbreak. Public education should be used as a tool to illustrate easily avoidable dangers, such as providing a local breeding ground for mosquitoes by storing water in small household containers. Educational plans should also be implemented at local health institutions so that healthcare workers are aware of their obstacles and options (ACSH, 1997). While the specific effects of climate change on human health cannot yet be determined, we do know enough at this point in time to say, with certainty, that most of the impacts will be negative. This puts lives at stake and illustrates the fact that we must make it a priority to pursue research, monitoring, and the development of preventative options. Even if climate change turns out to be less of a concern in the future than is currently thought, we nonetheless will always need a way to cope with infectious disease transmission. In this regard, implementing preventative policies now should be seen as a no regrets policy because it will benefit human life regardless of what happens in the future (ACSH, 1997)