Infectious Disease

Deer Tick, Lyme Disease VectorInfectious diseases like malaria, schistosomiasis, dengue fever, and zika virus are responsible for large burdens of disease globally and are highly sensitive to changes in environmental conditions, including temperature, soil moisture and precipitation patterns, deforestation, dams and irrigation projects, and others. It’s an urgent priority to better understand how land management practices alter the risk of these diseases in different settings and what types of interventions can reduce exposure to these diseases. Most emerging diseases globally are zoonotic diseases (with both human and animal hosts), and clearer understanding of anthropogenic influences on the emergence of zoonotic diseases (like HIV and Ebola) is another priority in planetary health research. Given the implications for food security and livelihoods, as well as for the state of global biodiversity, animal disease is also an important subtheme of disease ecology in the planetary health research context.

Learning Objectives

  • L1: Understand the environment-host-pathogen disease triangle and provide examples.
  • L2: Explain how environmental change can change the incidence, prevalence, geographical distribution, and/or severity of infectious diseases.
  • L3: Describe the criteria for an infectious disease hot spot and explain their characteristics with regard to environmental change.
  • L4: Recognize the interface between human and animal health in the contexts of environmental change and infectious diseases.

 

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Abdelwahab, M F. Changing pattern of schistosomiasis in Egypt 1935-79. Lancet [Internet]. 1979;2 :242-244. Publisher's VersionAbstract

A village in the Nile surveyed for schistosomiasis by J. A. Scott in 1935 was surveyed again in 1979. The same number of people as in the 1935 survey were randomly selected for investigation by the same parasitological techniques as those used by Scott. The prevalence of Schistosoma mansoni infection had increased from 3.2% to 73%, whereas S. haematobium infection, which had been very common in 1935 (74%), had almost disappeared (2.2%). In the local district hospital since 1972 the percentage of urine specimens found to contain S. haematobium ova has dropped from 30 to 9%, while the percentage of stool specimens containing S. mansoni ova has increased from 2 to 22%. In the local irrigation canals snail intermediate hosts for S. mansoni have outnumbered those for S. haematobium by a factor of 5--40 in the past 7 years. Changes in the proportions of snail vectors appear to be related to construction of the Aswan High Dam and to changes in the water-flow patterns of the Nile. The change in the relative frequencies of the two infections had important public-health implications, since the hepatosplenic schistosomiasis caused by S. mansoni is more difficult to treat and is associated with more morbidity and mortality than the urinary schistosomiasis caused by S. haematobium.

Desjeux P. The increase in risk factors for leishmaniasis worldwide. Trans R Soc Trop Med Hyg [Internet]. 2001;95 :239-43. Publisher's VersionAbstract

Economic development leads to changing interactions between humans and their physical and biological environment. Worldwide patterns of human settlement in urban areas have led in developing countries to a rapid growth of mega-cities where facilities for housing, drinking-water and sanitation are inadequate, thus creating opportunities for the transmission of communicable diseases such as leishmaniasis. Increasing risk factors are making leishmaniasis a growing public health concern for many countries around the world. Certain risk factors are new, while others previously known are becoming more significant. While some risk factors are related to a specific eco-epidemiological entity, others affect all forms of leishmaniasis. Risk factors are reviewed here entity by entity.

Patz JA, Graczyk TK, Geller N, Vittor AY. Effects of environmental change on emerging parasitic diseases. International Journal for Parasitology [Internet]. 2000;30 :1395-1405. Publisher's VersionAbstract

Ecological disturbances exert an influence on the emergence and proliferation of malaria and zoonotic parasitic diseases, including, Leishmaniasis, cryptosporidiosis, giardiasis, trypanosomiasis, schistosomiasis, filariasis, onchocerciasis, and loiasis. Each environmental change, whether occurring as a natural phenomenon or through human intervention, changes the ecological balance and context within which disease hosts or vectors and parasites breed, develop, and transmit disease. Each species occupies a particular ecological niche and vector species sub-populations are distinct behaviourally and genetically as they adapt to man-made environments. Most zoonotic parasites display three distinct life cycles: sylvatic, zoonotic, and anthroponotic. In adapting to changed environmental conditions, including reduced non-human population and increased human population, some vectors display conversion from a primarily zoophyllic to primarily anthrophyllic orientation. Deforestation and ensuing changes in landuse, human settlement, commercial development, road construction, water control systems (dams, canals, irrigation systems, reservoirs), and climate, singly, and in combination have been accompanied by global increases in morbidity and mortality from emergent parasitic disease. The replacement of forests with crop farming, ranching, and raising small animals can create supportive habitats for parasites and their host vectors. When the land use of deforested areas changes, the pattern of human settlement is altered and habitat fragmentation may provide opportunities for exchange and transmission of parasites to the heretofore uninfected humans. Construction of water control projects can lead to shifts in such vector populations as snails and mosquitoes and their parasites. Construction of roads in previously inaccessible forested areas can lead to erosion, and stagnant ponds by blocking the flow of streams when the water rises during the rainy season. The combined effects of environmentally detrimental changes in local land use and alterations in global climate disrupt the natural ecosystem and can increase the risk of transmission of parasitic diseases to the human population.

Sieswerda LE, Soskolne CL, Newman SC, Schopflocher D, Smoyer KE. Toward measuring the impact of ecological integrity on human health. Epidemiology [Internet]. 2001;12 :28-32. Publisher's VersionAbstract

Ecological integrity refers to the ability of environmental life-support systems to sustain themselves in the face of human-induced impacts. We used a correlational, aggregate-data study design to explore whether life expectancy, as a general measure of population health, is linked to large-scale declines in ecological integrity. Most of the data were obtained from World Resources Institute publications. Selected surrogate measures of ecological integrity and gross domestic product (GDP) per capita (as a socioeconomic confounder) were modeled, for the first time, using linear regression techniques with life expectancy as the health outcome. We found a modest relation between ecological integrity and life expectancy, but the direction of the association was inconsistent. When GDP per capita was controlled, the relation between ecological integrity and life expectancy was lost. GDP per capita was the overwhelming predictor of health. Any relation between ecological integrity and health may be mediated by socioeconomic factors. The effect of declines in ecological integrity may be cushioned by the exploitation of ecological capital, preventing a direct association between measures of exposure and outcome. In addition, life expectancy may be too insensitive a measure of health impacts related to ecological decline, and more sensitive measures may need to be developed.

Wu C, Maurer C, Wang Y, Xue S, Davis, D L. Water pollution and human health in China. Environmental Health Perspectives [Internet]. 1999;107 :251-256. Publisher's VersionAbstract

China's extraordinary economic growth, industrialization, and urbanization, coupled with inadequate investment in basic water supply and treatment infrastructure, have resulted in widespread water pollution. In China today approximately 700 million people--over half the population--consume drinking water contaminated with levels of animal and human excreta that exceed maximum permissible levels by as much as 86% in rural areas and 28% in urban areas. By the year 2000, the volume of wastewater produced could double from 1990 levels to almost 78 billion tons. These are alarming trends with potentially serious consequences for human health. This paper reviews and analyzes recent Chinese reports on public health and water resources to shed light on what recent trends imply for China's environmental risk transition. This paper has two major conclusions. First, the critical deficits in basic water supply and sewage treatment infrastructure have increased the risk of exposure to infectious and parasitic disease and to a growing volume of industrial chemicals, heavy metals, and algal toxins. Second, the lack of coordination between environmental and public health objectives, a complex and fragmented system to manage water resources, and the general treatment of water as a common property resource mean that the water quality and quantity problems observed as well as the health threats identified are likely to become more acute.

 

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