Understanding the interactions between infectious diseases and the environment requires interdisciplinary collaboration like that enabled by NCEAS. We have convened teams of ecologists, evolutionary biologists, medical researchers, and social scientists to explore the interactions between disease, humans, and their environments. Here is a sampling of their most influential papers.
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Disease Epidemics in Humans and Animals
Ecology drives the worldwide distribution of human diseases, and many disease organisms that threaten humans are in turn affected by human actions and non-human attributes of ecosystems. Our research teams have used ecological theory to better understand patterns of disease transmission to inform infectious disease control and public health policy.
Rabies is a viral disease that is contracted by mammals like bats, dogs, and raccoons and can be transmitted to humans through direct interaction. Smith et al. (2002) analyzed an extensive database documenting rabid raccoons to see how human demography and key habitat features influence the spatial dynamics of raccoon rabies epidemics. They found that large rivers act as semipermeable barriers to transmission, leading to a 7-fold reduction in local rates of disease spread, and that human population density had very little effect on the local spread of rabies.
When investigating epidemics, scientists often focus on the individual cases that are severe or fatal. However, an NCEAS study by King et. al. 2008 published in Nature suggests that mild or asymptomatic cases infecting large numbers of people are the key to understanding outbreak cycles of cholera —a mild to severe bacterial infection of the small intestine. This has important implications for the interpretation of epidemiological records, which often exclude the mild or asymptotic cases of infection.
The deadliest form of malaria in humans is also very sensitive to climate. Previous scientific studies estimated the optimal temperature for malaria transmission from mosquitos to humans at 31°C, but an NCEAS study found transmission to peak at much lower temperatures. Mordecai and colleagues (2013) developed a new mathematical model that accounts for the fact that both mosquitoes and parasites suffer in high temperatures. Their results predict malaria transmission to peak at 25°C and dramatically decrease above 28°C. Unlike previous models, the new model fits observations of malaria transmission in Africa very well, and will aid in understanding the effects of temperature on the spread of malaria and other diseases.
Grenfell et al. (2001) analyzed an exhaustive dataset of measles epidemics in England and Whales, and revealed recurring wave-like patterns of disease transmission throughout the region. They found that infections would begin in large core cities and then spread to smaller satellite towns. This paper was fundamental in quantifying the way that human settlement structures impact the spread of infection.
Ecosystems and Infectious Disease
Pathogens and parasites play important roles in the evolution and ecology of natural systems. Our research teams have improved understandings of the relationships between diseases, their hosts, and their environments to better understand the impacts to ecosystem processes.
A 2003 NCEAS study examined the relationship between disease risk and biodiversity, and its implications for conservation. Altizer et al. (2003) concluded that host genetic diversity plays an important role in buffering populations against widespread epidemics. Pathogens can also be a driving force behind maintaining biodiversity, so preserving interacting networks of coevolving hosts and pathogens is important to enable hosts to respond to future disease threats. Therefore, conservation programs should seek to maintain natural host-pathogen interactions as an important evolutionary process.
Many pathogens are highly sensitive to climate, and several NCEAS projects have examined the impact of seasonal forcing and climate change on infectious disease dynamics. A comprehensive study conducted at NCEAS was the first to analyze disease epidemics across entire ecosystems and investigate the impacts of climate change on disease risk. Harvell et al. (2002) concluded that climate change is triggering disease epidemics around the world, as warmer summers and milder winters favor the growth and spread of pathogens. This suggests that disease risk will increase with temperature for a wide range of hosts, including corals, oysters, plants, birds, and humans.
Success of introduced species is often attributed to escape from the pathogens of their native ranges. However, prior to two NCEAS studies published in Nature, there was a lack of clear, quantitative evidence to support this hypothesis. Mitchell et al. (2003) studied infection rates of invasive plants in their introduced and native ranges, finding that exotic plants were infected by 77% fewer fungus and virus pathogen species than their native counterparts. In addition, exotic plant species that are more completely released from their natural pathogens are more likely to be reported as harmful invaders of agricultural and natural ecosystems.
The same is true for animals. Torchin and colleagues (2003) compared the parasites of exotic animals in their introduced and native ranges using 26 host species of mollusks, crustaceans, fishes, birds, mammals, amphibians, and reptiles. They found that the number of parasites found in exotic populations is half that found in native populations.
Many factors, such as climate warming, pollution, harvesting, and introduced species can contribute to disease outbreaks in marine life. However, simultaneous increases in each of these makes it difﬁcult to attribute recent changes in disease occurrence to any one factor. Lafferty and colleagues (2004) synthesized studies of disease outbreaks in the ocean, and concluded that environmental degradation and climate change are increasing diseases in several marine taxa, while reducing disease incidence in others. For example, an increase in disease of Caribbean coral is postulated to be a result of climate change and introduction of terrestrial pathogens. In contrast, fishing and pollution may have reduced diseases in fishes. This study highlights the complexity of marine disease dynamics and stresses the importance of long-term studies in tracking changes in disease over time.
Available Data in the KNB Repository
In support of open science, NCEAS encourages data publication in online repositories. Below are a few examples of freely available NCEAS datasets pertinent to infectious disease research: