Disease ecology

Disease ecology

The interaction of the behavior and ecology of hosts with the biology of pathogens, as it relates to the impact of diseases on populations.

Threshold theorem

For a disease to spread, on average it must be successfully transmitted to a new host before its current host dies or recovers. This observation lies at the core of the most important idea in epidemiology: the threshold theorem. The threshold theorem states that if the density of susceptible hosts is below some critical value, then on average the transmission of a disease will not occur rapidly enough to cause the number of infected individuals to increase. In other words, the reproductive rate of a disease must be greater than 1 for there to be an epidemic, with the reproductive rate being defined as the average number of new infections created per infected individual. Human immunization programs are based on applying the threshold theorem of epidemiology to public health; specifically, if enough individuals in a population can be vaccinated, then the density of susceptible individuals will be sufficiently lowered that epidemics are prevented. See Epidemiology, Vaccination

In general, the rate of reproduction for diseases is proportional to their transmissibility and to the length of time that an individual is infectious. For this reason, extremely deadly diseases that kill their hosts too rapidly may require extremely high densities of hosts before they can spread. All diseases do not behave as simply as hypothesized by the threshold theorem, the most notable exceptions being sexually transmitted diseases. Because organisms actively seek reproduction, the rate at which a sexually transmitted disease is passed among hosts is generally much less dependent on host density.

Population effects

Cycles in many animal populations are thought to be driven by diseases. For example, the fluctuations of larch bud moths in Europe are hypothesized to be driven by a virus that infects and kills the caterpillars of this species. Cycles of red grouse in northern England are also thought to be driven by disease, in this case by parasitic nematodes. It is only when grouse are laden with heavy worm burdens that effects are seen, and those effects take the form of reduced breeding success or higher mortality during the winter. This example highlights a common feature of diseases: their effects may be obvious only when their hosts are assaulted by other stresses as well (such as harsh winters and starvation).

The introduction of novel diseases to wild populations has created massive disruptions of natural ecosystems. For example, the introduction of rinderpest virus into African buffalo and wildebeest populations decimated them in the Serengeti. African wild ungulates have recovered in recent years only because a massive vaccination program eliminated rinderpest from the primary reservoir for the disease, domestic cattle. But the consequences of the rinderpest epidemic among wild ungulates extended well beyond the ungulate populations. For example, human sleeping sickness increased following the rinderpest epidemic because the tsetse flies that transmit sleeping sickness suffered a shortage of game animals (the normal hosts for tsetse flies) and increasingly switched to humans to obtain meals.

It is widely appreciated that crop plants are attacked by a tremendous diversity of diseases, some of which may ruin an entire year's production. Diseases are equally prevalent among wild populations of plants, but their toll seems to be reduced because natural plant populations are so genetically variable that it is unlikely that any given pathogen strain can sweep through and kill all of the plants—there are always some resistant genotypes. But when agronomists have bred plants for uniformity, they have often depleted genetic diversity and created a situation in which a plant pathogen that evolves to attack the crop encounters plants with no resistance (all the plants are the same). For example, when leaf blight devastated the United States corn crop, 70% of that crop shared genetically identical cytoplasm, and the genetic uniformity of the host exacerbated the severity of the epidemic. See Plant pathology

Disease emergence

Humans are dramatically altering habitats and ecosystems. Sometimes these changes can influence disease interactions in surprising ways. Lyme disease in the eastern United States provides a good example of the interplay of human habitat modifications and diseases. Lyme disease involves a spirochete bacterium transmitted to humans by ticks. However, humans are not the normal hosts for this disease; instead, both the ticks and the bacterium are maintained primarily on deer and mouse populations. Human activities influence both deer and mice populations, and in turn tick populations, affecting potential exposure of humans to the disease. Much less certain are the impacts of anticipated global warming on diseases. There is some cause for concern about the expansion of tropical diseases into what are now temperate regions in those cases where temperature sets limits to the activity or distribution of major disease vectors. See Lyme disease, Population ecology

References in periodicals archive ?
Gene has published nearly 80 peer-reviewed articles and book chapters to date on shellfish health, oyster disease ecology, nonnative oysters, parasite phylogenetics, and related topics, in addition to many others on fish parasites.
He begins with the disease ecology of colonial Bengal as it was featured in European writings.
That tiger mosquito invasion is not predicted to increase LACV transmission or even human cases highlights an important issue at the interface between disease ecology and invasion biology.
Through this unique and long-standing multi-agency partnership, NIH continues to support disease ecology research focused on threats to human health, including research in resource-poor countries, with the goal of fostering locally-relevant solutions.
Burrowes' research on the disease ecology of Bd, the first fungus discovered to kill adult amphibians, has helped demonstrate just how difficult managing, or even predicting, outcomes of disease invasion can be.
SAHOTRA SARKAR, professor in the departments of Integrative Biology and Philosophy at the University of Texas, is a specialist in philosophy and history of science, conservation biology, and disease ecology.
In her disease ecology lab in College Station, Sarah Hamer, peering through a microscope, examines the newest addition to her expansive vector collection - an exotic tick the size of a poppy seed, typically only found in Central and South America.
NIVEDI presently under aegis of ICAR with 32 All India coordinating units is focused on research and development in livestock disease epidemiology, forewarning, forecasting, surveillance, monitoring and informatics, understanding infectious disease ecology, on-site diagnosis and strategic control technologies considering climate change and globalisation including zoonoses.
Finding solutions will require a combination of many approaches--spanning genomics, disease ecology, toxicology, behavior, ecological modeling, and economic analyses.
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This study demonstrates the potential for field-deployable eco-immunologic tools to inform infectious disease ecology research.
Along with profiles of Verhasselt's career, an international group of scholars in geography, community health, epidemiology, and various medical fields contribute discussion of conceptual and methodological aspects of the field; case studies of disease ecology, health care geography, development and health, and women's health; and essays on progress made in the field in Germany, Italy, and India.
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