Richard Preston’s The Hot Zone, painted a thrilling picture of an exotic disease festering in a remote jungle until it aggressively emerged into a vulnerable human population through an unconfirmed, but likely encounter with a reclusive animal. A more temperate, developed setting, however, is certainly not disease-free. Extremely virulent pathogens can lurk in your own backyard in natural reservoir species as common as the white-footed mouse. Zoonotic diseases, or those that can be transferred to humans from other vertebrate animals, are so common that the majority of the diseases that infect humans today originated in other animals. According to the CDC, approximately 75% of recently emerged infectious diseases, including West Nile virus, Eastern Equine Encephalitis, and Lyme disease, are zoonotic. The natural reservoir of these diseases is an animal that acts as a long-term host for the pathogen and for which an infection is non-lethal. The natural reservoir therefore acts as a continual source of a pathogen, which would otherwise eliminate itself by killing hosts too quickly to perpetuate the infection.
The emergence of new zoonotic pathogens in human populations has risen in recent years due to increased contact between humans and wildlife. As humans push back the margins of animal habitat for settlement and exploit animal resources through hunting and husbandry, we increase our contact with animals that may harbor pathogens, thereby increasing our exposure to zoonotic disease. Zoonotic pathogens do not require humans to complete their life cycle; under natural conditions the pathogen would stay within the animal population. If exposed to a disease reservoir or vector, however, humans can become an incidental host. Habitat destruction and resource exploitation increases our risk of infection by potentially deadly pathogens simply by accident.
My research partner and I were warned about disease exposure as we emptied Sherman traps, studying the relative abundance of the white-footed mouse (Peromyscus leucopus) and the deer mouse (P. maniculatus) in the margins of the woodland surrounding Colby College. Worldwide, rodents are natural reservoirs for over 35 diseases. The pathogens that cause these diseases can be spread to humans either directly through contact with saliva, urine, and feces; or indirectly via insect vectors. Lyme disease is a well-known and well-publicized disease that is caused by bacteria harbored in a mouse reservoir but spread by ticks as vectors. When summer ends and ticks overwinter on the forest floor, humans can still fall victim to the potentially deadly hantavirus pulmonary syndrome. Hantavirus is spread to humans by direct contact with the feces and urine of an infected mouse, causing renal failure and fatal hemorrhagic fever. While the symptoms sound like something straight out of The Hot Zone, there are no monkeys, fruit bats, or protective suits involved. Hantavirus can be contracted by inhaling aerosolized waste of common and abundant rodents, and hundreds of cases have been reported in the United States, especially in the western states.
While disease was not the primary focus of our research, it had very clear epidemiological implications. As humans encroach upon animal habitat, we upset the population balance of the ecological community by eliminating cover, introducing invasive species, and reducing food availability. Some species, such as the white-footed mouse, are generalists that can thrive in highly-disturbed, marginal habitat. Their populations tend to explode when specialist species populations decline. Therefore, as we push into deciduous and mixed forests, the white-footed mouse may benefit under the altered ecological conditions and increase in numbers, but overall biodiversity collapses. Incidentally, the white-footed mouse is a reservoir for both Lyme disease and hantavirus.
Zoonotic diseases are caused by pathogens that will readily infect a variety of different host species, but for which there are very few natural reservoir species. This means that only a few species can survive and spread the infection, thereby assisting the long-term survival of the pathogen. A high density of a natural reservoir species in a community would increase the presence of the pathogen and also increase the likelihood that the disease will be passed to another host or vector species. In contrast, high biodiversity would reduce the density of the natural reservoir species and increase the number of incompetent reservoirs that cannot spread the disease. It has therefore been hypothesized that biodiversity can prevent the spread of zoonotic disease and in this way protect human health.
In a ground-breaking 2000 paper in the journal Conservation Biology, Richard Ostfeld and Felicia Keesing applied this hypothesis to the spread of Lyme disease, calling it the “dilution effect.” They predicted that high species diversity within the host community would increase the likelihood that ticks would feed on animals that do not harbor the Lyme disease bacteria. As a result the infection power of the natural reservoir, the white-footed mouse, would be diluted. With a lower prevalence of infection in ticks, the vectors of the disease, Ostfeld and Keesing hypothesized that high species diversity would also reduce transmission of Lyme disease to humans. Sure enough, through analysis of state level ecological and epidemiological data, they found that states with a greater number of small mammal species in the host communities had fewer reported cases of Lyme disease per capita.
The work of Ostfeld and Keesling demonstrates that the dilution effect can apply to indirect infection through an insect vector. A recently published study in the journal PLoS One, however, took their theory one step further. While studying the relationship between small mammal biodiversity and the prevalence of hantavirus in Panama, scientists from the Museum of Southwestern Biology and the University of New Mexico found that the dilution effect holds up for direct infections as well. They theorized that when species diversity in a host community is reduced and the density of the natural reservoir species increases, infected reservoirs will have more encounters with uninfected reservoirs and therefore more opportunities to transmit the virus. In contrast, high species diversity would reduce the encounter rate and similarly reduce the transmission rate. After setting up experimental field plots, the researchers reduced species diversity within some of the plots by removing the non-reservoir species. When the number of species was reduced, both the density of the natural reservoir population and the prevalence of hantavirus among those individuals increased. From this finding one could hypothesize that high biodiversity would consequently reduce the rate of direct infection to humans who are exposed to the reservoir species.
The idea that biodiversity can offer a service to human health is not a new concept. In fact, when people mobilize to save large swaths of highly diverse habitat from destruction, human health is one of the most argued cases for its preservation. However, people usually argue that within these areas of high biodiversity may be un-studied organisms with compounds for potential pharmaceuticals or that can act as laboratory models. While certainly compelling, this case incentivizes the preservation of certain species of potential value, not necessarily biodiversity. The ability of a diverse community to dilute the presence of a zoonotic pathogen and reduce human exposure to some of the deadliest diseases is an even stronger correlation between high biodiversity and improved human health.
Ostfeld, Richard S. and Felicia Keesing. 2000. Biodiversity and Disease Risk: the Case of Lyme Disease. Conservation Biology. 14(3):722-278
Suzan, Gerardo, Erika Marce, J. Tomasz Giermakowski, James N. Mills, Gerardo Ceballos, Richard S. Ostfeld, Blas Armien, Juan M. Pascale, Terry L. Yates. 2009. Experimental Evidence for Reduced Rodent Diversity Causing Increased Hantavirus Prevalence. PLoS One. 4(5)
Photo: Peromyscus leucopus; Encyclopedia of Life
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