When we become ill from an infection, we tend to believe that we caught it from another person, who in turn caught it from someone else, and that the pathogen (the biological agent that causes an infectious disease such as a bacteria or virus) that made us ill never resided in any species other than our own. But this belief, it turns out, is false more times than not. For most human infectious diseases—some 60%—the pathogen has lived and multiplied in one or more other organisms at some stage of its life cycle.
Pathogens present in other organisms enter our bodies in a variety of ways, for example, when we eat contaminated meat or when we are exposed to the body fluids of infected animals. One of the most common ways occurs when vectors, such as mosquitoes or ticks, transmit pathogens by injecting them into us. Still other organisms called hosts or reservoirs, where pathogens multiply and are available for transmission, are involved in these vector-borne diseases. All of these organisms, including the pathogens, depend for their survival on the healthy functioning of the ecosystems they are a part of, and on their interactions with each other and with other organisms sharing those ecosystems. As a result, ecosystem disruption and the loss of biodiversity have major impacts on the emergence, transmission, and spread of many human infectious diseases.
Let us look at three areas that illustrate some of the ways these impacts work:
While deforestation typically reduces forest mosquito diversity, the species that survive and become dominant, for reasons that are not well understood, almost always transmit malaria better than the species that had been most abundant in the intact forests. This has been observed essentially everywhere malaria occurs—in the Amazon, East Africa, Thailand, and Indonesia. In the Amazon, for example, in the past few decades, deforestation has led to a proliferation of Anopheles darlingi, a mosquito species that is highly effective at transmitting malaria, and that has, in some instances, replaced some twenty other less effective Anopheles species that were present before the forests were cut down.
Deforestation can also influence diseases carried by certain snails. As with mosquitoes, it has been shown that deforestation alters snail diversity in the forests, with few of the original snail species able to adapt to the new, deforested conditions. But the ones that do adapt to the more open, sunlit areas are generally also those better able to serve as intermediate hosts for the parasitic flatworms that cause the disease schistosomiasis in people.
Deforestation can affect the emergence and spread of human infectious diseases in other ways as well. With forest loss comes a loss of habitat and food for some species that serve as reservoirs for human diseases. The original outbreak of Nipah virus infections in Malaysia provides an example. Fruit bats, such as the Malayan Flying Fox, driven from the forest by deforestation, were drawn to mango trees at the edges of pig farms There they transmitted Nipah virus, present in their saliva and their excrement, to the pigs, which, in turn, infected 257 people, killing 105 of them. The large size of new pig farms where the outbreak occurred may have been a contributing factor.
BUSHMEAT AND HIV-AIDS
It is now well established that the virus causing HIV-AIDS, which currently infects more than 30 million people worldwide, and which has killed more than 25 million since 1981, was transmitted to human beings as a result of people in West-Central Africa being exposed, sometime between 1910 and 1950, to the body fluids of infected chimpanzees, most likely during butchering of their meat. Research has demonstrated that by eating primate bushmeat, people in this region are now being exposed to several other primate viruses, some closely related to the HIV virus, and there is great concern that future human infections, and perhaps even future pandemics, could eventually result from these exposures.
While the eating of bushmeat has been practiced for millennia, it is now on the rise for at least three reasons:
1. expanding populations and the need for food have driven up demand;
2. deforestation for logging and mining has opened up new, previously inaccessible, areas of the forest, providing greater access for hunters;
3. and the depletion of Atlantic fish stocks off the coast of West-Central Africa, secondary to decades of over-harvesting by large-scale industrial fishing, has forced residents to replace fish, which had been one of their main protein sources, with bushmeat.
SPECIES DIVERSITY AND THE “DILUTION EFFECT”
Just as some animals are better vectors than others for transmitting infections to people, so too are some better, or more competent, hosts. That is, they are more capable, when infected by a pathogen, of infecting a vector that bites them. For many diseases, only a few species are competent hosts.
Greater animal diversity in a particular area is generally associated with a greater proportion of incompetent hosts available for vectors to bite. In these cases, pathogens are “diluted” in hosts poorly able, or unable, to pass them on to new vectors, thereby interrupting the infection cycle and reducing the chance that people will become infected in these areas. This is the case for Lyme disease, the most common vector-borne disease in the United States (also found in other parts of the world, especially Europe), the disease in which this “dilution effect” was first discovered. People are at greater risk for getting Lyme disease in, and at the edge of, fragmented forests and other degraded habitats, which favor mice that are highly competent hosts for Lyme. Ticks are the vectors for Lyme. By contrast, large, intact forests are associated with greater vertebrate diversity, more incompetent hosts, fewer infected ticks, and less disease risk. Lyme infections, if left untreated, can cause serious heart, joint, and central nervous system disease.
The protective effect of greater species diversity on the risk of human infection has been shown in other diseases as well, including West Nile virus disease, hantavirus infections, and schistosomiasis, and may, in fact, be a common feature of many human vector borne diseases.
Photo from USDA of female Black-legged Tick. Males lack red stomach. Both are the size of a sesame seed and transmit Lyme Disease.