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Emerging Infectious Diseases

Today’s bio-diversity crisis is not just one of lost habitats and extinct species. It is also a crisis of emerging infectious diseases (EID’s), such as HIV in humans, Ebola in humans and gorillas, West Nile virus and Avian Influenza in humans and birds, chytrid fungi in amphibians, and distemper in sea lions. There is every reason to take these events seriously, because EID’s appear to have a long evolutionary history.

This is because many pathogens are capable of infecting a range of hosts, but evolved in places where only some of those hosts live. Pathogens also have specialized means of transmission from host to host. If, for example, a pathogen is transmitted by an insect that lives in the tops of trees, susceptible hosts will not be infected if they never leave the ground.

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For humans, HIV, Ebola, West Nile virus, and Avian flu are only the latest in a long line of EID’s. When our ancestors moved out of the African forest onto the savannah more than a million years ago, they rapidly became effective predators. Sharing prey with pre-existing carnivores, they acquired tapeworms that originally inhabited only hyenas, large cats, and African hunting dogs.

When humans began to move out of Africa, they carried some of their pathogens to new areas, where they switched over to native hosts while native pathogens took up residence in the newly arrived humans. Agriculture and urbanization subsequently exposed us to yet more pathogens. If doctors were around back then, they would have called all these occurrences EID’s.

Geographical restriction and specialized transmission mean that in most time periods, most pathogens occur in a small number of host species, often only one, but retain the ability to infect more. But climate change alters everything. Species move out of their areas of origin and ecosystems change. Pathogens come into contact with susceptible hosts that they have never before encountered, and that never had the opportunity to evolve resistance. As a result, EID’s are not just possible; they are inevitable. Indeed, every episode of climate change has produced them.

Anticipating a problem is always more effective in terms of time and cost than responding to a crisis, no matter how viable the response. Unfortunately, we usually fight existing diseases but neglect looking forward.

For example, white-tailed deer living in northwestern Costa Rica host six species of tick. None of the ticks is known to carry Lyme Disease, which has never been reported in Costa Rica. As a result, there are no public health advisories about Lyme Disease there. But some of the ticks are closely related to known carriers, and thus are potential carriers themselves. This debilitating – and difficult to diagnose – illness could thus be introduced to Costa Rica inadvertently by something as simple as an asymptomatic eco-tourist.

The information about Costa Rican deer ticks comes from an inventory of parasites of vertebrates in northern Costa Rica, but it is the only inventory of its kind ever undertaken. We must do better. More than 50% of the species on this planet are parasites of some form, including pathogens affecting humans, livestock, crops, and wildlife. But, relative to our overall knowledge of biodiversity, we have documented less than 10% of the world’s pathogens. The remaining 90% are potential EID’s.

We face a potential crisis, therefore, that stems from our fundamental ignorance about the biosphere, for it is impossible to be proactive about species of pathogens whose existence has not been documented. This makes many pathogens “evolutionary land mines” awaiting us as we relocate to novel habitats, move species around, and alter existing ecosystems. Nevertheless, most resources are still being allocated for responses to known EID’s rather than to assessing the risk of potential EID’s.

Simply put, we must complete the global inventory of species. Now.

Five hundred thousand years of experience in hunting and gathering, coupled with cheaper and faster molecular analysis, performed by faster and cheaper computers makes this task economically feasible. Rapid identification using genetic “bar codes” can increase the rate of discovering species and determining the transmission dynamics of potential EID’s.

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Pathogens have highly specialized transmission modes, and groups of closely related species tend to be very similar. All species of malaria, for example, are transmitted by mosquitos. Once classified, we will then be able to make predictions about two closely related species based on incomplete information about either, buying time and saving money. Finally, this information should be digitized and made freely available over the Internet to all concerned researchers and parties.

If EID’s were rare, management through crisis response might be cost-effective. But EID’s are not rare at all. Rather, they are a common outcome of geographic dispersal associated with large-scale environmental changes. So we must cease being ignorant and reactive, and become informed and proactive. As a common sports adage puts it: never change a winning game, but always change a losing one.