Christine O'Connor

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Can we control the emergence of zoonotic infectious disease?

Seventy five percent of the world’s emerging infectious diseases are zoonotic, or animal, in origin. Familiar examples include HIV/AIDS, Ebola, SARS, Lyme disease and West Nile Virus.

Humans play a big part in the emergence and spread of these and other infectious diseases. Our ever-expanding encroachment on forests – due to deforestation and increases in bushmeat hunting – increases the potential for disease transmission from animals to humans. Urbanization and global travel more easily allow for the spread of a disease from person to person and across the globe. These factors combine to greatly increase the probability of successful emergence and spread of an infectious disease.

Despite the clear importance understanding and controlling zoonotic diseases we have not had much success eradicating them, or even limiting their spread. It is hard to eradicate what we can’t find. Fore example, despite many years of study, researchers still are not sure where Ebola comes from. Outbreaks can be traced to handling of dead chimpanzees and gorillas, but neither appears to be the reservoir host (a species that is a source of infection but not affected by the disease) for Ebola.

What should be done to stop or control the emergence and spread of these diseases before they become global pandemics? Should we try and limit close contact with wild animals, perhaps via habitat preservation? Should we better control the bushmeat trade and/or educate hunters and consumers of bushmeat and other types of wild animal markets of the risks involved with such meat? Or, is a more proactive approach possible? Is there a way to pre-emptively identify such emerging diseases, via monitoring of wild animals and those who regularly have close contact with them?

Souce: Spillover by David Quammen

Also see: Q&A on swine flue with virus hunter Nathan Wolfe "We've created the perfect storm for viruses"

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    Jun 4 2013: An important approach to limiting the spread of zoonotic diseases could be providing villagers that depend on bushmeat for a source of food and income with alternative sources of protein. People need to eat, and if there are not other options they are likely to continue extracting wild meat even if it poses a health threat. The Wildlife Conservation Society posted an article about developing snail farms in Nigeria to help alleviate some of the hunting pressure on Gorillas. Apparently, they are a good source of protein and fetch a nice price at the market, providing a possible source of income as well as food.

    More programs like this, that provide a reliable source of food, could be successful in reducing the take from the forest,and reducing transmissions between humans in animals. However, I'm not sure if there are any negative impacts of snail farming or diseases associated with them?? .
    • Jun 4 2013: From what it sounds like in your link, they want to utilize the African giant snail, which - if it is what I think it is (Achatina fulica) - is actually considered an invasive species! Although a great idea to help alleviate hunting pressures on gorillas and other primates for protein. I also found an article on one of the other species the author of your WCS article may be discussing, Archatina marginata, that did a study on school-aged children and their mothers preferences with "snail pie". Basically, the children and mothers were given a beef pie and a pie made of the edible parts of the snail. Both the parents AND kids enjoyed the snail pie; it topped beef for texture, appearance, taste and flavor. It also was higher in protein and iron. I got a good laugh from that. But in all seriousness, it would be interesting to explore alternative sources of protein and iron, like snails, as a replacement for bushmeat. As long as there is no snail-human transmission of another zoonotic infectious disease. Plus, the snail farm seems relatively cheap to maintain (WCS article: $87 per year) and like you said Sandra, could be a source of income!

      Invasive species:
      "Snail pie":
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        Jun 5 2013: Shelby-- huh, snail pie, that's a new one to me! I was so surprised, I looked to see if there were any diseases associated with the giant African snail, and it turns out they act as a vector for a parasitic nematode worm that can cause a form of meningitis in humans!...they actually act as a host and can pass it from rats to humans.

        You're right, it sounds like they reek havoc as an invasive species in other areas, and interestingly are used in skin creams.
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      Jun 4 2013: I'm interested in discussing the morality and theoretical sustainability of bushmeat hunting versus current large-scale commercial livestock practices. It is my personal opinion that we may be like the pot calling the kettle black, since the land cover change associated with agriculture is one of the largest causes of global extinctions. I am not convinced that we should be calling for native Africans to curtail their traditional hunting practices until after we control our own appetite for excessive consumption. In addition, conventional livestock operations are, in my opinion, callous and inhumane industries that have reduced the value of a life to abstract cellophane packages of meat.
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        Jun 4 2013: The rainforest ecosystems need to be treated differently. It's one thing to use grasslands in the U.S. for livestock, but to clear rainforest is sort of counterproductive. It's a difficult problem to tackle because human populations are increasing around forested habitats, and we obviously want to shift them away from relying on the forest for its meat while also preserving the forest. Many of the villages suffer from poor access to urban areas, so meat transportation is very difficult. If this were too improve, it'd be much easier to provide rural areas with protein from areas that did not require the destruction of rainforests. Ultimately, better access to sustainable resources, and a reduction in the human population will alleviate much of the strain on the environment, which will also minimize contact between humans and disease carrying wild animals.
        • Jun 4 2013: Ryan, I think that yes ideally that if there was greater access to sustainable resources that some of the strain on the environment would be eliminated, however what kind of policy do you think would help minimize the contact between humans and disease carrying wild animals that isn't a complete change of norms for these people?

          Here is an article review of game meat safety:
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        Jun 4 2013: Okay, let me first clarify, I am not by any means suggesting we start going and clearing large amounts of tropical rainforest to start growing beef and poultry. I personally have disengaged in supporting the livestock industry for a long period of time now for the same reason you listed above Gwynne, and additionally for the implications it has on excessive resource use CO2 emissions. Also, all the reading I did for our presentation suggested that livestock did not hold well in these areas anyways due to susceptibility of tropical diseases and other factors.

        However, if people in these areas are not able to limit their hunting they may push many local species to extinction. In lots of areas the hunters now have to travel long distances to find meat and ecologists are calling it the "empty forest" syndrome. If wild animals continue to be extracted at the same rate there may be none left for these people to depend on, and a new source of food will have to be implemented anyways. It does not have to be large scale production, other alternative might be finding vegetables or legumes that fair well in poor tropical soils, or start to research edible plants that grow naturally in the areas.

        Additionally, lots of people use bushmeat as a source of income in these areas because there is not many other options. If transportation of meat to these areas did improve they would still have to purchase it. If they cannot afford to purchase it, there is not much point in bringing it to them. Unless someone plans on subsidizing it.

        empty forest article
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          Jun 4 2013: True, the people in these areas may be over-harvesting the resource. I simply think that it is a minimal global impact compared to the impact that is attributable to our own lifestyles. I am wondering what is the best long-term solution for this? We cannot fix things with the same type of thinking that caused the problem in the first place. Better access to cities means that people living their traditional lifestyles will be more likely to begin adopting more Western practices - driving more, consuming more. If logging is one of the primary roots of the problem, then why don't we boycott unsustainably harvested tropical timber? Why don't we encourage higher income nations to not purchase bushmeat and trophies from wild animals? Perhaps what we need is not more restrictions upon the people trying to eke out a subsistence living, but more education and restrictions upon the populations of industrialized nations. I strongly believe that the change should start at home and we should be looking at ourselves before pointing fingers at others.

          This discussion reminds me of the TED Conversation that was up a couple weeks ago: "Why are YOU killing the planet?" (
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          Jun 5 2013: The problem basically lies in the increasing human population. Trying to limit hunting and deforestation is going to be problematic while the population continues to rise. Unless the people are provided jobs that do not involve utilizing the forest, providing them with an income that can pay for food transported in from long distances, limiting human growth is really the main challenge. If scientists are able to determine that certain amounts of forest can be cleared/utilized without much disturbance to the ecosystem, that could be an approach as well, but you still have to be cautious when populations continue to expand.
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      Jun 5 2013: I read this report of potential alternatives to bushmeat. I certainly don't claim to be an expert of the topic, but we cannot just say, "Remove a significant proportion of the protein in your diet because we say so."
  • Jun 3 2013: One way to be proactive in epidemic prevention is to hold governments accountable for the health of their people by encouraging them to make disease surveillance a priority.
    According to Stephen S. Morse, PhD, a professor of epidemiology at Colombia University, “governments are often reluctant to report disease information for fear of political embarrassment, economic repercussions, or concern that it may make the government look ineffectual.”
    Dr. Morse is a founder of the Program for Monitoring Emerging Diseases (ProMED) “a scientist-to-scientist network connecting more than 30,000 subscribers in 155 countries, and the World Health Organization's Global Outbreak and Response Network (GOARN).”
    I think if disease surveillance were a priority of governments they wouldn’t be able to ignore the factors contributing to disease transmission such as deforestation and bushmeat hunting. This could have positive repercussions for not only control of disease, but other environmental issues as well.
    Here’s the article about infectious disease monitoring:
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      Jun 3 2013: It also has to be considered how smaller villages may not report diseases to the government as well. Isolated villages often have little to do with the workings of their government and would be wary of dealing with outsiders, even when faced with disease.
      • Jun 5 2013: You and Sandra make great points that dealing directly with villagers might prove difficult. What the governments need to address are the underlying causes, such as foreign demand for bushmeat and timber that causes deforestation. Infection may be difficult to control at the local level, but perhaps with greater surveillance we can prevent global epidemics.
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      Jun 3 2013: Good point Janielle. I think another problem that might make surveillance programs hard to implement is that most of the wild meat hunted is harvested illegally. The surveillance method mentioned in the article we posted for the class to read talked about making catalogs of blood samples of hunters and the animals they catch. I feel like people might not willingly bring forth the animals they catch for fear of getting in trouble.
      • Jun 4 2013: Perhaps, or perhaps Sandra it is the fear of also being blamed for bringing the disease to the village? I feel like getting in trouble to them is outweighed by the benefit of the money they make/ the food they bring back to feed themselves and their families with, hence they continue to hunt bushmeat. However, I do not know if they would feel the same if the animal they killed was responsible for an outbreak of some disease in the village. Thoughts?
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          Jun 5 2013: Hmm, I didn't think about that Sonya. Maybe knowing the potential risks could prompt people to participate in surveillance systems. I was surprised when I watched the Nathan Wolfe ted talk, that the hunter they came across had the blood samples with him. I feel like that is a good sign that their efforts were paying off.
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    Jun 3 2013: How do the effects of climate change factor in to zoonotic disease spread? When the vectors for disease can range farther or into new areas, the likelihood of transmission increases. Maybe it is not just increased contact with animals in their historic ranges but also their spread into previously marginal habitat that contributes to increased disease transmission. There was an article in Veterinary World 2010 entitled "Effect of climatic changes on the prevalence of zoonotic diseases" that explores some of these issues.
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      Jun 3 2013: Climate change will almost certainly affect the spread of zoonotic infectious diseases. Unfortunately, scientists are not sure how - depending on the animals affected, disease transmission mode, original range etc., the probability of disease spread could increase or decrease. (For a possible increase, see this article about Denge fever re-emergence in the south from Slate:

      However, on a possible bright side, the transmission of some infectious diseases can be controlled, but widespread control and elimination seems to depend on organization and effort from the government. As shown by this article from the CDC most of the US used to be endemic for malaria, and now I don't think anywhere is.
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        Jun 4 2013: These articles give me mixed emotions. I think it is great that we were able to rid the US of malaria, but I'm not sure that spraying DDT on the interior of homes is a healthy solution. I was surprised to learn that dengue has had such a presence here, but if its only a matter of time before it re-establishes itself, I wonder if we will be facing lots of large scale insecticide dispersals in the future?...I'm picturing "Silent Spring" all over again
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    Jun 3 2013: This comment is mainly concerned with the effects of Ebola but I think that it is an interesting subject. In this paper that I found they mention that the Ebola haemorrhagic fever has contributed to the roughly 98% local population decline of the great apes I wonder how that has affected the biodiversity in those areas? It would be interesting to see if there were any historical records of the biodiversity before EHF hit those areas and how the community dynamics have changed.
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      Jun 3 2013: I thought the piece from "Spillover" was very interesting, regarding Ebola and the severity with which it can curtail ape populations. I did not even know that Ebola was zoonotic until that reading. Just hearing about researchers being unable to find a single sign of gorillas in prime gorilla habitat in an area where they used to thrive, or finding the normally trafficked Bais completely bereft of gorillas, and the very very few found could not even be approached is quite illustrative. Also the clusters, or "piles" of apes fallen dead in the jungle was a really haunting image, and captures the speed with which an Ebola outbreak can ravage these apes. 98%, damn.

      It would/will be interesting to see which species starts filling the niche of the extirpated apes, maybe even just other apes immigrating in, or a reestablishment of local populations from resistant survivors/uninfected refugees. In any case, that's a huge ecological event, losing practically all of a large-bodied animal, quickly.
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        Jun 3 2013: That is exactly what I was wondering. How would the large animal niche be filled and how do the plant communities change?
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          Jun 3 2013: Nick, you might like this article about seed dispersal by lowland gorillas. It discusses how their nesting sites might benefit seed survival because they are usually in more open areas, that allow for more light availability. Its a little long, but most of the good stuff is in the discussion section.

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          Jun 4 2013: Sometimes the niche won't be filled by another large animal. Often times smaller animals are able to exploit the niche left behind by the disappearance of a large animals. But large rainforest animals such as primates don't necessarily have other animals that occupy that same niche (outside of other primates). If chimps or gorillas are wiped out from a forested area because of disease, it's very hard to replace those animals and their ecological impact.
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        Jun 3 2013: Sometimes I wonder if it would even be possible to 're-fill' any empty niches left by extirpation of great apes and other large primates with other populations of the same species. So many of those species are already limited to small and scatted populations and they need so much space to thrive. And of course, primates are not a very specious group in the first place.
        I have a horrible feeling that we could see the extinction of a great ape species in our lifetimes.
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          Jun 3 2013: Unfortunately, I think your horrible feeling might be right. I've heard predictions that chimpanzees, our closest relatives, might be extinct in 30 years!!!!!!!
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    Jun 4 2013: Do you think even if the hunters were educated, they would change their actions? Do you think the costs of contracting a deadly disease would outweigh the benefits in their eyes? There also may be other factors involved besides not fearing contracting an infectious disease. Here is something I find interesting. There was an article published by Popular Science that was related to this topic of zoonotic diseases. It had an emphasis on the Marburg virus which is related to Ebola.In 2007 a team of miners who were working in a cave in Uganda filled with Egyptian fruit bats contracted Marburg and died. After their death a response team including members from the CDC, NICD, and WHO were sent to the cave. What I find interesting is that the response team dawned full protective clothing including Tyvek suits, rubber boots, goggles, respirators, gloves, and helmets; clothing all of us would expect to wear in a cave potentially housing a deadly disease. However, the local miners that were leading the team throughout the cave wore shorts, T-shirts, and sandals...

    Christine, have you found anything throughout your research regarding the thoughts of any of the locals, not just the bushmeat hunters on how they view something like disease? I have a feeling that a lot of people, including hunters just simply aren't aware of threats of disease, but the ones that do, as in the case of these miners seem to deal with it as a normal part of nature and are willing to take the risks.

    Here is the article:
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      Jun 5 2013: Laurel, no I haven't found anything specifically regarding the thought of locals in areas with high incidences of emerging diseases.
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    Jun 4 2013: In the United States, we are also seeing an increase in the incidences of some infectious diseases due to our changing interactions with our environment and changing the environment itself. Emerging zoonotic diseases aren’t just a problem in other parts of the world! Lyme disease is a bacterial pathogen that humans get from ticks and which is prevalent in parts of the Midwest and Northeast in the US and its incidence is on the rise. A study by Allen et al. (2003) found that risk of Lyme disease increased in areas of NY state with decreasing forest patch size. This is thought to happen because mice, a reservoir animal for Lyme disease, increase in population density at forest edges. As forest areas become more fragmented and smaller, the amount of area considered an edge increases. Where there are more forest edges there are more mice, and then there are also more ticks feeding on the larger mouse population and potentially becoming carriers for the Lyme disease bacterium and then passing it on to humans via biting.
    These sorts of patches can be created and exacerbated by suburban and exurban development, I believe, where development encroaches on standing forest and people want to have some forest in their backyard, so there is lots of close contact with small patches of forest.

    Allan, B et al. Effects of forest fragmentation on Lyme disease risk. 2003. Conservation Biology 17: 267-272;jsessionid=296280AF2D2E5992B7CFAB05B0B77D27.d01t01
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    Jun 4 2013: According to this 2004 article by Ostfeld and Holt (, the majority of zoonotic reservoirs are rodents. The paper suggests that top-down regulation by predators would actually be one way to curb the increase in emerging zoonotic diseases.

    Also, another similar article that discusses how more biodiversity may play a role in reducing zoonotic disease transmission by diluting the rate of contact between susceptible individuals and the reservoir species (;2).
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    Jun 5 2013: In population ecology, one of the first models taught is the density-independent, exponential growth model, where a population exhibits continued geometric growth. While this type of growth is present in real life, it often only occurs for short periods of time because the growth is so rapid it is unsustainable and eventually something becomes limiting. However, this is the type of growth demonstrated by the human population so far. Have we exceeded our carrying capacity? Or have we increased it by inventing ways to create more food or extract more resources? Either way it cannot be maintained at this rate for much longer.

    In Alan Hastings’ book, Population Biology: Concepts and Models, he lists several hypotheses for mechanisms of population regulation, one of which is parasites and diseases.

    Perhaps the emergence of zoonotic pathogens is just form of population regulation that nature is inducing on the human population. Maybe we won’t be able to suppress it, or maybe by suppressing it we are only buying more time until another mechanism begins to kick in?

    this blog post discusses relationships between overpopulation and the emergence of diseases, but I think Gwynne in particular might like the last paragraph where he questions if overpopulation is the cause of emergence, or over consumption by industrialized nations.
  • Jun 4 2013: I think Gwen has an excellent point that people in industrialized/developed nations are only aggravating the
    problem by creating demand for an industry that is hurting Africa's biodiversity.

    The over-harvesting problem could be framed or categorized as a "tragedy of the commons" type problem (for a discussion: Today in the developed world people's priorities are different since their day to day needs are primarily met in terms of food and shelter and has longed since morphed into a more materialistic society that we know today. However to acquire beyond the basics we too have transformed the area we live in to the determent of local biodiversity. As people become more aware/ educated about the impact of an unsustainable lifestyle people have altered their habits to be more "green" or eco-friendly, but all of this took time and some people are still not mindful of this.
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      Jun 5 2013: I second Gwynne's point as excellent.
      Unfortunately, I think we are far far away from a sustainable lifestyle in the developed world. Some 'green' technologies and practices are becoming more common, but we still haven't really solved our biggest problem - reliance on fossil fuels and everything we use that comes from petroleum. Not to mention the difficulties of creating more 'green' lifestyles in the US as many aspects of renewable energy or even acknowledging climate change as a thing are political in nature for some people .
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      Jun 5 2013: Sonja, your response made me think of the "environmental Kuznets curve" theory - where the beginning stages of industrialization and economy growth are "dirty," but as people become better off financially they begin to care more about a clean environment and are willing to pay for it. In looking for the article where I remembered reading that, I ran across this other one ( discussing the transitions from hunter-gatherer and agrarian societies to industrial. The article concludes that our typical technological band-aids will not succeed in moving us toward sustainability, and that a fundamental reconsideration of the relationship between human societies and the environment will be necessary. The article may not be relevant for the discussion about zoonotic disease transmission, but it's great for thinking about solutions to our larger problems.
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    Jun 4 2013: I understand a need for Zoos in the 1st place, but honestly I'd love to see them outlawed in the future.
    There serve only a purpose of serving us, and that's very misguided way of thinking to pass down to one another.
    We could better protect them in a larger reserve, with the same zoo visitors bending their nature life to see them rather the other way around. Maybe this way we could limit the unnatural diseases these awesome animals can get.
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    Jun 3 2013: I wonder if it is a side effect of globalization? Maybe it will subside as defenses come about?
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      Jun 3 2013: Absolutely globalization influences the transmission of infectious disease. The more contact rate between people the higher the probability of transmission. This paper looks like it describes the models they use for disease transmission pretty clearly If you look at the disease threshold model there are several factors that are involved. A high contact rate, a high infection rate and mortality rates will all determine how the disease is spread through a population.
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      Jun 4 2013: In addition to globalization, population growth and urbanization by themselves are also hugely important factors. More people equal more possible contacts with diseased animals and urbanization means its easier for a disease to spread from person to person.
      I don't know if the emergence of infectious diseases will ever really subside. Some of this increase in emergence is almost certainly due to increased reporting and awareness of disease, but much of the increase is certainly a 'real' increase, as we humans are changing out interactions with our environments and global warming permits the spread of some diseases to new locations. (See the Slate article I posted a link to above, about Dengue fever in the US:
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        Jun 4 2013: I'm glad you pointed out how the human factors, like urbanization and changing interactions with the environment, are critical to consider. This article ( uses the term "biocomplexity" to encompass the interactions among physical, chemical, biological and social dimensions that all come together to influence the earth's systems.
        • Jun 4 2013: Here is an article by a law professor on behalf of the CDC that explores what measures are taken within a country as well as international laws that are in place to limit the exposure/spreading to infectious diseases:

          I found it particularly interesting that some of the recommendations are considered to be "soft-norms" or recommendations by the World Health Organization as a way to give guidance to countries on guidelines, practices or policies a country should consider to enact.
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        Jun 4 2013: Along similar lines, this paper on "Engineering Responses to Pandemics" (written by folks at MIT) has a lot of really interesting information about social adaptations and behaviors that are used by people to avoid contracting an infectious disease:

        It also discusses in detail how these "soft" responses affect the R_0, "reproductive number," of the infection and goes through quite a bit of the math, if anyone is interested.
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      Jun 4 2013: Although globalization is an element, one thing that needs to be done (in the case of hunting and the transfer of diseases between hunters and their prey) is the education of hunters. From my experience with Ivorian and Liberian hunters, many are not aware of the inherent dangers of the unsanitary conditions of handling meat. Since we currently cannot prevent outbreaks through medication, educating hunters and consumers of wild meat would be the simplest and most cost effective approach currently.
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        Jun 4 2013: I completely agree! The hope would be that once some of the hunters that are educated on the consequences of unsanitary or diseased meat would eventually spread their knowledge to others. It wouldn't solve the problem of disease transfer but it would be a help a great deal.