Where the Wild Things Are: Unknown and Invisible Killers

You are quite the busy bee! You are overwhelmed by the paperwork piling up on your desk, your children’s obligations at school and the many meetings today you are preparing for. As your mind cycles through its to-do list, your body is coming into contact with millions of infectious organisms that threaten to derail your productivity at any moment. Fortunately, your immune system has seen them before and can fight (most) of them off.

Despite the impressive repertoire of immune defenses that we have, there are many other microorganisms that are lurking in the wild which our immune systems would be completely unable to recognize. Approximately 75% of all infectious diseases that affect humans originate in animals, and transmission of diseases between animals and humans is facilitated by close contact and disruption of natural habitats. Our genetic similarity to certain animals, such as chimpanzees, bonobos and gorillas, lends an ideal mixing bowl that can enable easy spread of diseases from one species to another. 

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One study, published in the Journal of Virology by a large team of  scientists from around the world, sought to compare adenovirus strains from wild non-human primates to those isolated in captive primates at zoos and primate facilities. It also sought to clarify whether strains were being passed to and from humans and non-human primates. For healthy individuals, adenovirus itself is typically not very dangerous, causing some annoying but non-life threatening symptoms such as fever, coughing, sore throat and eye and ear infections. It can also cause some yucky gastrointestinal issues. But for those whose immune systems are weakened (say, through HIV infection), adenovirus infection can be fatal.

All adenoviruses specifically infect only closely-related species. 30 different adenoviruses were detected in primate facilities and zoos across North America, and most were found to be very similar to human viruses. In and of itself, the presence of these viruses in these facilities provides a viable scenario in which disease transmission can occur between species. Most of what we know about viruses that affect non-human primates comes from captive animals, but little is known about the strains that are circulating in wild populations.

In this study, one chimpanzee strain was 99.2% identical to a human strain that causes conjunctivitis and upper respiratory symptoms, and another was 99.8% identical to a human strain that causes gastrointestinal issues. The study also reported a recent respiratory illness of a scientist who was infected with an unknown adenovirus while investigating a fatal outbreak of pneumonia and hepatitis in Titi monkeys at a primate research center in California. He wound up being infected with a simian virus, which up to that point, had never been detected in human populations. This all indicates likely transmission between non-human primates and humans and recent divergence of human and simian strains.

This study also illustrates the potential for viruses of all families to spread between species. So what should we, as a population, do? Certainly, we cannot avoid animals altogether. And we love taking our children to the zoo. I must highlight that the risk of disease transmission from that cute chimpanzee or marmoset to your little one is very low and no cause for alarm; however, we do need to be aware of the diseases that could be unleashed if we continue to disrupt these animals’ natural habitats. This adds another dimension to the conservation movement- the immediate human benefit.

Dr. William Karesh, Executive Vice President for Health and Policy at EcoHealth Alliance, noted that “it’s one world, and we only have one health and our lives are inextricably linked to the health of the environment.” Living in an environmentally sustainable manner and informing others of the human toll of habitat disruption can prevent new diseases from springing up in humans.

  

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Laura Zambrano, MPH is a Ph.D. student in Environmental Health Sciences at the Emory University Rollins School of Public Health. She is particularly interested in studying the impact of how human activities impact the environment, and how environmental change can facilitate disease transmission between animals and humans. That said, she enjoys playing with puppies, watching squirrels and cuddling with kittens... and still thinks monkeys are pretty darn cute.

Wevers, D, S. Metzger, F. Babweteera, M. Bieberbach, C. Boesch, K. Cameron, E. Couacy-Hymann, M. Cranfield, M. Gray, L. Harris, J. Head, K. Jeffrey, S. Knauf, F. Lankester, S. Leendertz, E. Lonsdorf, L. Mugisha, A. Nitsche, P. Reed, M. Robbins, D. Travis, Z Zommers, F. Leendertz & B. Ehlers. 2011. Novel adenoviruses in wild primates: A high level of genetic diversity and evidence of zoonotic transmission. J Virology 85(20): 10774-10784