Zoonoses are infectious diseases that can be transmitted between humans and animal species. The infectious agent may be a bacteria, virus, fungus, protozoa or helminth. Zoonoses may be transmitted directly, indirectly or be vector-borne (e.g. via an insect bite). Humans have lived in close contact with animal species for thousands of years, and so while zoonoses are not a new phenomenon, they are receiving more focus recently because of the role they are playing in emerging infectious diseases.
Most emerging infectious diseases (EIDs) are zoonotic pathogens. In fact since the 1940s more than three hundred human diseases have emerged from animals, and of the pathogens which affect humans, the majority (61%) are zoonotic. Many of the zoonoses have reservoirs of infection within wildlife populations. A reservoir of infection is often a group or species of animals which can carry and possibly transmit organisms infectious to humans, without displaying symptoms of disease. (Taylor et al. 2001) EIDs are receiving attention because of the negative impact on human and animal health, with potential associated economic consequences. They may affect farm animal health and welfare as well as wildlife conservation efforts.
The role of wildlife in zoonoses is multifaceted. Anthropogenic factors are driving major changes of wildlife habitats. The increasing global human population and the associated requirement for food, shelter and fresh water creates more opportunities for wildlife interaction and therefore infection.
Climate change and global warming provide conditions where pathogens may be able to exploit a new geographic niche or host species. Most microorganisms have optimum temperature ranges, and environmental changes can lead to alterations in transmission, environmental persistence, or virulence. The consequences of these changes are not always predictable. (Bengis et al. 2004) In countries where water is a limited commodity, water sources can become a location for direct and indirect infection of humans by the wildlife species with which they share this essential resource.
Direct destruction of habitats by humans, means that the wildlife residents are required to relocate, learn new foraging behaviour, or rely on novel food species, all of which creates potential conflict and interactions with humans.
One devastating example of the interconnectedness of wildlife, humans, livestock and disease, and the importance of forests for human health, occurred in Malaysia in the late 90s. Slashing and burning of vast swathes of tropical rainforest provide land for pig farming. The fruit bats who had been displaced from the forest, were forced to forage for fruit on the trees near the pig farms. Pigs ate the pieces of fruit contaminated with bat saliva and urine which fell into their enclosures. The pigs ingested the Nipah virus, which is carried asymptomatically and shed by the bats, and developed severe respiratory and neurological symptoms. For many pigs the first symptom was sudden death. The Pig farming industry collapsed at a massive financial loss to the farmers and the Malaysian economy. In addition to this there were hundreds of humans infected by the virus and more than one hundred people lost their lives. (Aguirre et al. 2012, Kahn, 2011) However, there needs to be caution in the response to identifying the source of infection or the reservoir species. Bats for example, provide many essential ecosystem roles including pollination, insect consumption and seed dispersal; they should not be thought of as primarily a source of infection.
As the human population increases, some communities employ varied means of finding food. In some countries of the world this means that bush meat is being hunted, traded and eaten on a regular basis. Currently over 500 million wild animals each year. Not only is this a threat to several iconic and critically endangered species, but the opportunities for human infection arising from the contact with and consumption of infected animals is increasing. Ebola virus causes a haemorrhagic disease of both humans and great apes; and with no specific treatment, mortality rates in both groups can be high. (Bermejo, et al. 2006) HIV is also thought to have entered the human population via bushmeat consumption. (Bengis et al. 2004)
Wildlife trade is also big business, with live animals or animal products shipping round the globe. Millions of birds and fish are transported live each year, as well as tens of thousands of primates and reptiles. The trade is estimated to be worth $6 billion. The increase in intensive farming means that an outbreak of disease in a single species can be devastating.
As well as being a source, wildlife can be adversely affected by zoonotic diseases. The impact on human health of Ebola, Rabies, and HIV, which are all zoonotic pathogens, cannot be underestimated as they cause significant morbidity and mortality. However, Humans are also able to infect vulnerable wildlife species. Gorilla tourism in Africa has become a popular way to raise funds to support conservation of this critically endangered species. However Gorilla are susceptible to pathogens any human visitors may be carrying, particularly respiratory viruses. Gorilla have also sustained marked losses due to the recent Ebola outbreaks. (Bermejo, et al. 2006)
Travel to almost everywhere on the globe is possible; the world is increasingly connected. This means that pathogens, diseased people and animals may also move around with relative ease, meaning that infectious diseases can be difficult to contain or to then trace to a source when an epidemic is under way. A concerted, multidisciplinary, global effort is required to reduce the impact of zoonotic diseases worldwide. There must be collaboration between human health professionals, ecologists, animal health professionals, as well as other involved parties to try to predict, monitor and document diseases. The role that biodiversity plays, in maintaining health cannot be overlooked, and I plan to cover these topics further in future posts.
Aguirre, A., Daszak, P. & Ostfeld, R. (2012). New directions in conservation medicine applied cases of ecological health. Oxford ; New York: Oxford University Press
Bengis RG, Leighton FA, Fischer JR, Artois M, Mörner T, Tate CM. (2004) The role of wildlife in emerging and re-emerging zoonoses. Rev Sci Tech. Aug;23(2):497-511.
Bermejo M., Rodríguez-Teijeiro, J., Illera G., Barroso, A. Vilà, C. Walsh, P. (2006) Ebola Outbreak Killed 5000 Gorillas. Science: Vol. 314, Issue 5805, pp. 1564 DOI: 10.1126/science.1133105
GAO analysis of USGS data (2011) https://www.gao.gov/assets/590/586047.pdf
Kahn, LH, (2011) https://thebulletin.org/2011/02/deforestation-and-emerging-diseases/
Taylor LH, Latham SM, Woolhouse ME, Risk factors for human disease emergence. Philos Trans R Soc Lond B Biol Sci. 2001 Jul 29;356(1411):983-9. https://doi.org/10.1098/rstb.2001.0888