By Sheila Wertz-Kanounnikoff, Senior Forestry Officer, FAO, and Kristina Rodina, Forestry Officer, FAO
Aside from claiming high numbers of victims, what do Human Immunodeficiency Virus (HIV/AIDS), Ebola and COVID-19 have in common? The answer is that all three are zoonoses, the term given to an infectious disease caused by a pathogen that has jumped from animals to humans. Nearly three-quarters of recognized emerging infectious diseases (EID) are known to have originated from animals, generally from wildlife, and as the world has attempted to digest the profound implications of COVID-19 in recent months, much has been written about wet markets in China, thought to have been the starting point for significant human to human transmission.
However, strong evidence points to another connection – namely, forest degradation and habitat loss, or more specifically the way that nature is used to meet societal needs. Research has revealed that a small but significant percentage of emerging infectious diseases have a direct connection with forests; one study found that some 15 percent of approximately 250 EIDs analysed were linked with forests, with several of these, such as HIV/AIDS and Ebola, related to high costs in terms of human lives.
Behind the link between zoonoses and forests is the rich biodiversity that these latter generally host, particularly in tropical regions. That biodiversity is in turn a major source of potential pathogens – the infectious agents that cause disease when they spill over into humans– and of the vectors that transmit them. Many emerging infectious diseases appear to be transmitted among non-human primate hosts or insect vectors, and involve a variety of potential intermediate hosts, including domestic animals. However, forests also have an invisible secret weapon when it comes to zoonoses. So long as they and their ecosystems are left undisturbed, they can act as a buffer to disease transmission between animals and humans, through the so-called dilution effect, as observed for tick-borne Lyme disease where species richness protects against infection from zoonotic pathogens.
The tipping point comes when humans intervene and disrupt that equilibrium, whether it be through logging or cutting swathes of forest to grow crops or rear livestock. One concerning statistic is that activities associated with agricultural intensification, such as the building of dams, factory farms and irrigation installations, have been associated with up to 50 percent of all infectious diseases transmitted from animals to humans since 1940.
A case in Malaysia in the late 1990s illustrates the potentially lethal chain of events that can unwittingly be triggered when humans meddle with natural landscapes. A decision to place a commercial mango plantation next to an industrial-scale pig farm led to the arrival of Pteropus fruit bats, a species that is a natural reservoir for Nipah virus, named after the village where the events occurred. This zoonotic disease, which can cause acute respiratory infection and fatal encephalitis, led to an outbreak among pig farmers on the neighbouring farm, transmitted via sick pigs or their contaminated tissues. Millions of pigs were culled in an effort to contain the disease, adding massive economic collateral damage to the tragedy of the human lives lost.
Ebola virus disease, which causes an often fatal haemorrhagic fever in humans and whose primary host is again believed to be fruit bats from the Pteropodidae family, killed more than 11,000 people during an epidemic that ravaged Guinea, Liberia, Sierra Leone and other West African countries between 2014 and 2016. Research into this particularly contagious disease has shown a clear relationship with deforestation, pinpointing the average onset of human infection as two years after forest clearance has taken place.
The practice of logging can increase the spill-over risk and thus the spread of zoonoses, partly since it leads to wildlife habitat fragmentation, but also because it often goes hand in hand with increased wildlife or wildmeat hunting, facilitated by the opening up of roads. Studies also point to the role of wildmeat, not so much the consumption itself – which in fact constitutes a vital source of protein for many indigenous and local communities – but when rising urban demand for wildmeat as a luxury item drives overexploitation and enhanced risk of animal-to-human disease transmission. More dangerous still is the multi-million dollar global trade in wildlife for pets, food and wildlife-based products sold for alleged medicinal purposes, with its far-flung customer base, much of it in Asia, and the often uncontrolled nature of its operation, including the transport and sale of live animals.
Emerging infectious diseases are a growing threat to global health, economies and security. They are a wake-up call to the urgent imperative of protecting the world’s forests, and the wildlife that these host. At the Food and Agriculture Organization of the United Nations (FAO), our work with partners on mainstreaming biodiversity across agricultural sectors or restoring degraded ecosystems within the scope of the upcoming UN Decade on Ecosystem Restoration (2021-2030) offers an opportunity to build back better: for better health, better nature and better lives.
In the framework of a consortium, we run the Sustainable Wildlife Management Programme, which addresses the nexus of wildmeat consumption, food security, wildlife conservation and the spread of zoonotic diseases. With the World Health Organization and the World Organisation for Animal Health, FAO promotes the One Health approach, which can reduce disease transmission risks and improve the health and well-being of all people, wildlife and livestock.
It is well known that the benefits of forests span a far greater range than straightforward timber production. They serve as a source of food, medicine and fuel for more than one billion people. They act as carbon sinks, alleviating the effects of climate change; they nurture our soils, play a crucial role in water and air purification and are home to more than three-quarters of the world’s terrestrial biodiversity. As we recover from the COVID-19 pandemic, it is worth reflecting that forests also act as a critical bulwark against infectious diseases. One event of the COVID-19 FAO Forestry Webinar Week (22-25 June 2020) therefore addresses how forestry dynamics and considerations associated with health and livelihoods of forest-dependent people can be strengthened in the One Health approach to build back better. We must put the brakes on ecosystem degradation, deforestation and biodiversity loss to stem the emergence of future pandemics.
Chua K, Bellini W, Rota P, Harcourt B, et al. (2000). Nipah virus: A recently emergent deadly paramyxovirus. Science. 288:1432–1435.
Despommier, D., Ellis, B.R. & Wilcox, B.A. (2006). The Role of Ecotones in Emerging Infectious Diseases. EcoHealth 3, 281–289.
Epstein JH, Field HE, Luby S, Pulliam JR, et al. (2006). Nipah virus: Impact, origins, and causes of emergence. Curr Infect Dis Rep 8:59–65.
Fa, J.E., Peres, C., Meeuwig, J. (2002a). Bushmeat exploitation in tropical forests: an intercontinental comparison. Conservation Biology 16, 232-237.
Food and Agriculture Organisation of the United Nations. (2011). Investigating the role of bats in emerging zoonoses: Balancing ecology, conservation and public health interests. Edited by S.H. Newman, H.E. Field, C.E. de Jong and J.H. Epstein. FAO Animal Production and Health Manual No. 12. Rome.
Guégan J-F, De Thoisy B, Ayouba A., and Cappelle J. (2018). Forêts tropicales, changements d’usage des sols et risques infectieux émergents. Revue Forestière Française, 70, 2-3-4, p. 209-230.
Jones, K. E., Patel, N. G., Levy, M. A., Storeygard, A., Balk, D., Gittleman, J. L., & Daszak, P. (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990–993.
Johnson, Pieter & Thieltges, David. (2010). Diversity, decoys and the dilution effect: How ecological communities affect disease risk. The Journal of experimental biology. 213. 961-70.
Kilonzo, C, Stopka T & Chomel B. (2013). Illegal Animal and (Bush) Meat Trade Associated Risk of Spread of Viral Infections. 10.1002/9781118297469.ch10.
Nasi R and Fa J. (2020). COVID-19-led ban on wild meat could take protein off the table for millions of forest dwellers. DG’s Column. Center for International Forestry Research. Bogor. Indonesia (https://forestsnews.cifor.org/64855/covid-19-led-ban-on-wild-meat-could-take-protein-off-the-table-for-millions-of-forest-dwellers?fnl=en, accessed 3 April 2020).
Olival, K., Hosseini, P., Zambrana-Torrelio, C. et al. (2017). Host and viral traits predict zoonotic spillover from mammals. Nature 546, 646–650.
Olivero, J., Fa, J. E., Real, R., Márquez, A. L., Farfán, M. A., Vargas, J. M., Gaveau, D., Salim, M. A., Park, D., Suter, J., King, S., Leendertz, S. A., Sheil, D., & Nasi, R. (2017). Recent loss of closed forests is associated with Ebola virus disease outbreaks. Scientific Reports, 7(1), 14291.
Pulliam JRC, Epstein JH, Dushoff J, Rahman SA, et al. Agri- cultural intensification, priming for persistence and the emergence of Nipah virus: A lethal bat-borne zoonosis. (2012) J R Soc Interface; 9:89–101.
Rohr, J. R., Barrett, C. B., Civitello, D. J., Craft, M. E., Delius, B., DeLeo, G. A., Hudson, P. J., Jouanard, N., Nguyen, K. H., Ostfeld, R. S., Remais, J. V., Riveau, G., Sokolow, S. H., & Tilman, D. (2019). Emerging human infectious diseases and the links to global food production. Nature Sustainability, 2(6), 445–456.
Rulli, M. C., Santini, M., Hayman, D. T. S., & D’Odorico, P. (2017). The nexus between forest fragmentation in Africa and Ebola virus disease outbreaks. Scientific Reports, 7(1), 41613.
Siembieda, J.L., R.A. Kock, T.A. McCracken and S. H. Newman (2011). The Role of Wildlife in Transboundary Animal Diseases. Animal Health Research Reviews. DOI:10.1017/S1466252311000041.
Wilcox, B.A. and Ellis, B. (2006). Forests and emerging infectious diseases of humans. Unasylva 224/57:11-18.