by Niina Heikkinen
As the H1N1 virus continues to spread around the globe, the importance of understanding zoonoses—pathogens that can be transmitted from one species to another—becomes increasingly clear. Not only do zoonoses account for 60 percent of emerging diseases, in the past quarter century about 75 percent of ‘new’ human diseases have come from animal populations (Tomley & Shirley, 2009). Although wildlife has been increasingly identified as playing an important role in zoonotic disease transmission, for the most part, responses to zoonosis transmission from animal to human populations has focused on risks associated with working with livestock and companion animals. In order to be able to effectively halt the spread of zoonoses, much greater emphasis must be placed on understanding and preventing the causes of disease transmission in wildlife populations.
Identifying Causes of Zoonotic Transmission
One of the first steps to tackling zoonotic disease transmission is identifying how zoonoses enter human populations from wildlife species. Many researchers in the field of infectious disease attribute the recent increased incidence of zoonotic disease transmission to rising urbanization, destruction of wildlife habitat, globalized trade, and the increased migration of humans and animals (Cabello & Cabello, 2008, abstract). A number of researchers have found that global warming increases the incidence of zoonotic disease transmission. Milder temperatures allow temperature-sensitive viruses and bacteria to live longer, increasing their capacity to infect wildlife. Animals and disease-carrying insects are able to travel greater distances, introducing new diseases to different geographic regions. Meanwhile, a more temperate climate has led to greater population density, and as a result, more disease transmission, (Parkinson & Butler, 2005 abstract). Also, both subsistence and highly industrialized animal management practices as well as lack of strict health standards during wildlife importation play a potentially significant role in increasing the risk of disease transmission from wildlife (Tomley & Shirley, 2009) (Pavlin et al. 2005). The wide range of factors contributing to the spread of zoonotic disease illustrates the complex global health issue facing the international health care community. Recent research in zoonosis transmission highlights the need for collaboration between agencies not directly involved in human and animal health, such as urban development groups and environmentalists.
Despite growing evidence that wildlife play a key role in zoonotic disease transmission, there is almost no scientific literature about how diseases are transmitted within wildlife populations. Even though some species such as birds, bats and small mammals have been identified as common initial or reservoir hosts of virulent zoonoses like avian influenza, rabies and plague, little is known about how diseases are transferred within these species. Zoonotic diseases in large mammals and primates can serve as a particular risk to human populations as these species often serve as a food source in developing nations, yet disease transmission is not well understood in these species either (“Survey Wild Animals,” 2009). Therefore, another key component in preventing the spread of zoonoses would be to increase funding for research on how diseases are transmitted in these species.
Part of the reason for this lack of research in wildlife may be because in the short term, addressing human outbreaks alone has often worked to control the spread of disease in human populations. Human vaccination, anti-viral and antibacterial treatments, as well as international reporting of confirmed cases of zoonoses have proven to be mostly effective in slowing the global spread of some zoonoses. (Childs & Gordon, 2009, pp. 422-423). Thus there may seem to be little motivation for international governments to invest millions of dollars in additional research projects.
However, reactive treatment to outbreaks in the human population does nothing to prevent future outbreaks of viruses that may undergo further mutation, nor does it prevent the introduction of completely new zoonoses into the human population. Dr. Nathan Wolfe, Director of the Global Viral Forecasting Initiative and known for discovering that retroviruses can spread from nonhuman primates to humans, argued in a TED conference that the global health community can no longer afford to ignore the mechanisms for cross-species disease transmission.
To illustrate his point, Wolfe discussed the origins of HIV, one of the world’s most deadly viruses, which first infected chimpanzees in the Congo. Wolfe said his research has revealed that the virus had likely infected thousands of people who hunted chimpanzees for meat in Brazzaville, Congo as early as 1929. Wolfe pointed out that had the international community discovered HIV when it had first infected humans, the current AIDS pandemic might have taken a much different and less virulent turn (“Nathan Wolfe’s Jungle…” 2009). With the rapidly growing global human population reaching approximately 6.5 billion in 2008, the percentage of cross-species disease infection is only likely to increase as humans have even greater contact with wildlife through expansion of urban areas and destruction of wildlife habitats (Tomley & Shirley, 2009).
Finding A Solution
To gain a better understanding of the transmission of zoonotic diseases in wildlife, researchers must identify high-risk areas where zoonotic disease transmission is most prevalent and then establish systematic methods for tracking the spread of disease in these areas. Early detection of virulent zoonotic disease, coupled with increased research on effective treatment methods for wildlife will greatly reduce the likelihood of new global pandemics. While researchers differ on the methods that should be employed to track these diseases, all agree that establishing more effective disease surveillance methods is essential for protecting animal and human health. One of the main concerns with establishing national and international wildlife surveillance is that since there are few organizations to build from, most projects must be established from scratch, at very high cost. In order to make costs less prohibitive, some researchers suggest limiting surveillance efforts to diseases that pose clear pandemic threats, such as H1N1. Few nationwide wildlife population estimates exist, therefore the use of target studies in combination with statistical modeling and geographical information system technology could be used in order to create maps of high-risk areas (Childs & Gordon, 2009, p. 423).
A second major challenge in controlling zoonotic disease is determining how to manage infected wildlife populations. Traditionally, infected wildlife have been subject to culling through shooting, trapping, poisoning or use of pathogenic agents (Cooper & Larsen, 2006). Such methods often involve the culling of animals that are healthy but at risk for catching the disease. Action of this kind has raised the ire of animal welfare groups. These groups demand more humane methods of population control such as vaccination and immunocontraception. Other researchers maintain that the use of immunocontraception, which creates immune resistance to the body’s own gametes, is too ineffective. Not only must every animal in the population be caught and injected individually with the vaccine, some species have been shown to be resistant to it, and most require one or more booster shots in order for the vaccine to remain effective. Critics fear that species that have natural resistance to the contraception vaccine will create inbred populations that will be exceptionally susceptible to zoonotic disease because of their lack of genetic diversity (Cooper & Larsen, 2006, p. 822). Therefore, while humane, immunocontraception seems to be am impractical solution to halting zoonosis transmission. However more research needs to be done on finding effective ways to either vaccinate or treat wildlife for zoonotic disease.
International Collaboration
In order to increase the effectiveness of cross-species disease transmission control, organizations involved in national and international health of human and animal species must increase their collaboration so that the can respond more swiftly and accurately in cases of potential zoonotic disease pandemics. One of the only models in national health for this kind of collaboration is evident is the surveillance of rabies. All hospitals, veterinary clinics and wildlife organizations in the U.S. have a national mandate to report confirmed cases of rabies to the Centers of Disease Control and Prevention. Unfortunately, rabies is the exception to the rule. It is the only disease about which the Centers of Disease Control and Prevention have gathered extensive data concerning animal as well as human transmission (Childs & Gordon 2009, p. 422). Increased collaboration between federal agencies like the Centers of Disease Control and Prevention, U.S. Department of Agriculture, U.S. Department for International Development, U.S. Department of Defense, the World Heath Organization, USAID, and private organizations such as the Wildlife Conservation Society and Wildlife Trust is necessary for fully understanding and preventing the spread zoonotic disease.
The Global Viral Monitoring Initiative, lead by Wolfe, provides an effective model for identification and surveillance of zoonotic disease on an international scale. Wolfe’s research team works mainly in field sites in Cameroon, where they follow the effects of consumption of bush meat (hunting of wild game for food) in the transmission of zoonotic disease from animals to humans. Not only do the researchers test game as well as hunters for the spread of zoonotic diseases, one of their main functions is to educate people on the dangers of acquiring diseases from the animals they hunt. In order to help hunters protect themselves and their families, the organization distributes filter papers that provide a simple test for blood and body fluids to see if an animal is infected with the disease or not. Each animal that is caught is tested for disease before it is consumed, to reduce the risk of disease transfer to humans. Their approach has proven effective in not only identifying new zoonotic diseases previously undiscovered, it has also stopped the spread of known pathogens (“Nathan Wolfe’s Jungle…” 2009).
Although Wolfe’s project is geared specifically towards identifying disease transmission from wild game to hunters, similar systematic testing of wildlife using filter papers could be implemented in wildlife conservation areas, to test wildlife caught for importation to zoos, and at animal control agencies. Also, similar field sites could be set up in countries identified as high-risk areas to monitor global spread of zoonotic disease. Once zoonotic diseases have been identified, more effort and resources must be placed on finding effective treatments for animals as well as humans so that outbreaks can be halted before they reach pandemic proportions. With the majority of emerging diseases developing from animal species and transferring into the human populations, it is essential that the international health community collaborate with veterinary and wildlife organizations to control and monitor zoonotic diseases as they develop in wildlife and livestock populations. Efforts to control disease transmission to humans from animal species will greatly reduce the likelihood of a devastating global zoonotic pandemic.
References
Cabello, C.C. & Cabello C.F. (2008). Zoonoses with wildlife reservoirs: a threat to public health and the economy. Servicio Agricol Ganchero, 136 (3), 385-393. PubMed.gov. PMID: 18575667
Childs, J. E. Gordon, & Elizabeth R. (2009). Surveillance and control of zoonotic agents prior to disease detection in humans. Mount Sinai Journal of Medicine: A Journal of Translational and Personalized Medicine, 76 (5), 421-428. doi: 10.1002/msj.20133
Cooper, D.W., & Larsen, E. (2006). Immunocontraception of mammalian wildlife: ecological and immunogenetic issues. Reproduction: The Journal of the Society of Reproduction and Fertility, 132, 821-828. doi: 10.1530/REP-06-0037
Nathan Wolfe’s jungle search for viruses (2009, March). TED.com. Retrieved Nov. 20, 2009 from www.ted.com/talks
Parkinson, A.J., & Butler, J.C. (2005). Potential impacts of climate change on infectious diseases in the Arctic. International Journal of Circumpolar Health, 64 (5), 478-86. PubMed.gov
Pavlin, B. I. & Schloegel, L. M. & Daszale, P (2005). Risk of Importing Zoonotic Diseases Through Wildlife Trade, United States. Emerging Infectious Diseases, 15, 1721-1727. doi: 10.3201/eid1511.090467
Survey Wild Animals (2009). Global Viral Forecasting Initiative. Retrieved Nov. 20, 2009 from www.gvfi.org
Tomley, F. M., & Shirley, M. W. Livestock infectious diseases and zoonoses. (2009). The Philosophical Transactions of Royal Society of Biological Sciences, 364 (1530). doi: 1530 2637-2642