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Ecology, drivers and dynamics of zoonoses
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More than 60% of human infectious diseases are caused by pathogens shared with wild or domestic animals. Human pathogens from all taxa contain zoonotic species. Roughly 80% of viruses, 50% of bacteria, 40% of fungi, 70% of protozoa and 95% of helminths that infect human beings are zoonotic.
Arnon Shimshony
Some of these pathogens were responsible for the most devastating disease outbreaks such as the 1918 Spanish flu pandemic. It has become apparent that nearly two-thirds of the roughly 400 emerging infectious diseases that have been identified since 1940 are zoonotic.
This is the background to a recent series in The Lancet on zoonoses, in which researchers contributed three review papers addressing the following issues:
- Ecology of zoonoses (natural and unnatural): Zoonotic disease organisms include those that are endemic in human populations or enzootic in animal populations with frequent cross-species transmission to people. Some of these diseases have only emerged recently. Endemic and enzootic zoonoses cause about a billion cases of illness in people and millions of deaths every year. Emerging zoonoses are a growing threat to global health and have caused hundreds of billions of US dollars of economic damage in the past 20 years. The paper aims to review how zoonotic diseases result from natural pathogen ecology, and how other circumstances, such as animal production, extraction of natural resources and antimicrobial application change the dynamics of disease exposure to human beings. In view of present anthropogenic trends, a more effective approach to zoonotic disease prevention and control will require a broad view of medicine that emphasizes evidence-based decision making and integrates ecological and evolutionary principles of animal, human and environmental factors. This broad view is essential for the successful development of targeted surveillance and strategic prevention. Engagement of partners outside the medical community will help improve health outcomes and reduce disease threats.
- Drivers, dynamics and control of emerging vector-borne zoonotic diseases: Many vector-borne pathogens have appeared in new regions in the past 2 decades while many endemic diseases have increased in incidence. Although introductions and emergence of endemic pathogens are often considered to be distinct processes, many endemic pathogens are actually spreading at a local scale coincident with habitat change. The researchers draw attention to key differences between dynamics and disease burden that result from increased pathogen transmission after habitat change and after introduction into new regions. Local emergence is commonly driven by changes in human factors as much as by enhanced enzootic cycles, whereas pathogen invasion results from anthropogenic trade and travel where and when conditions (hosts, vectors and climate) are suitable for a pathogen. Once a pathogen is established, ecological factors related to vector characteristics can shape the evolutionary selective pressure and result in increased use of people as transmission hosts. Challenges inherent in the control of vector-borne zoonotic diseases and some emerging non-traditional strategies that could be effective in the long term are described.
- Prediction and prevention of the next pandemic zoonosis: Most pandemics originated in animals, are caused by viruses and are driven to emerge by ecological, behavioral or socioeconomic changes. Despite their substantial effects on global public health and growing understanding of the process by which they emerge, no pandemic has been predicted before infecting human beings. This paper reviews what is known about the pathogens that emerge, the hosts that they originate in and the factors that drive their emergence. Challenges to predict and control pandemics are discussed, surveillance to the most crucial interfaces targeted prevention strategies identified. New mathematical modeling, diagnostic, communications and informatics technologies can identify and report hitherto unknown microbes in other species, and thus new risk assessment approaches are needed to identify microbes most likely to cause human disease. The authors lay out a series of research and surveillance opportunities and goals that could help to overcome these challenges and move the global pandemic strategy from response to pre-emption.
In the third paper, the authors adopted a three-stage model to assess pandemic potential: Stage 1 (pre-emergence, no human infection); stage 2 (localized emergence of human infection, spillover); and stage 3 (widespread transmission and global dissemination). The frequency at which stages 1 and 2 occur is unknown, but probably high.
During stage 1, the putative pandemic pathogen is still in its natural reservoir. Ecological, social or socioeconomic changes (change in land use) alter the dynamics of pathogen transmission within the host or between hosts and allow the pathogen to expand within its host population, spread to a new region or be transmitted to another non-human host population or species. Each of these changes increases the likelihood of the pathogen making contact with human beings and thus progress to stage 2, during which wildlife or livestock pathogens spill over to people. Causes range from handling of butchered wildlife to exposure to fomites in wildlife markets or livestock farms or in the wild. Outcomes vary widely, from small clusters of human cases (Menangle virus) to large outbreaks, some with limited person-to-person transmission (Ebola virus) and some without (Hendra virus).
During stage 3 (full pandemic emergence), sustained person-to-person transmission and large-scale spread occur, often aided by global air travel (HIV/AIDS, SARS) or the international movement of reservoir hosts or vectors through trade (West Nile virus). Stage 3 pandemics are rare because even pathogens capable of some person-to-person transmission might not be able to maintain long enough chains of transmission to spread (Nipah virus in Bangladesh).
In their conclusions, the researchers indicate that understanding of the process of emergence and spread of emerging zoonoses has moved from anecdotal through analytical to potentially predictive. Researchers are positioned to move from a paucity of data to a wealth of information on potential pathogens in nature. The challenge is to develop the basic research agenda to allow potential pathogens to be distinguished from harmless microbes by use of molecular sequence data only (the most commonly collected information) or information that can be deduced from these data (structures of key proteins).
Global Networks
Political will for countries to act together to strengthen global networks against pandemic emergence also seems to have become positive. This new approach to pandemic prevention is shown by the handling, since its discovery in 1997 of the highly pathogenic influenza A (H5N1) virus of avian origin that infects people (precursor of the now widespread H5N1 avian influenza). Since then, and despite the virus’s continued inability to transmit effectively between people, the public health community has recognized that a lethal virus circulating only in wildlife and domestic animals creates extraordinary opportunities to mitigate future risk. The response to H5N1 has implicated not only clinical, diagnostic and therapeutic advances, but also better understanding of avian ecology, the economics of poultry production in low-income countries, and the ecology of the virus across the virus’s broad host range. For perhaps the first time, the response to a zoonotic pandemic has included development agencies improving individual countries’ abilities to identify new zoonoses early and mitigate quickly any new health threats arising within their borders. Zoonotic diseases, by definition, should be a key mission of human-health agencies, agricultural authorities and producers, and natural resource managers — all working cooperatively.
References:
Kilpatrick AM. Lancet. 2012;380:1946-1955.Morse SS. Lancet. 2012;380:1956-1965.
For more information:
Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases Zoonoses Moderator. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.Disclosure: Shimshony reports no relevant financial disclosures.