Date Published: September 15, 2016
Publisher: Public Library of Science
Author(s): Waithaka Mwangi, Paul de Figueiredo, Michael F. Criscitiello, Carolyn B Coyne.
It is estimated that 60% of recently emerging human diseases, including HIV-1 and pandemic influenza, originate from animals [1–4]. The increasing pressures of zoonoses, which are infectious diseases of animals that can be naturally transmitted to humans, have their roots in many causes. MacCready has estimated that since the dawn of human agriculture, the terrestrial vertebrate biomass has shifted from humans and their domesticated species accounting for ~0.1% 10,000 years ago to 98% today . These approximations demonstrate the exponential growth of opportunities for pathogens to spread with increasing ease from the animals upon which we depend to us. Besides growth of human and animal populations, many other factors drive zoonoses. These include habitat destruction and the resultant increased contact between humans and wildlife; bushmeat consumption, which was linked to HIV-1 infections in humans , and climate change, which influences the geographic range of many disease vectors. Range expansion into areas heavily populated by humans and human encroachment into the habitats of animal reservoirs also increase the risk of human infection . The global economy has enabled the rapid spread of people, animals, plants, and agricultural products across the world. This mobility has contributed to more frequent outbreaks of zoonotic diseases and infections of naive populations . To address these diverse challenges, innovative ways of thinking about health from an integrated perspective that countenances human, animal, and environmental factors must be developed.
The Ebola outbreak of 2013–2015, which started in Guinea and raised the specter of pandemic spread , provides a useful lens through which to analyze the One Health approach. Previous Ebola outbreaks were self-limiting due, in part, to the fact that they occurred in remote regions . However, the scale and the spread of the recent outbreak were unprecedented. The infection threatened health providers in international gateway cities, and contingencies for satellite spread burdened global health care infrastructure . In fact, health care systems in the affected regions in West Africa were rendered dysfunctional to the extent that other common diseases, such as malaria, caused additional deaths, and it was suggested that this added mortality exceeded the outbreak itself . Analysis of real-time responses to the outbreak highlight many challenges in modeling not only the environmental facilitation, zoonosis, and human spread of Ebola but also the related dynamics of other disease (e.g., through interruption of vaccination programs) . Economic activities also slowed down or came to a halt, thereby negatively impacting the livelihood of residents of affected regions . This “perfect storm”  was made possible by several interconnected factors that were responsible for not only the initial outbreak in humans but also the rapid spread of the virus from the epicenter.
The domestic and international response to the recent Ebola crisis was complex and multimodal. It included the implementation of public health, public awareness, and clinical intervention measures . As a result of these measures, the outbreak was eventually contained. However, the crisis revealed several significant gaps in disease awareness and management that can be addressed in responses to future outbreaks of emerging diseases. These gaps include the following: (1) insufficient monitoring and ecological modeling of zoonotic infection and transmission, (2) insufficient systems for rapid dissemination of and community education about the ecological aspects of disease outbreak and management, and (3) insufficient resources committed to enhancing food security to limit environmental encroachment and exposure to zoonotic disease in the wild.
First, it is not clear exactly how humans were initially exposed to Ebola, but available information suggests that it could have been through handling infected fruit bats . Despite increased knowledge of cultural practices and bat behavior in Ebola endemic regions, more studies are needed to generate the data required to create computational models that will accurately predict outbreaks [27,28]. Evidence of asymptomatic infection in fruit bats suggests that these mammals may be natural Ebola reservoirs , but there is a need to empirically identify ecological drivers of virus spillover and how these drivers influence infection of other susceptible hosts, including humans. Recently, the virus has been discovered to reside in immune-privileged organs (e.g., eyes and testes) several months after initial infection [29,30]. This raises questions about how long infectious forms of the virus can be maintained in infected hosts and if there are factors that promote latency. In addition, it is important to determine whether Ebola causes acute or chronic infection in natural reservoirs and whether certain factors, such as stress, can lead to high viral loads that are shed in wastes or contaminate the environment, including wild fruits on which bats and other wildlife feed . Such pulses have been found in Marburg, Nipah, and Hendra shedding from bats [32–34]. Furthermore, seasonal monitoring of Ebola virus in wildlife will reveal the temporal profile of viral load in these hosts and these data will reveal when there is an increased probability of zoonotic infection and transmission.
The implications of implementing the One Health vision are manifold, especially in the developing world, where the impact of zoonotic and emerging diseases is most acutely experienced. For example, human, animal, and environmental health are essential for long-term economic prosperity, reduction in foreign aid dependency, and political stability. In terms of impact on the community of researchers investigating host–pathogen interactions, the One Health approach implies a recalibration of training to address the relative paucity of investigators prepared to address the threat of global zoonotic infections, including hemorrhagic diseases, in which proficiency working at high biocontainment levels needs to be cross-trained with a One Health consideration of animal and environmental factors. Funding agencies must prioritize host–pathogen interactions as well as the broader health of reservoir and vector species and the environments containing them.