Zoonotic diseases are those that originate in animals and cross the species barrier to infect humans. Zoonotic pathogens may be bacteria, viruses, parasites, fungi, or unconventional agents such as prions. They can cause both familiar diseases (e.g., salmonellosis, rabies) and more exotic ones (e.g., Ebola, anthrax). While domestic animals can transmit infections to humans, wildlife (e.g., non-human primates, bats, rodents, ticks, and insects) tend to be significant disease reservoirs.
The majority of all emerging infectious diseases originate in wildlife. Spillover events, though rare, have the potential to cause global outbreaks because they are, essentially, novel diseases (i.e., they have acquired a new host). Disease processes differ between animals and humans, so it can be difficult to understand or predict the pathogenicity, virulence, and course of treatment for zoonotic diseases using only animal data. Additionally, many zoonotic diseases are caused by viruses, which are inherently difficult to treat. Some agents, such as prions (the infectious proteins responsible for bovine spongiform encephalitis, also known as mad cow disease in cattle, and Creutzfeldt-Jakob disease in humans) are not only poorly understood but have no known treatment in any host.
Recent examples of emerging zoonotic disease outbreaks include the 2003 Severe Acute Respiratory Syndrome (SARS) and 2012 Middle East respiratory syndrome coronavirus (MERS-CoV) outbreaks, believed to have originated from bats and camels, respectively, and the 2009 H1N1 pandemic, believed to have originated from pigs. And while the natural reservoir of Ebola virus disease is still unknown, the 2014 outbreak has been traced back to a single patient who was believed to have been infected by a bat.
Despite the potential for zoonotic pathogens to cause global outbreaks, it is thought that many events go unnoticed and that human hosts often provide a dead-end of sorts for disease transmission. Sometimes, but not always, the genetic changes that allow a zoonotic disease to cross the species barrier render it less infective or increase the virulence so much that initial infection and sustained human-to-human transmission cannot occur. If humans show no sign of infection and/or cannot transmit the disease, the pathogen will burn out in its new host. The worry is that a highly pathogenic and virulent strain will emerge that is capable of sustained transmission, in which case international travel and trade provide favorable conditions for the disease to quickly spread across the globe.
The relationship between animals, the ecosystem, and humans is important and may be key to understanding how zoonotic diseases are spread from reservoirs to hosts and then across species. Under the One Health approach, which recognizes that the health of one component affects the health of another, scientists from multiple disciplines have increased their collaboration to strengthen disease detection and response in both human and animal populations. The One Health community includes public health professionals, epidemiologists, veterinarians, and environmental scientists, among others, who work together to better understand how population, individual, and environmental health affects the prevalence of zoonotic infections.
Collaborative disease surveillance efforts have shown that increases in population density and subsequent human-wildlife interactions create optimum conditions for spillover to occur. There are specific geographic areas where wildlife and humans are in close contact and where ecological niches are favorable to facilitate disease emergence and transmission. These areas, termed “global hotspots”, include parts of developing countries such as tropical Africa and Southeast Asia. While not the only areas where emerging diseases may occur (the 2009 H1N1 outbreak began in Mexico), these hotspots remain a focus for disease surveillance efforts. Environmental changes within these geographic regions (e.g., deforestation, rainfall patterns, changes in farming practices) allow some disease vectors to flourish and enable humans to encroach on previously uninhabited areas, bringing wildlife and humans even closer together and providing a greater statistical chance that a disease will pass from one species to another. Zoonotic outbreaks are usually, though not always, first seen in animals. Animals may get sick before any human infections occur. These sentinel events can serve as important warning signs that conditions are favorable for spillover to occur
In an effort to better understand the dynamics of what occurs at the animal-human interface, scientists have developed specific modeling techniques that examine the spatial and temporal patterns of disease. Climate models are also useful in predicting variables such as temperature and rainfall that affect the ability of certain vectors to flourish. But because disease processes aren’t always well understood, and real-world systems are hard to duplicate, the utility of models as a discrete approach to understanding and anticipating spillover events is limited. However, when combined with advanced diagnostic tools and the ever-increasing communication between the animal, environmental, and human health communities, results from these models contribute valuable information and help strengthen the One Health approach.
Infection from many zoonotic diseases can be avoided by following appropriate safety precautions. Maintaining awareness of and educating communities on the public health implications of human-wildlife interactions is an important first step preventing zoonotic outbreaks. As our understanding of disease processes advances, so too will our ability to accurately identify and hopefully predict future spillover events.