Infectious diseases constitute one of the leading causes of death globally, killing over 6 million people per year. These diseases are caused by bacterial, fungal, parasitic, and viral pathogens, which can be transmitted between humans, through vectors, or from animals. Over 60% of identified human pathogens are zoonoses, diseases transmitted between animals and humans, highlighting the role of wildlife in infectious diseases and outbreaks.
Both wildlife and domesticated animals serve as reservoirs for pathogens. Reservoirs are characterized by their ability to maintain the pathogen within a population, can consist of multiple host species, and do not necessarily include the natural host. Continued propagation of the pathogen drives selective evolution, introducing genetic variations that increase diversity while fine-tuning transmissibility and pathogenicity in their hosts. Importantly, this evolution supports host switching, the transmission and adaptation of a pathogen to a new host species. Host switching is critical to development of zoonoses and is facilitated by frequent direct contact between the host species and potential hosts. Influenza A viruses were originally endemic in waterfowl, but close interactions of these birds with domesticated animals and humans enabled host switching to humans. Circulation in human populations has led to pandemics, such as the H5N1 outbreak in 1997.
Given that close interaction between humans and wildlife plays a role in zoonoses, what factors contribute to disease emergence? One of the key drivers in the spread of infectious disease is the translocation of pathogens into new geographical locations. Armed with the ability of flight, birds support the worldwide dispersal of pathogens through their migratory activities. Some outbreaks of West Nile Virus in Europe have been attributed to the movement of infected birds. Similarly, bats contribute to pathogen spread and are hosts to a myriad of viruses. Their long lifespans, high population densities, and close proximity to human and domestic animals make them ideal reservoirs. Nipah virus, a pathogen that circulates among fruit bats, was identified during the 1998 outbreak in Malaysia, with a fatality rate of over 30% of reported cases. Direct contact with bats or infected pigs was determined to be the predominant method of transmission. On the other hand, the global invasion of rodents has been facilitated by intercontinental travel and urbanization. Known to carry over 60 zoonotic diseases, rodents spread these diseases directly through contact, bites, or excretions, such as feces, urine or saliva, or indirectly through vectors. As an example, rats play a crucial role in spread of Lassa fever, an endemic illness in West Africa, resulting in more than 5000 deaths per year.
Human behaviour also has profound impact on the spread of disease through direct influence on animal reservoirs. Encroachment into wildlife habitats, wildlife trade, and animal domestication cause ecological changes that increase wildlife-human contact and facilitates pathogen adaptation to humans. For instance, the re-emergence of Lyme disease in the United States has been attributed to increased human contact with deer, deer mice, and ticks as a result of suburbanization. Other human activities such as global trade and travel supports artificial introductions and translocations of pathogens, vectors, and reservoir hosts. Bovine tuberculosis was introduced to South Africa through importation of cattle from Europe and has since become endemic in the local wildlife. By-products of human activity also affect disease spread. Plastic waste serves as breeding habitats for mosquitoes, vectors of many infectious diseases, and has been shown in some studies to support the spread of dengue fever worldwide. Moreover, research has revealed that climate change contributes to outbreaks and impacts epidemiology of infectious diseases. Notably, the migration and abundance of mosquitoes are affected by precipitation levels and temperature fluctuations. These climate changes have been attributed in part to human activity, such as industrialization, and play a pivotal role in outbreaks of Rift Valley fever, bluetongue virus, and tick-borne encephalitis.
The prevalence of zoonoses is a significant burden on healthcare systems and a threat to global health security. In particular, the unpredictable emergence of novel pathogens results in rapid spread and poses difficult challenges due to high morbidity, lack of specific treatment options, and slow global mobilization of infection control measures. Identification and constant surveillance of animal reservoirs and understanding patterns of disease emergence will be critical for predicting and responding to future outbreaks. Other control measures include development and worldwide distribution of vaccines and treatments for known pathogens. Increasing global awareness of zoonoses and the contribution of human behaviour to the emergence of these diseases will be valuable to global infection control.
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