by Isadora Rivera-Janer | Edited by Nannette Beckley
Zoonotic diseases have become some of the most pressing issues in current events, with diseases such as monkeypox and Covid-19 emerging to be extremely widespread and devastating. Though zoonotic diseases are not at all a new phenomenon, much of the concern happens to be that we are entering an era where zoonotic diseases are becoming much more frequent, infectious events. It is estimated that worldwide, zoonoses cause 2.5 billion cases of human illness and 2.7 million human deaths yearly.
A zoonotic disease is a pathogen that can be transmitted from human to animal or from animal to human. About 61% of emerging human pathogens are zoonoses, so they are extremely common. Zoonotic diseases can arise through several vectors of infection, such as bacterial zoonoses, viral zoonoses, parasitic zoonoses, and fungal zoonoses and can arise from any interactions between humans and animals, such as livestock, pets, aquatic animals, as well as from the consumption of animals. Therefore, zoonotic diseases are not only important to understand because they are remarkably prevalent, but also because they are being fueled by the relatively recent breakdown of barriers between humans and nature.
One factor is urbanization, with almost half of human populations living in cities and is on track to increase in the next decades. Animals such as mammals and birds, also are able to live in high densities within urban centers while simultaneously being some of the animals most likely to be virus reservoirs (sources) of infectious zoonotic diseases. Urbanization then creates a perfect storm for the spread of zoonotic diseases, with less opportunity for containment measures of emerging infectious disease, as well as increasing the proximity of animals and humans. The fact that many of the animals that live in close contact with humans are mammals is also important to note, as that means that viruses that live within mammals have to navigate much less difficulties in transmission to humans, as well as giving these viruses an opportunity to learn how to more efficiently infect us. Specifically, a disease such as COVID foreshadows the potential for “generalist” coronaviruses that can infect many different mammalian species.
Anthropogenic land-use changes also can contribute to the increase of zoonotic diseases. Rises in zoonotic diseases all stem from expansion in geographical, host or vector range that result in high rates of human-animal contact. Land use and agricultural expansion disrupt natural ecosystems, and this not only puts humans in contact with new pathogens, but can also promote practices among animal husbandry that make transmission within livestock extremely common. Domestic animals can function as intermediate hosts of diseases such as tuberculosis and anthrax from the environment through consumption. Wild animals are largely affected by how land use irrevocably affects their habitats by decreasing environmental services, space, and resources. Many animals deal with this by coming into human residences to supplement themselves. This drives pathogen evolution and exposure by increasing interactions of both humans and animals with unfamiliar environmental factors.
Climate change is also suggested to affect the incidence of zoonotic disease. Many animals will have to negotiate new environmental borders, shifting their ranges. This means that as animals may move to places where they were formerly absent they will spread the diseases associated with them to areas unaccustomed to those pathogens. The rising temperatures also may affect population sizes through which will increase the rates of zoonotic disease as animals used to temperate climates will have higher survival rates or generalists will move into places where animals who could not adapt have died out, increasing the size and range of animal populations. Humans will most likely also be navigating new environmental boundaries, for as the environment starts to change due climatic factors, people may begin to encroach on formerly pristin areas, increasing the exposure to previously unknown pathogens. Many of the solutions suggested promote more surveillance of the animal-human interface, with more resources invested into research of new infectious events, treatment, and vaccinations. We should also invest more into studying and regulating human-animal contact, of understanding which species are reservoirs or vectors for certain diseases and how to decrease the possibility of transmission. Learning from the monkeypox epidemic, even though some diseases may initially only be endemic in relatively few areas, many zoonotic diseases have the possibility of becoming global outbreaks that disrupt people’s health and lives, and should be taken very seriously.