Zoonotic Diseases

Zoonotic diseases, or zoonoses, are infections that are naturally transmitted between vertebrate animals and humans. They span bacterial, viral, parasitic, and fungal agents and include many of the infections that spill over from animal reservoirs to cause emerging epidemics. Understanding zoonoses links human infectious-disease epidemiology to the ecology of animal hosts and the conditions that favour cross-species transmission.

Definition

A zoonosis is an infection or infectious disease that is naturally transmissible between vertebrate animals and humans, the animal serving as a reservoir or source of the pathogen.

Scope

This entry covers what defines a zoonosis, the reservoirs and spillover pathways through which animal pathogens reach humans, and why zoonoses account for a large share of emerging human infections. It is a reference overview connecting transmission ecology to human disease and does not provide management guidance for any individual zoonotic infection.

Core questions

• What distinguishes a zoonosis from a human-adapted infection?
• Through which reservoirs and routes do animal pathogens spill over into humans?
• Why do zoonoses, and especially wildlife-origin pathogens, dominate emerging infections?
• What ecological and human factors raise the risk of cross-species transmission?

Key concepts

• Reservoir host
• Spillover (cross-species transmission)
• Emerging and re-emerging infections
• Wildlife and livestock reservoirs
• Vector-borne zoonoses
• One Health
• Pandemic potential

Key theories

Zoonotic spillover and pathogen emergence

Most emerging human infections originate in animals, with wildlife an important reservoir; emergence is driven by ecological and human-activity factors that bring people into contact with animal pathogens and allow cross-species transmission.

Mechanisms

A zoonotic pathogen maintained in an animal reservoir reaches humans through direct contact, contaminated food or water, environmental exposure, or an arthropod vector. Whether such spillover leads to onward human-to-human spread depends on the pathogen's biology and on opportunities for transmission. Surveys of human pathogens show that a majority are zoonotic and that zoonotic agents are disproportionately represented among emerging infections, with wildlife a prominent source (Taylor, 2001; Jones, 2008; Daszak, 2000). Ecological disruption, land-use change, animal trade, and intensified human-animal contact increase the frequency of spillover, motivating surveillance strategies aimed at anticipating the next pandemic zoonosis (Morse, 2012).

Clinical relevance

Recognising that an infection may be zoonotic helps situate it within its animal reservoir and exposure context and explains why surveillance, animal-health measures, and One Health approaches are part of control. This entry describes the category and ecology of zoonoses in general terms and is not a basis for diagnosing or treating any specific zoonotic infection.

Epidemiology

Analyses of emerging infectious-disease events have found that the majority arise from zoonotic pathogens and that a large share of these originate in wildlife, with emergence events concentrated where ecological and socioeconomic drivers increase human-animal contact (Jones, 2008; Taylor, 2001). High-consequence outbreaks of recent decades — including emerging coronaviruses and influenza viruses of animal origin — illustrate the pandemic potential of spillover events (Daszak, 2000; Morse, 2012).

History

The recognition that many human infections are shared with animals is long-standing, but the modern, quantitative view of zoonoses crystallised around 2000, when systematic surveys established that most human pathogens and a still larger share of emerging infections are zoonotic. Subsequent work linked emergence to wildlife reservoirs and ecological change and framed prediction and prevention of the next pandemic zoonosis as a global priority, contributing to the One Health perspective that integrates human, animal, and environmental health.

Debates

Can the next pandemic zoonosis be predicted and pre-empted?

Proposals to forecast spillover by surveilling wildlife and high-risk interfaces are promising but contested in terms of feasibility, cost, and predictive accuracy, given the diversity of potential pathogens and the complexity of emergence.

Key figures

• Mark Woolhouse
• Kate Jones
• Peter Daszak
• Stephen Morse
• Louise Taylor

Seminal works

• taylor-2001
• jones-2008
• daszak-2000

Frequently asked questions

What makes a disease a zoonosis?

A zoonosis is an infection that is naturally transmissible between vertebrate animals and humans, with an animal acting as the reservoir or source of the pathogen.

Why are zoonoses important for pandemic preparedness?

Most emerging human infections originate in animals, and many of the largest recent outbreaks began as spillover from animal reservoirs, so monitoring zoonotic pathogens is central to anticipating new epidemics.