Zoonotic Diseases

Zoonotic diseases are among the most important health challenges at the intersection of animals, humans, and the environment. These diseases, commonly referred to as zoonoses, are infections that can be naturally transmitted between vertebrate animals and people. Throughout history, zoonotic pathogens have influenced public health, agriculture, wildlife conservation, global trade, and food production. Today, zoonotic diseases remain a major focus of veterinary medicine, human medicine, epidemiology, and public health because many emerging infectious diseases originate in animal populations before spreading to humans (Kahn, 2006; Karesh et al., 2012).

The relationship between humans and animals has become increasingly complex. Companion animals share homes with people, livestock production systems support growing populations, and human activities continue to alter wildlife habitats. These interactions create opportunities for pathogen transmission across species boundaries and contribute to the emergence of new infectious threats (Plowright et al., 2017; Esposito et al., 2023).

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Understanding zoonotic diseases requires more than studying individual pathogens. It involves examining disease ecology, environmental influences, animal reservoirs, transmission pathways, surveillance systems, and prevention strategies. Modern approaches increasingly recognize that human, animal, and environmental health are interconnected, forming the foundation of the One Health framework (Shaheen, 2022; Erkyihun & Alemayehu, 2022).

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As a major pillar of the CountryVetMom Veterinary Knowledge System, this page serves as a central educational hub that introduces the science of zoonoses and connects readers to more specialized topics related to companion animals, livestock, wildlife, and public health.

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What This Major Pillar Covers

 

This major pillar explores the science, ecology, and public health significance of zoonotic diseases. Topics include:

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• Definitions and classifications of zoonoses

• Pathogen transmission between animals and humans

• Companion animal zoonoses

• Livestock-associated zoonotic diseases

• Wildlife reservoirs and spillover events

• Emerging infectious diseases

• Disease surveillance and monitoring

• One Health approaches to prevention and control

• Global public health implications

 

This pillar also serves as the parent hub for the following educational resources:

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Understanding Zoonoses

 

Understanding the mechanisms of pathogen transmission is fundamental to zoonotic disease science. Learn more in our guide to Understanding Zoonoses.

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Companion Animal Zoonoses

 

Companion animals play an increasingly important role in human society and are a key component of zoonotic disease ecology. Learn more in Companion Animal Zoonoses.

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Livestock and Wildlife Zoonoses

 

Agricultural systems and wildlife populations play critical roles in the emergence, maintenance, and spread of zoonotic pathogens. Learn more in Livestock and Wildlife Zoonoses.

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Why This Area Matters

 

Zoonotic diseases affect millions of people and animals worldwide every year. Their impacts extend beyond individual illness and can influence food security, agricultural productivity, biodiversity conservation, economic stability, and healthcare systems (Rahman et al., 2020; Belay et al., 2017).

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Many well-known infectious diseases have zoonotic origins. Examples include rabies, salmonellosis, leptospirosis, avian influenza, brucellosis, toxoplasmosis, and numerous emerging viral diseases. Research suggests that approximately 60% of known human infectious diseases are zoonotic, and up to 75% of emerging infectious diseases originate from animal populations (Karesh et al., 2012; Rahman et al., 2020).

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Several global trends continue to increase zoonotic disease risks:

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• Urban expansion into wildlife habitats

• Global movement of people and animals

• Intensification of livestock production

• Climate change

• Environmental degradation

• Changes in biodiversity

• Increased human-animal contact

 

These factors can alter disease ecology and create new opportunities for pathogen spillover between species (Johnson et al., 2020; Meurens et al., 2021).

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Because zoonotic diseases exist at the intersection of veterinary medicine, human medicine, and environmental science, they are among the clearest examples of why interdisciplinary collaboration is essential for protecting health at local, national, and global scales.

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How This Major Pillar Relates to One Health & Public Health

 

This pillar is part of the broader One Health & Public Health Overview knowledge system.

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The One Health approach recognizes that human health, animal health, and environmental health are interconnected. Rather than viewing diseases through a single-species lens, One Health promotes collaborative efforts among veterinarians, physicians, epidemiologists, ecologists, public health officials, and policymakers to address shared health challenges (Kahn, 2006; Shaheen, 2022).

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Zoonotic diseases provide one of the strongest justifications for the One Health model because pathogen transmission frequently occurs across species and environmental boundaries. Effective surveillance, risk assessment, outbreak response, and prevention strategies often require coordination across multiple sectors (Desvars-Larrive et al., 2024; Erkyihun & Alemayehu, 2022).

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Public health agencies increasingly emphasize:

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• Integrated disease surveillance

• Early detection systems

• Veterinary public health programs

• Wildlife monitoring

• Food safety initiatives

• Biosecurity measures

• International collaboration

 

These efforts contribute to global health security and help reduce the impact of emerging infectious diseases (Belay et al., 2017; Bowsher et al., 2021).

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Key Concepts Within This Pillar

 

Several foundational concepts appear throughout zoonotic disease research and public health practice.

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Reservoir Hosts

 

A reservoir host is an animal species that maintains a pathogen in nature and serves as a source of infection for other animals or humans. Wildlife species frequently act as reservoirs for emerging pathogens (Karesh et al., 2012).

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Spillover Events

 

Spillover occurs when a pathogen successfully crosses species barriers and infects a new host species. Spillover events are central to the emergence of many zoonotic diseases (Plowright et al., 2017).

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Transmission Pathways

 

Pathogens can move between animals and humans through:

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• Direct contact

• Indirect environmental exposure

• Foodborne transmission

• Waterborne transmission

• Aerosol transmission

• Vector-borne transmission

 

(Lloyd-Smith et al., 2009; Guo & Jin, 2026)

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Emerging Infectious Diseases

 

Emerging infectious diseases are diseases that are newly recognized, newly evolved, or increasing in incidence or geographic range. Many emerging diseases have zoonotic origins (Haider et al., 2020).

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Disease Surveillance

 

Surveillance systems monitor animal and human populations to identify disease trends, detect outbreaks, and inform public health interventions (Qiu et al., 2023).

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One Health Collaboration

 

One Health emphasizes interdisciplinary cooperation among professionals working in veterinary medicine, human healthcare, environmental sciences, wildlife biology, agriculture, and public health (Sharma et al., 2023).

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Understanding Zoonoses

 

Zoonoses are infectious diseases that are naturally transmitted between animals and humans. The term encompasses a diverse group of bacterial, viral, fungal, and parasitic diseases—as well as unconventional agents like prions—that can cross species barriers (Bauerfeind et al., 2015; Zucca et al., 2022).

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The relationship between humans and animals has shaped disease transmission for thousands of years. Domestication of livestock, urbanization, agricultural development, wildlife trade, habitat modification, and global travel have all influenced how zoonotic pathogens emerge and spread (Recht et al., 2020; Bendrey & Martin, 2021).

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Zoonotic diseases are commonly classified according to their causative agents:

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Bacterial Zoonoses

 

Examples include:

• Salmonellosis

• Campylobacteriosis

• Brucellosis

• Leptospirosis

 

These diseases remain important causes of foodborne illness, occupational exposure, and environmental transmission worldwide (Chlebicz & Śliżewska, 2018; Asante et al., 2019).

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Viral Zoonoses

 

Viruses represent some of the most significant emerging zoonotic threats. Examples include influenza viruses, rabies virus, coronaviruses, and numerous wildlife-associated viral pathogens (Tomori & Oluwayelu, 2023; Kibenge, 2023).

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Parasitic Zoonoses

 

Protozoa and helminths continue to contribute substantially to the burden of zoonotic diseases worldwide. Companion animals, livestock, and wildlife may all participate in transmission cycles (Otranto et al., 2021; Baneth et al., 2016).

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Fungal Zoonoses

 

Although less frequently discussed than bacterial or viral diseases, fungal pathogens are increasingly recognized as important components of One Health research (Kundu et al., 2024).

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The ecology of zoonotic disease transmission is rarely simple. Multiple animal hosts, environmental reservoirs, vectors, and human activities often interact to influence pathogen movement across populations. Understanding these complex relationships remains a central goal of modern zoonotic disease research (Lo Iacono et al., 2016; Desvars-Larrive et al., 2024).

 

Companion Animal Zoonoses

 

Companion animals occupy a unique position within zoonotic disease ecology because they live in close association with humans. Dogs, cats, birds, rabbits, reptiles, small mammals, and other household pets provide companionship and contribute positively to human well-being. However, close human-animal interactions also create opportunities for pathogen transmission across species boundaries (O’Neil, 2018; Gamble et al., 2023).

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The vast majority of pet ownership experiences do not result in zoonotic disease transmission. Nevertheless, companion animals remain important participants in the epidemiology of bacterial, viral, fungal, and parasitic infections that affect both animal and human populations (Basit et al., 2024).

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The Human–Companion Animal Interface

 

Modern companion animals often share homes, furniture, recreational spaces, and daily routines with people. These close interactions strengthen human-animal bonds but also increase opportunities for exposure to infectious agents through direct contact, environmental contamination, bites, scratches, saliva, feces, urine, and ectoparasites (Reaser et al., 2008; Warwick, 2020).

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Several factors influence zoonotic disease risks associated with companion animals:

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• Animal species

• Husbandry practices

• Environmental conditions

• Geographic location

• Human behavior

• Pathogen prevalence

• Public awareness

 

(Rijks et al., 2016)

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Common Companion Animal Zoonoses

 

Dogs and cats may participate in the transmission cycles of numerous zoonotic pathogens. Examples include:

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Bacterial Diseases

 

• Salmonellosis

• Campylobacteriosis

• Leptospirosis

• Bartonellosis

• Pasteurellosis

 

(Goni et al., 2018; Ebani & Mancianti, 2023)

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Viral Diseases

 

• Rabies

• Certain emerging viral infections

• Selected vector-associated viral pathogens (such as tick-borne encephalitis or West Nile virus)

 

(Reperant et al., 2016; Tomori & Oluwayelu, 2023)

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Parasitic Diseases

 

• Toxocariasis

• Giardiasis

• Cryptosporidiosis

• Toxoplasmosis

• Echinococcosis

 

(Baneth et al., 2016; Pal et al., 2023)

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Fungal Diseases

 

Certain dermatophytes and other fungal organisms can be transmitted between animals and people, particularly through close contact or contaminated environments (Ebani & Mancianti, 2023).

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Beyond Dogs and Cats

 

Although dogs and cats receive the greatest attention in companion animal zoonosis research, numerous other household species may also carry zoonotic pathogens.

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Birds, reptiles, amphibians, rabbits, ferrets, rodents, and exotic companion animals contribute to the diversity of potential zoonotic exposures in domestic environments (Bursee, 2023; Qureshi et al., 2024).

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The growing popularity of unconventional pets has expanded scientific interest in species-specific disease risks and surveillance needs.

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Vector-Borne Companion Animal Diseases

 

Many companion animals serve as hosts for vectors such as fleas, ticks, mosquitoes, and sandflies. These vectors can facilitate pathogen transmission among animals and, in some situations, contribute to human exposure.

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Vector-borne diseases represent an increasingly important research area because changing climates, environmental conditions, and vector distributions can alter disease patterns over time (Maggi & Krämer, 2019; Chacón, 2023).

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Companion Animals Within One Health

 

Companion animals occupy a central position within One Health because they bridge human, animal, and environmental systems. Surveillance programs increasingly incorporate companion animal populations into broader zoonotic disease monitoring efforts (Bayko et al., 2023; Waugh & Mullaney, 2019).

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Recent research also highlights the importance of monitoring companion animal infectomes, pathogen diversity, and emerging infectious disease risks associated with pets and peri-domestic wildlife populations (Wu et al., 2024; Grieve et al., 2025).

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Companion animals therefore represent more than individual pets. They are important participants in disease ecology, public health surveillance, and the broader One Health framework.

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Livestock and Wildlife Zoonoses

 

Livestock and wildlife populations play critical roles in the emergence, maintenance, and transmission of zoonotic pathogens. Many zoonoses originate or persist within animal populations before infecting humans through direct contact, environmental exposure, food systems, vectors, or spillover events.

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Understanding livestock and wildlife zoonoses requires examining agricultural systems, ecosystem dynamics, wildlife reservoirs, environmental change, and human-animal interactions simultaneously (Karesh et al., 2012; Meurens et al., 2021).

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Livestock as Reservoirs and Amplifiers

 

Cattle, pigs, poultry, sheep, goats, and other production animals support global food systems while also serving as hosts for numerous zoonotic pathogens.

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Examples of important livestock-associated zoonoses include:

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• Brucellosis

• Salmonellosis

• Campylobacteriosis

• Avian influenza

• Q fever

• Leptospirosis

• Tuberculosis

• Cryptosporidiosis

 

(Libera et al., 2022; Klous et al., 2016)

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Livestock-associated zoonoses influence both animal health and public health. In addition to human disease burdens, they can affect food production, international trade, animal welfare, and agricultural economics (Rahman et al., 2020).

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Agricultural Systems and Disease Ecology

 

Modern livestock production systems create complex epidemiological environments. Population density, animal movement, environmental conditions, feed systems, water sources, and management practices all influence pathogen transmission dynamics.

Research continues to examine how concentrated animal production systems, farming practices, and environmental conditions affect the emergence and spread of zoonotic diseases (Guo et al., 2022; Majiwa et al., 2024).

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Agricultural biosecurity has therefore become a major component of zoonotic disease prevention and disease surveillance efforts worldwide.

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Wildlife Reservoirs and Spillover Events

 

Wildlife species serve as natural reservoirs for numerous pathogens with zoonotic potential. Bats, rodents, primates, carnivores, birds, and many other wildlife taxa contribute to the ecology of emerging infectious diseases (Johnson et al., 2020; Thompson, 2013).

Wildlife reservoirs often maintain pathogens without showing severe clinical disease, allowing infectious agents to persist within ecosystems for extended periods.

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Under certain ecological conditions, pathogens may cross species barriers through a process known as spillover. Spillover events represent a critical stage in the emergence of many zoonotic diseases (Plowright et al., 2017).

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Human Activities and Emerging Zoonoses

 

Research increasingly demonstrates that human-driven environmental changes can influence the emergence of zoonotic diseases.

Important factors include:

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• Deforestation

• Urban expansion

• Agricultural encroachment

• Habitat fragmentation

• Wildlife trade

• Climate change

• Biodiversity loss

 

These activities can increase contact among wildlife, livestock, companion animals, and people, creating opportunities for pathogen transmission (Esposito et al., 2023; Galindo-González, 2023).

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The movement of pathogens from forests and natural ecosystems into agricultural and urban environments remains a major focus of disease ecology research (Latif et al., 2023).

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Wildlife–Livestock Interfaces

 

Interactions between wildlife and livestock populations have become an important area of scientific investigation.

These interfaces may facilitate:

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• Pathogen maintenance

• Cross-species transmission and bi-directional spillover 

• Emergence of novel diseases

• Regional disease persistence

 

Research suggests that wildlife-livestock interactions are among the most significant pathways for the emergence of future zoonotic diseases (Meurens et al., 2021; Johnson et al., 2020).

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Global Public Health Implications

 

Many recent emerging infectious diseases have highlighted the importance of understanding zoonotic disease ecology across wildlife, livestock, and human populations.

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Global surveillance initiatives increasingly focus on:

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• Early pathogen detection

• Wildlife monitoring

• Veterinary intelligence

• Integrated surveillance systems

• Pandemic preparedness

• One Health collaboration

 

(Belay et al., 2017; Bowsher et al., 2021)

 

The study of livestock and wildlife zoonoses therefore extends beyond agriculture or conservation alone. It represents a critical component of global health security, disease prevention, and public health preparedness.

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Current Research Themes

 

Zoonotic disease research continues to evolve as scientists investigate emerging pathogens, environmental drivers of disease transmission, and strategies that strengthen global health security. Modern research increasingly recognizes that zoonoses are not isolated biological events but complex ecological processes involving humans, animals, and the environment.

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One Health Integration

 

One of the most influential research themes is the continued development of the One Health framework. Researchers are exploring methods to integrate veterinary medicine, human medicine, wildlife biology, environmental science, and public health into coordinated disease surveillance and response systems (Desvars-Larrive et al., 2024; Erkyihun & Alemayehu, 2022).

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Emerging Infectious Diseases

 

Scientists continue to investigate the ecological and biological factors that contribute to disease emergence. Research focuses on identifying high-risk interfaces where wildlife, livestock, companion animals, and humans interact and where pathogen spillover is most likely to occur (Plowright et al., 2017; Johnson et al., 2020).

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Wildlife Surveillance

 

Wildlife monitoring has become a major component of zoonotic disease preparedness. Researchers are studying wildlife reservoirs, pathogen diversity, migration patterns, and ecosystem changes that may influence future disease emergence (Kibenge, 2023; Thompson, 2013).

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Companion Animal Surveillance

 

Companion animal populations are increasingly recognized as valuable components of disease surveillance systems. Current research explores infectious disease monitoring in pets, pathogen diversity within companion animal populations, and the potential role of these animals as sentinels for emerging zoonotic threats (Bayko et al., 2023; Grieve et al., 2025).

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Climate Change and Disease Ecology

 

Climate-sensitive zoonotic diseases represent an expanding area of research. Scientists are evaluating how changes in temperature, rainfall patterns, habitat shifts, and vector distributions influence disease transmission dynamics (Hobson et al., 2025; Rees et al., 2021).

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Antimicrobial Resistance

 

The emergence of antimicrobial-resistant pathogens in animal and human populations poses a significant challenge to the veterinary and public health sectors. Researchers continue to examine how antimicrobial resistance develops, spreads, and affects zoonotic disease risks across species (Asante et al., 2019; Gupta et al., 2024).

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Reverse Zoonoses and Bidirectional Transmission

 

While zoonoses traditionally focus on animal-to-human transmission, researchers are increasingly studying reverse zoonoses (zooanthroponoses), in which humans transmit pathogens to animals. This bidirectional perspective strengthens understanding of disease ecology and One Health interactions (Messenger et al., 2014; Noman et al., 2024).

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Frequently Asked Questions

 

What are zoonotic diseases?

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Zoonotic diseases, or zoonoses, are infectious diseases that can be naturally transmitted between animals and humans. They may be caused by bacteria, viruses, parasites, fungi, or other infectious agents (Zucca et al., 2022).

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How are zoonotic diseases transmitted?

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Transmission can occur through direct contact with animals, environmental exposure, contaminated food or water, vectors such as ticks and mosquitoes, aerosols, or contact with contaminated animal products (Lloyd-Smith et al., 2009; Guo & Jin, 2026).

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Are all animal diseases zoonotic?

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No. Many animal diseases affect only animals and cannot be transmitted to humans. A disease is considered zoonotic only when natural transmission between animals and humans is possible (Bauerfeind et al., 2015).

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Why are zoonotic diseases important to public health?

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Zoonotic diseases influence human health, animal health, food security, economic stability, and global health preparedness. Many emerging infectious diseases originate in animal populations before affecting humans (Belay et al., 2017; Rahman et al., 2020).

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What is a spillover event?

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A spillover event occurs when a pathogen successfully crosses from one host species into another. Spillover events play a critical role in the emergence of many zoonotic diseases (Plowright et al., 2017).

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What role do companion animals play in the transmission of zoonotic diseases?

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Companion animals can participate in the transmission cycles of certain bacterial, viral, fungal, and parasitic pathogens. They are also increasingly included in disease surveillance and One Health research programs (O’Neil, 2018; Bayko et al., 2023).

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Why is wildlife important in zoonotic disease research?

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Wildlife populations serve as reservoirs for numerous pathogens with zoonotic potential. Understanding wildlife disease ecology helps researchers identify and monitor emerging infectious disease threats (Johnson et al., 2020; Thompson, 2013).

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What is the One Health approach?

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One Health is a collaborative framework that recognizes the interconnected nature of human, animal, and environmental health. It promotes interdisciplinary approaches to addressing shared health challenges, including zoonotic diseases (Shaheen, 2022).

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Explore Related Topics

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Continue exploring the One Health & Public Health Veterinary Knowledge System:

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Understanding Zoonoses

 

Explore disease classifications, transmission pathways, pathogen ecology, reservoirs of infection, and spillover mechanisms in Understanding Zoonoses.

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Companion Animal Zoonoses

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Learn about zoonotic diseases associated with dogs, cats, birds, rabbits, reptiles, and other household pets in Companion Animal Zoonoses.

 

Livestock and Wildlife Zoonoses

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Examine the ecology of zoonotic diseases at the livestock–wildlife–human interface in Livestock and Wildlife Zoonoses.

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One Health & Public Health Overview

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Return to the parent system page: One Health & Public Health Overview.