Viral Zoonoses: Spillover, One Health & Emerging Threats

Hazara University Mansehra | Department of Zoology

Viral Zoonoses: Spillover, One Health, and Emerging Threats

Shahab Uddin

Dr. Hamid Ur Rahman

Department of Zoology — Hazara University Mansehra

18 June 2026

Human–Animal–Environment Spillover

Wildlife → Livestock → Humans

Environment and land-use change shape risk

Detection → Prevention → Response

Based on WHO One Health Framework & CDC Zoonotic Exposure Categories

02

Introduction

Why Viral Zoonoses Matter

Zoonoses are infectious diseases naturally transmissible from vertebrate animals to humans.

More than 60% of emerging infectious diseases reported globally come from animals, both wild and domestic.

Human activities and stressed ecosystems increase spillover risk — animal trade, agriculture, urbanization, climate change, habitat fragmentation.

Viral zoonoses can cause recurrent outbreaks, severe epidemics, and pandemics. Examples: Ebola, avian influenza, rabies, and COVID-19.

Why this topic matters

Emerging risk

Outbreak potential

Pandemic potential

One Health relevance

Source: WHO One Health fact sheet · WHO Zoonoses fact sheet

Foundations

03

Definition and Scope

<b>A zoonosis</b> is any disease or infection naturally transmissible from vertebrate animals to humans.

<b>Zoonotic pathogens</b> may be bacterial, viral, parasitic, or involve unconventional agents; they can spread by direct contact, food, water, or the environment.

<b>Viral zoonoses</b> are the subset caused by viruses — transmission shaped by animal contact, bites/scratches, contaminated environments, vectors, or food systems.

<b>This presentation</b> focuses on viral zoonoses relevant for outbreak preparedness, clinical recognition, and One Health surveillance.

Animal Reservoir

Human Exposure

Infection

Possible Human Spread

Zoonosis → Viral Zoonosis

Source: WHO Zoonoses fact sheet · CDC Yellow Book

One Health Framework

04

Human Health

Disease surveillance

Clinical detection

Outbreak response

Human<br>Health

Animal<br>Health

Ecosystem<br>Health

ONE<br>HEALTH

Spillover

Amplification

Control

Animal Health

Veterinary surveillance

Livestock biosecurity

Wildlife monitoring

Integrated approach balancing health of people, animals, and ecosystems

Human, domestic animal, wild animal, plant, and environmental health are interdependent

Supports prevention, detection, preparedness, response, and management

Spillover risk rises when the human–animal–environment interface becomes unstable

Source: WHO One Health · OHHLEP spillover prevention framework

Drivers of Spillover

Main anthropogenic factors behind emerging viral zoonoses

05

nature

Deforestation

Habitat loss brings humans closer to wildlife reservoirs.

pets

Wildlife Trade

Cross-species contact increases at markets and transit points.

agriculture

Livestock Intensification

Dense farming amplifies human–animal interfaces.

location_city

Urbanization

Expanding settlements encroach on wild habitats.

thermostat

Climate Change

Shifts vector ranges and alters reservoir ecology.

flight

Global Travel & Trade

Rapid spread of novel pathogens across regions.

Habitat fragmentation and extractive industries further reshape disease ecology

WHO One Health fact sheet · WHO/FAO/OIE joint consultation on emerging zoonoses

Roadmap of the Presentation

How viral zoonoses are organized in this talk

06

Direct Contact Zoonoses

Bite / Scratch Zoonoses

Inhalational & Respiratory Zoonoses

Vector-Borne Zoonoses

Food- & Water-Borne Zoonoses

High-Consequence Viral Families

Rabies, Orthopoxviruses, B virus

Rabies, Cowpox, B virus

Influenza A, Coronaviruses, Henipaviruses

Flaviviruses, Alphaviruses, Arboviruses

Hepatitis E and zoonotic foodborne viruses

Filoviruses, Arenaviruses, Nairoviruses, Henipaviruses, Coronaviruses

Exposure route &rarr; Virus family &rarr; Clinical syndrome &rarr; Prevention

Source: CDC Yellow Book zoonotic exposure chapter

Rabies and Lyssaviruses: The Deadliest Viral Zoonosis

Family: Rhabdoviridae | Genus: Lyssavirus

07

Reservoir & Transmission

Main reservoirs: dogs, bats, foxes, raccoons, skunks, wild carnivores

Transmission: bites, scratches, saliva on broken skin or mucosa

Dog bites/scratches account for ~99% of human rabies cases

Dog / Bat /<br>Wild Carnivore

Bite or<br>Scratch

Peripheral<br>Nerves

Brain / CNS

Fatal<br>Encephalitis

Epidemiology

Kills tens of thousands annually — concentrated in Asia and Africa

Once symptoms begin: essentially 100% fatal

One of the clearest examples: severe but preventable

Dog vaccination &nbsp;|&nbsp; Wound washing &nbsp;|&nbsp; Post-exposure prophylaxis (PEP) &nbsp;|&nbsp; High-risk group vaccination &nbsp;|&nbsp; Elimination at dog source is most sustainable

Source: WHO Rabies fact sheet · WHO Rabies prevention page

Zoonotic Influenza Viruses and Pandemic Risk

Family: Orthomyxoviridae | Segmented RNA genome

08

Reservoir & Transmission

wild aquatic birds, poultry, swine, other animal hosts

Human infection via direct/indirect contact with infected animals or contaminated environments

Segmented genome enables reassortment → novel strains

Key Subtypes & Epidemiology

H5N1, H5N6, H7N9, H9N2, swine-origin variants

mild flu-like illness to severe pneumonia, ARDS, and death

Novel influenza A = pandemic risk because humans lack immunity

Wild Birds

Pigs

Human Exposure

Novel Influenza A

Outbreak / Pandemic Risk

Reassortment creates pandemic-capable strains

Animal surveillance

Biosecurity

Rapid testing

Genomic monitoring

H5 virus surveillance remains a major public health priority

CDC Novel Influenza A surveillance · CDC Yellow Book influenza page

Zoonotic Coronaviruses: From Spillover to Epidemics

Family: Coronaviridae &nbsp;|&nbsp; Broad host range & frequent recombination

09

<strong style="color: #1B4D3E;">Natural reservoirs:</strong> bats

<strong style="color: #1B4D3E;">SARS:</strong> intermediate host — civets

<strong style="color: #1B4D3E;">MERS:</strong> intermediate host — dromedary camels

<strong style="color: #1B4D3E;">Transmission:</strong> close contact and respiratory exposure

<strong style="color: #1B4D3E;">SARS and MERS:</strong> demonstrated capacity of animal viruses to cause severe epidemics

<strong style="color: #1B4D3E;">MERS remains zoonotic:</strong> camels are primary reservoir, ongoing risk

<strong style="color: #1B4D3E;">COVID-19:</strong> clearest example of animal-to-human spillover with global consequences

Reservoir Spillover

Epidemic Amplification

<span style="color: #2B5E4A;">Prevention:</span> Wildlife surveillance &nbsp;|&nbsp; Animal interface monitoring &nbsp;|&nbsp; Respiratory infection control &nbsp;|&nbsp; Early genomic detection

WHO Coronavirus page

Henipaviruses: High-Fatality Emerging Zoonoses

Family: Paramyxoviridae | Nipah & Hendra

10

Reservoir & Transmission

Key reservoir: fruit bats (Pteropus spp.)

Spillover via contaminated fruit, date palm sap, or livestock intermediates

Human-to-human transmission documented for Nipah

No approved drugs or vaccines currently available for Nipah

Epidemiology

Outbreaks: Malaysia, Bangladesh, India, Australia

Acute encephalitis and severe respiratory disease

High mortality; neurological complications

WHO: Nipah is a priority pathogen for R&D

Fruit Bats

Contaminated fruit / date palm sap / livestock

Human: acute encephalitis + severe respiratory illness

Person-to-person spread possible

Avoid raw date palm sap | Farm biosecurity | Monitor bat–livestock interfaces | Rapid outbreak detection essential — mortality is high

Source: WHO Nipah virus · WHO henipavirus pages

Filoviruses: Ebola and Marburg Hemorrhagic Fevers

Family: Filoviridae | Severe viral hemorrhagic fever

11

Reservoir and Transmission

<strong style="color: #1B4D3E;">Ebola:</strong> linked to fruit bats; spillover via infected wildlife such as chimpanzees, gorillas, forest antelope

<strong style="color: #1B4D3E;">Marburg:</strong> natural host is Egyptian fruit bat

<strong style="color: #1B4D3E;">Human-to-human:</strong> blood and body fluids, contaminated surfaces, close contact

Fruit Bats / Wildlife

Human spillover

Body fluids contact

Household and healthcare spread

Immune dysregulation <span style="color: #D97706; margin: 0 6px;">&rarr;</span> Vascular leakage <span style="color: #D97706; margin: 0 6px;">&rarr;</span> Multi-organ failure

Epidemiology

Major outbreaks in sub-Saharan Africa

Ebola often fatal; Marburg severe and often fatal

Very high case fatality rate without early control

Requires special-pathogen containment measures

<strong style="color: #1B4D3E;">Prevention:</strong> Isolation <span style="color: #A3B5AA; margin: 0 8px;">|</span> PPE <span style="color: #A3B5AA; margin: 0 8px;">|</span> Contact tracing <span style="color: #A3B5AA; margin: 0 8px;">|</span> Safe burials <span style="color: #A3B5AA; margin: 0 8px;">|</span> Lab biosafety <span style="color: #A3B5AA; margin: 0 8px;">|</span> Vaccination where available <span style="color: #A3B5AA; margin: 0 8px;">|</span> Early supportive care improves survival

Source: WHO Ebola fact sheet &middot; WHO Marburg fact sheet

Rodent-Borne Arenaviruses and Hemorrhagic Fever

Family: Arenaviridae (Mammarenaviruses) | Lassa fever and related HF viruses

12

Reservoir & Transmission

<strong style='color:#1B4D3E;'>Main reservoirs:</strong> rodents (Mastomys rats for Lassa fever)

<strong style='color:#1B4D3E;'>Transmission:</strong> urine, feces, contaminated food, household items, aerosolized excreta

<strong style='color:#1B4D3E;'>Spread:</strong> Some mammarenaviruses allow person-to-person spread

Spillover Pathway

Mastomys Rat

Contaminated food / excreta / aerosols

Human Hemorrhagic Fever

South American HF viruses also rodent-associated.

Epidemiology

<strong style='color:#1B4D3E;'>Lassa fever:</strong> acute viral hemorrhagic illness — endemic in West Africa

South American HF viruses also rodent-associated

Often underrecognized; can be severe

<strong style='color:#1B4D3E;'>Key indicators:</strong> fever, weakness, hemorrhage, organ involvement

<strong style='color:#1B4D3E; font-weight:700;'>Prevention:</strong> Rodent control | Food protection | Environmental hygiene | Laboratory diagnosis & biosafety especially important in suspected cases

Source: WHO Lassa fever · WHO Arenaviridae R&D roadmap

Bunyavirales: Tick-, Rodent-, and Ruminant-Associated Viral Threats

CCHF | Hantavirus | Rift Valley Fever | Tick-borne Encephalitis

13

Hantaviruses:

rodent-maintained

CCHF:

tick-transmitted; livestock-associated exposure

Rift Valley fever:

linked to livestock and mosquitoes

Tick-borne encephalitis:

transmitted by infected ticks

Hantavirus

Pulmonary syndrome / HF

CCHF

Hemorrhagic fever / human-to-human possible

RVF

Fever / HF / encephalitis

CCHF:

endemic in Africa, Balkans, Middle East, Asia; CFR up to 40% (WHO)

Hantavirus:

often fatal depending on type and geography

RVF:

affects animals and humans; causes livestock losses

CCHF:

can spread human-to-human

Prevention:

Tick control | Livestock surveillance | PPE | Safe handling of animal blood/tissues | Environmental and occupational exposure are central to risk

WHO CCHF · WHO Hantavirus · WHO RVF · CDC Yellow Book tick-borne encephalitis

Flaviviruses and Other Arboviral Zoonoses

West Nile | Zika | Dengue | Yellow Fever | Japanese Encephalitis | TBE

14

Reservoir and Transmission

Viruses circulate in complex mosquito- and tick-mediated cycles with vertebrate hosts

Key vectors: Aedes, Culex mosquitoes and ticks

WHO lists dengue, yellow fever, Japanese encephalitis, West Nile, TBE as key vector-borne threats

Vertebrate Host (birds/mammals)

Mosquito / Tick Vector

Human (dead-end/amplifying host)

Fever → Encephalitis / Hemorrhage / Congenital disease

Epidemiology

West Nile, Zika, dengue, yellow fever, Japanese encephalitis, TBE

Zika during pregnancy: congenital Zika syndrome including microcephaly

Large burden of dengue globally

Climate change and vector ecology expanding risk areas

Prevention: Vector control | Personal protection | Vaccination where available | Surveillance | Climate change expanding risk zones

WHO vector-borne diseases · WHO Zika · WHO Japanese encephalitis · WHO yellow fever

Alphaviruses: Emerging Arboviral Threats

Family: Togaviridae | Chikungunya, VEE, EEE, WEE, Mayaro

15

Mosquito-borne viruses in sylvatic and urban cycles

Examples: chikungunya, Venezuelan equine encephalitis (VEE), eastern equine encephalitis (EEE), western equine encephalitis (WEE), Mayaro

Mayaro: sylvatic cycle involving non-human primates and mosquitoes

Chikungunya: major outbreak-prone mosquito-borne disease

WHO priority pathogens: chikungunya and Venezuelan equine alphavirus

Several alphaviruses have epidemic and pandemic potential

Geographic spread influenced by mosquito ecology and human movement

Prevention: Vector control | Surveillance | Expand diagnostic capacity | Geographic spread influenced by mosquito ecology and human movement

Source: WHO chikungunya · WHO Mayaro · WHO Togaviridae prioritization

Poxviruses: Mpox and Cowpox

Family: Poxviridae | Genus: Orthopoxvirus

16

Reservoir & Transmission

<strong style='color:#1B4D3E; font-weight:700;'>Mpox:</strong> zoonotic — animal-to-human via bites, scratches, hunting, handling carcasses, eating infected animals

Animal reservoir of monkeypox virus not fully known

<strong style='color:#1B4D3E; font-weight:700;'>Cowpox:</strong> associated with rodents; spillover via infected cats or direct rodent contact

Path 1: Mpox Spillover

Wild Animals<br><span style='font-size:13px; font-weight:400; opacity:0.85;'>(unknown reservoir)</span>

Human:<br>Mpox

Person-to-person<br>spread

Path 2: Cowpox Spillover

Rodents

Infected<br>cats

Human:<br>Cowpox

Clinical Presentation

Fever <span style='color: #D97706; margin: 0 8px;'>|</span> Painful rash <span style='color: #D97706; margin: 0 8px;'>|</span> Lymphadenopathy <span style='color: #D97706; margin: 0 8px;'>|</span> Skin lesions

Epidemiology

<strong style='color:#1B4D3E; font-weight:700;'>Mpox:</strong> outbreaks beyond traditional endemic regions

Now a globally recognized One Health concern

<strong style='color:#1B4D3E; font-weight:700;'>Cowpox:</strong> rodent exposure, occasional human infections

Some orthopoxviruses spread person-to-person after zoonotic spillover

<strong style='color:#1B4D3E; font-weight:700;'>Prevention:</strong> Avoid animal exposure <span style='color: #B5CFC1; margin: 0 8px;'>|</span> Outbreak isolation <span style='color: #B5CFC1; margin: 0 8px;'>|</span> PPE <span style='color: #B5CFC1; margin: 0 8px;'>|</span> Vaccination in appropriate settings<br><strong style='color:#1B4D3E; font-weight:700;'>Mpox:</strong> globally recognized One Health concern

Source: WHO mpox · CDC Yellow Book zoonoses

High-Value Zoonotic Viruses Often Missed in Reviews

Hepatitis E Virus (HEV) and Herpesvirus B (B virus/Macacine herpesvirus 1)

17

Hepatitis E Virus — Zoonotic Genotypes 3 and 4

Pigs, wild boar, deer, rabbits

Undercooked meat (especially pork products)<br>Contaminated food and water

Pig / Wild Boar

Undercooked Meat / Food

Human: Hepatitis

Hepatitis; can occasionally become severe

HEV genotype 3 especially relevant for zoonotic foodborne transmission

B Virus — Macacine Herpesvirus 1

Macaques (Macaca species)

Bites, scratches, body fluid contact from macaques

Macaque

Bite / Scratch / Fluid

Human: Encephalitis / Death

Rare but can cause severe brain damage or death if untreated

Occupational risk for researchers, animal handlers working with macaques

Food safety | Animal-source monitoring | Strict biosafety around macaques <br> <span style="font-size: 20px; color: #4A5650; display: block; margin-top: 4px;">These viruses matter because they are clinically important but easily overlooked.</span>

WHO Hepatitis E · CDC B virus page

Clinical Spectrum of Viral Zoonoses: Syndromes, Not a Single Disease

Same spillover event — different organ systems, different outcomes

18

Viral zoonoses span multiple syndromes with different tissue tropisms and severity profiles.

WHO treats these threats at the viral family level — not as isolated pathogens.

Some viruses cause sporadic spillover; others amplify into epidemics or pandemics.

High-consequence families: filoviruses, arenaviruses, nairoviruses, henipaviruses, highly pathogenic coronaviruses.

Encephalitis

Rabies

Nipah

Japanese encephalitis

West Nile

Hemorrhagic Fever

Ebola

Marburg

Lassa

CCHF

Respiratory Disease

Influenza A

SARS

MERS

Hendra/Nipah

Hepatitis

Hepatitis E (zoonotic genotypes)

Rash / Mucocutaneous

Mpox

Cowpox

Febrile Arthralgia

Chikungunya

Related alphaviruses

Early syndrome recognition → narrows likely exposure source → improves biosafety decisions &nbsp;|&nbsp; Essential for outbreak triage and special-pathogen escalation

Source: CDC Yellow Book zoonotic exposure routes · WHO pathogen-prioritization framework

Diagnostic Workflow for Suspected Viral Zoonoses

Exposure history → Syndrome → Specimen → Test → Biosafety

19

Exposure History

Animal contact, bites, scratches, travel, vector exposure, food risks, caves, livestock markets, slaughtering, wildlife, occupational risks

Syndrome Recognition

Match clinical picture: encephalitis, HF, respiratory, hepatitis, rash, arthralgia

Specimen & Timing

Choose correct specimen based on syndrome and day of illness

Molecular / Antigen / Serology

PCR, antigen detection, or serology as appropriate

Biosafety + Referral

Isolate early if high-consequence pathogen suspected; notify public health

Start with detailed history: livestock markets, caves, slaughtering, wildlife exposure, occupational risks

Zoonotic infections follow: bites, scratches, inhalation, ingestion, or contact with body fluids

Early recognition critical for: rabies, hemorrhagic fevers, and special pathogens

High-risk viral families: filoviruses, arenaviruses, nairoviruses, henipaviruses, highly pathogenic coronaviruses

Biosafety + referral + isolation decisions must be made EARLY | Pair diagnostic pathway with infection prevention and public health notification

Source: CDC Yellow Book exposure and returned-traveler evaluation chapters

Prevention and Control: A One Health Pyramid

From upstream animal surveillance to individual protection and outbreak response

20

Risk communication — Community engagement — HCW protection

PEP — Outbreak isolation — Contact tracing

PPE — Safe handling — Avoid animal blood/saliva/urine/feces

Surveillance — Animal vaccination — Vector control — Food safety

Upstream Prevention

Animal vaccination where available

Vector control

Wildlife and livestock surveillance

Food safety and safe animal handling

Public Health Control

Post-exposure prophylaxis where available

Outbreak isolation and contact tracing

Risk communication and community engagement

Healthcare worker protection and laboratory biosafety

For high-risk viral zoonoses: prevention is often more effective than treatment — countermeasures may be limited. WHO pathogen-prioritization is built around this exact challenge.

CDC Yellow Book zoonotic exposures · WHO One Health prevention framework

Surveillance, Genomics, and One Health Governance

Integrated surveillance across human, animal, and environmental data

21

Surveillance Interface

Integrated surveillance combines human, animal, and environmental signals

Animals serve as early warning sentinels at the interface

FAO: One Health surveillance captures signals earlier by moving beyond siloed data

Fragmented systems miss early spillover signals

Policy Significance

WHO: close links between human, animal, plant, and environmental health demand cross-sector collaboration

WHO priority-pathogen process: 200+ scientists from 50+ countries, 28 viral families evaluated

One Health is a coordination model, not only a scientific idea

Genomics: detect spillover, track transmission chains, identify family-level risk

Human Health Data

Animal Health Data

Environmental Data

Integrated One Health Surveillance

Genomic Analysis

Early Warning Signal

Public Health Response

Strongest surveillance combines: field ecology + veterinary data + clinical reporting + laboratory genomics + public health response

Source: WHO One Health · FAO early-warning material · WHO pathogen-prioritization

Conclusion: What Viral Zoonoses Teach Us

Spillover → Syndrome → Surveillance → One Health → Prevention

22

Core Idea

Viral zoonoses are spillover phenomena shaped by animal reservoirs, environmental change, and human behavior.

WHO and CDC view these as infections transmitted between animals and people, with increasing public health importance.

Spillover<br><span style="font-size:14px; font-weight:500; color:#5A6D63; margin-top:2px;">(Reduced Risk)</span>

Syndrome<br>Recognition

Surveillance

One Health<br>Response

Prevention

One Health

Public Health Significance

WHO pathogen-prioritization focuses on whole viral families — more effective than treating pathogens in isolation.

One Health links human, animal, and ecosystem health with integrated cross-sector coordination.

Essential for managing high-consequence families with epidemic or pandemic potential globally.

Spillover is <strong style="color: #D4AF37;">PREVENTABLE</strong>

Syndromic thinking <strong style="color: #D4AF37;">IMPROVES</strong> diagnosis

One Health is the <strong style="color: #D4AF37;">FUTURE</strong> of zoonotic disease control

Source: WHO One Health · WHO pathogen-prioritization | Hazara University Mansehra — Department of Zoology