Wildlife Diseases

Basic Principles &

Concepts

OutlineMotivations to study wildlife disease

Defining & quantifying disease

Epidemiological factors

Impacts of wildlife disease

Motivations to study Wildlife Disease

Human health

Livestock health

Game management

Rare species management

Economics

Human HealthZoonotics

West Nile VirusEncephalitisFever

Birds Mosquitoes Humans

Infection ImpactsIdentified in 19991000’s of human cases / year since 2003

Human Health Anthropozoonitics

Humans Wildlife

Spread to other primatesRespiratory virusesMeaslesInfluenza Parasites? Polio? Yaws

Livestock HealthBovine Tuberculosis

Wildlife reservoirsWhite – tailed deer (MI, USA)European badgers (UK)

Agricultural Impacts (Michigan)Loss of TB-free status Restrictions on tradeRequirements for monitoring

Game ManagementChronic Wasting Disease

Wisconsin’s GoalsMinimize the affected areaMinimize prevalence

Management – decrease deer populationDelineate CWD management zonesRestructure hunting seasons

Management – decrease disease spreadRestrict cervid baiting & feedingRegulate farmed cervid industry

Rare Species ManagementDevil Facial Tumor Disease

Contagious cancerFatal

WidespreadOccurs over ~ 60% of rangeLocal prevalence up to 83%

Large population declines64% on average Up to 95% locallyPotential for extinction within 20 years

EconomicsZoonotics $100’s M

Rabies: > $300M / yr in US on detection, prevention, controlSARS: Canada lost millions in tourism after 2003 casesWest Nile Virus: $20M in 2002 public health costs to Louisiana

Bovine Tuberculosis $15M / yr Lost from MI agriculture without TB free status

Chronic Wasting Disease $10’s MLost from hunting-related revenue and commerce in WI in 2003 hunting season

Anthrax $M’sLost from tourism as Chobe NP closed during 2004 outbreak, and other African parks lost valued animals

Wildlife Disease Study Requires Interdisciplinary Science

Human Medicine

Veterinary Medicine

Wildlife EcologyWildlife Disease

What is Wildlife Disease ?What is it?

How is it transmitted?

How do we quantify occurrence?

What is Wildlife Disease ?Disease

“Any impairment that interferes with or modifies the performance of normal functions, including responses to environmental factors such as nutrition, toxicants, and climate;infectious agents; inherent or congenital defects; or combinations of these factors” (Wobeser 2006)

Zoonosis (zoonotic disease)Infectious disease that is transmitted naturally between animals and humans

Endemic / EnzooticA disease that occurs with predictable regularity and rate in a given population / area / time frame

Epidemic / EpizooticA disease that is occurring in a species / place / time where it is unexpected, or at a higher rate than expected

Types of Disease

Herbicides, insecticides-Human-synthesized chemicalsLead, selenium-Human-distributed chemicalsCyanobacterial toxins-Naturally occurring chemicals

ChemicalsHeat, cold, traumaPhysical agentsMalnutrition, vitamin deficiencyNutritional deficienciesDiabetesMetabolic alterationsInbreeding depression, mutationsGenetic disorders

NON-INFECTIOUS

Nematodes, flukes-EndoparasitesFleas, ticks-Ectoparasites

MetazoaMalaria - plasmodiumProtozoaChytrid fungusFungiMycobacterium spp.BacteriaRabies virusViruses

INFECTIOUSFamiliar ExamplesDisease Agents

Modes of Disease Transmission

Horizontal Vertical

Direct Indirect

Close EnvironmentalContact Contamination

-Intermediate host-Vector

-Skin-to-skin-Aerosol / droplet-Secretions / excretions-Sexual activity-Contacting carcasses

-Air-borne-Water-Fomites-Transport host-Paratenic host

-Parent – offspring

Defining Disease CasesMortality (field observations)

CarcassesIs dead animal with disease killed by disease?How many dead exist for every carcass recovered?

Infected (field observations, laboratory tests)

Clinical signs Pre-clinical

Exposed (laboratory tests)

ImmuneRecoveredCarriers

Quantifying OccurrenceIndividual Measures

Parasite load• # parasites / host

Case – fatality rates• # deaths / # infected

Population MeasuresIncidence• # new cases / # in pop• over specified time period

Prevalence• # existing cases / # in pop

Incident cases

Mortality / Recovery

Prevalence

The Epidemiological Triangle

HOST

PATHOGEN ENVIRONMENT

Vect

or

Host TypesPrimary / Definitive

The host in which the agent undergoes replication

IntermediateHost in indirect transmission cycles in which the agent does not reproduce

ReservoirA species in which the agent is maintained, usually without developing disease, and may be passed to others

CarriersHarbors infectious agent, lacks clinical signs, but can transmit infection to others

Dead end hostsA species that can be infected with the agent acquired from outside sources, but does not maintain the agent

Host FactorsBiology

AgeSexGeneticsSocial statusBreeding status

Immune StatusSusceptibilityImmunity

ConditionNutritionStress

BehaviorContactsMovements

Pathogen FactorsInfectivity

Ability to enter host

Life CycleHost requirementsReproductive rates

PathogenicityMortality / morbidity

SurvivalPersistence in environment

Vector Factors

Life CycleHost requirementsReproductive ratesEnvironmental effects - climate

SurvivalPersistence in environment

Environment FactorsStressors

ClimateSeason

LocationHabitat qualityPopulation densityProximity to infected animals

ExposuresPathogen reservoirsToxins

Impacts of Wildlife DiseaseCellular injury

Individual productivity loss

Individual mortality

Population impacts

Alteration of ecological communities

Impacts of Wildlife Disease

Energy Intake

Energy Assimilated

Production StorageRespiration

Individual ImpactsIncreased energy demands

ThermoregulationInflammatory / immune responseAltered behavior

Decreased energy intakeBehavioral• Lethargy or hiding decrease foraging• Avoidance of certain foods / feeding areas• Time / energy devoted disease Physical• Lack of appetite• Reduced mobility / ability to forage• Decreased digestive efficiency

• Increased excretions

Energy Intake

Energy Assimilated

Individual ImpactsRespiration less efficient

Poor thermoregulationDecreased activity

Decreased reproductionAbortions Smaller littersDecreased ability / attempts to mateInjury to reproductive system / organsAbnormal young or infant mortality

Less energy for storageDecreased growthDelayed maturity

Energy Intake

Energy Assimilated

Production

Storage

Respiration

Individual ImpactsDirect mortality

Indirect mortalityPredationStarvationHypothermia Other diseasesAccidentsDepressed growth

Energy Intake

Energy Assimilated

Production

Storage

Respiration

Impacts of Wildlife DiseaseCellular injury

Individual productivity loss

Individual mortality

Population impacts

Alteration of ecological communities

“A disease becomes important to a population when it causes sufficient reproductive failure or mortality to reduce breeding numbers below what would otherwise occur.” (Newton 1998 from Wobeser p 143)

Population ImpactsLimitation

Disease is a factor prohibiting population increase

RegulationDisease holds population near carrying capacity if contact, transmission & infection increase when animals are more densely populated

Stochastic eventsSmall populations will be especially vulnerable to extinction from highly pathogenic disease, or increased susceptibility to environmental changes due to disease stress

Population ImpactsPopulation size

May be decreased by disease

Spatial distributionAnimals may avoid habitat with disease riskSegments of the population may be extirpated

Age structureDisease risk and survival may vary by age

Sex ratioDisease risk and survival may vary by sex

Life history traitsBreeding strategies and age at maturity may determine resilience of population to disease mortality

Population ImpactsPopulation size

ReproductionDirectly impacted by disease via decreased reproductive success of individuals

MortalityDirectly impacted by disease mortality and additive mortality due to increased susceptibility to starvation, predation, accidents, etc.

Additive vs compensatory mortality

Immigration & EmigrationMay be affected by changes in behavior and activity

Ecological ImpactsLocal population extinction

Species rarity / extinction

Lack of prey availability

Lack of predator regulation

Open niches filled by invasive species

Ecological ImpactsLynx – snowshoe hare cycles

Hare density increasesHare become nutrient limitedIncreased disease (helminthe parasites) Increase susceptibility to predationHare density decreasesLynx population declinesPredation pressure decreases

Ecological ImpactsHawaiian bird communities

Humans bring non-native birds & mosquitoesNon-native birds bring diseases• Avian pox• Avian malariaNative populations restricted to high elevations free of mosquitoesLowland forests free for expansion of non-native populations

Discussion Questions