Pathogen Life Cycles

Pathogens/Parasites

Alternate Between Animal Hosts

Schistosomiasis: Macroparasite

Zoonotic Disease: Human Infection, “Accident”

Lyme Disease West Nile Fever

Lyme Disease

Most Common Vector-Borne Infection in

Northern Hemisphere

Northeast: 4+ Species: Enzootic Cycle

Borrelia (pathogen)

Black-legged Tick (vector)

Small, Large Mammal Hosts

Ixodid Tick

Year 1

Egg

Larva

Blood Meal

White-footed Mouse,

Other Small Mammal

Infection?

Molt/Quiescent

Year 2

Nymph

Blood Meal

Small Mammal

Zoonotic Infection

Adult

Blood Meal (♀), Deer

Reproduce

Enzootic Cycle, Lyme (Penn State Univ.)

Ixodid Ticks (CDC)

NYS: Lyme Disease; Glavanakov et al 2001

0

200

400

600

800

1000

1200

1400

1600

1800

2000

88 89 90 91 92 93 94 95 96

YEAR

INC

IDE

NC

E R

AT

ES

0

5000

10000

15000

20000

25000

30000

CU

MU

LA

TIV

E C

AS

ES

Lyme Disease: NYS, Cases & Rates

Schistosomiasis

200 x 106 Infections Worldwide

Tropical, Freshwater Snails

5 Macroparasites

Complex Lifecycle: 2 Hosts,

2 Reproductive Modes

Schistosomiasis

(Originally) Disease of Herbivores

Infects Cattle, Jumped to Humans?

Human Parasitic Disease:

Chronic Infection/Cognition

Impact Second to Malaria/Economic

Schistosome Life Cycle

Schistosome Life Cycle

Snail: Mother Sporocyst, Asexual Reproduction,

Daughter Sporocysts, Daily Release of Cercariae

Human: Schistosomules Mature as Male or Female, Sexual Reproduction

West Nile Virus

WNV: Arbovirus, Arboviral Encephalitis

N America: First Noted 1999

Across Continent, Central America

Reservoir: Birds

Vectors: Mosquito Spp.

Secondary Infection – Mammals

Humans: Zoonosis

WNV: Geographic Distribution (CDC)

WNV Birds - 2009 (USGS)

WNV: Human cases, 2012

WNV Life Cycle (Stanford)

WNV Life Cycle

RNA Virus: High Mutation Rate

Mosquito: Innate Immunity

Viral Master Sequence

Mutant Cloud: Quasispecies, High Diversity

Bird: Vertebrate Immunity

Master Sequence, Low Diversity

WNV Life Cycle

Viral R0 : Must Reproduce Sequentially in Two Different Environments

Trade-offs?

WNV Vectors

Mosquito Spp and Zoonosis Risk

Enzoootic Cycle: Culex pipiens

Ornithophilic; Fewer Mammal Meals

“Bridge Vectors” Prefer Mammals; Risk to Humans?

Aedes Spp.

WNV Vectors

1. Abundance

2. Fraction Blood Meals from Mammals

3. WNV Prevalence

Pr[Fed Infected Bird & Acquired WNV]

Fraction of Mosquito Sp. Carrying WNV

4. Vector Competence

Given Infection, Pr Will Transmit to New Host

WNV Vectors (Kilpatrick et al. 2005)

Bridge (Aedes) Ornithophilic

Abundance 21% 37%

Mammal Meals 86% 19%

WNV Prevalence 5% 95%

Competence 17% 38%

Risk 4.5% 80.2%

WNV Vectors/Zoonosis Risk

Control Ornithophilic Spp.

Small Set of Breeding Sites

Tires, Gutters, Open Pools

Around Human Habitations

Wetland Intervention Unnecessary