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
Year 2
Egg
Larva
Blood Meal
White-footed Mouse,
Other Small Mammal
Infection?
Molt/Quiescent
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
2000
30000
25000
1600
1400
20000
1200
1000
15000
800
10000
600
400
5000
200
0
0
88
89
90
91
92
YEAR
93
94
95
96
CUMULATIVE CASES
INCIDENCE RATES
1800
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%
5%
95%
Competence
17%
38%
Risk
4.5%
80.2%
WNV Prevalence
WNV Vectors/Zoonosis Risk
Control Ornithophilic Spp.
Small Set of Breeding Sites
Tires, Gutters, Open Pools
Around Human Habitations
Wetland Intervention Unnecessary