The Ecology of Lyme Diseaseoaks, moths, mice, gypsy moths, and lyme disease
Gypsy Moth Defoliation (1975-2008)
The white-footed mouse can regulate gypsy moth populations
While gypsy moth populations have been down the incidence of Lyme
disease has increased
What is going on?
Lyme Disease Biology
Caused by the spirochete bacteria Borrelia burgdorferi
Vectored by ticks (in California Ixodes pacificus)
Vector: any agent that carries and transmits an infectious agent
A Community of Borrelia Hosts
dusky-footed woodrat
western fence lizard
white-footed mouseeastern chipmunk
white-tail deer
mom and dadour best friends
Target Hosts
Lyme Disease Symptoms
Within a few days:Skin rash 3-30 days after tick bite
Within days or weeks:FatigueChillsFeverHeadacheMuscle and joint achesSwollen lymph nodesBell’s Palsy (loss of muscle tone on one side of the face)
Long-term neurological problems:Problems with concentrationShort-term memory lossSevere arthritis and joint pain
Erythema migrans (~70% cases)
Lyme Disease Historydid it just appear in the 1970s?
1883: The first record of a condition like Lyme Disease
1921: Joint problems disease is associated with Ixodes scapularis ticks
1975: Cluster of cases of rashes and swollen joints in Lyme, Connecticut
1982: Borrelia burgdorferi is discovered by Dr. Willy Burgdorfer
How do you show that a particular microbe causes a disease?
Koch’s postulatesTo establish that an organism causes disease, is
must be:
1. found in all cases of the disease examined, while absent in healthy organisms
2. prepared and maintained in a pure culture (some practical difficulties here)
3. capable of producing the original infection (some ethical difficulties here)
4. retrievable from an inoculated animal and cultured again
Tick life cycle (Ixodes scapularis)
Eggs uninfected
Larvae uninfected
Nymphal Stage
Adult Stage
feedingone chance to pick up Borrelia infection before nymphal stage
feedinghumans are at risk
feedingmostly deer
Risk to humans determined by:1. density of nymphal ticks2. infection prevalence in nymphal ticks3. human behavior
How Does the Host Community Affect Disease Riskdisease risk = lyme disease cases
Increased Disease Risk Decreased Disease Risk
AmplificationAdding a species to a community increases the total abundance of hosts for the pathogen, increasing the disease risk to the target host
DilutionAdding a species to a community decreases the abundance of more competent hosts, decreasing the disease risk to the target host
Each tick feeds only once per stage
Feedings spent on poorly-competent hosts are wasted for the disease
Competence (for hosts)The efficiency with which a host acquires and spreads a pathogen
How Does the Host Community Affect Disease Riskdisease risk = lyme disease cases
White footed mice are a preferred host = more ticksWhite footed mice are a competent host = more infected ticks
How Does the Host Community Affect Disease Riskdisease risk = lyme disease cases
Increased mammal diversity = more ticks, less ticks, who knowsIncreased mammal diversity = less infected ticks
Biodiversity and Disease Risk
Biodiversity and Disease Risk
Tick life cycle (Ixodes pacificus)
Reservoir competenceHosts differ in their efficiency at acquiring and spreading the Borrelia spirochete
highly competent somewhat competent totally incompetent
dusky-footed woodratNeotoma fuscipes
deer mousePeromyscus maniculatus
western fence lizardSceloporus occidentalis
The Lizard That Fights Lyme Disease
Proteins found in the blood of S. occidentalis kill the Borrelia spirochete
Tick life cycle (Ixodes pacificus)
The Lizard That Fights Lyme Disease
Prevalence of the Borrelia spirochete ≈ 1% - 2%
Prevalence of the Borrelia spirochete ≈ 30% - 60%
Does the Forest Community Affect Disease Risk?
Does the Forest Community Affect Disease Risk?
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What about gypsy moths?
Mast production, or masting, refers to the synchronous, episodic production of heavy seed crops by a population of plants
What are the ecological benefits of masting?
Red oaks (Quercus rubra) mast every 3-5 years
Type II Functional ResponseHandling-Mediated Specialization
0 10 20 30 40 50 60 70 80 90 1000
5
10
15
Density of Prey Population
Num
ber o
f Pre
y C
onsu
med
h = 0.05h = 0.5
Tick life cycle (Ixodes scapularis)
disease risk is maximized 2 years following mast production