The Ecology of Lyme Disease o aks, moths, mice, gypsy moths, and lyme disease

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