Download 11 - Lyme Disease

The Ecology of Lyme Disease
oaks, 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
western fence lizard
dusky-footed woodrat
white-footed mouse
eastern chipmunk
Target Hosts
our best friends
white-tail deer
mom and dad
Lyme Disease Symptoms
Within a few days:
Skin rash 3-30 days after tick bite
Within days or weeks:
Fatigue
Chills
Fever
Headache
Muscle and joint aches
Swollen lymph nodes
Bell’s Palsy (loss of muscle tone on one side of the face)
Long-term neurological problems:
Problems with concentration
Short-term memory loss
Severe arthritis and joint pain
Erythema migrans (~70% cases)
Lyme Disease History
did 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 postulates
To 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)
Risk to humans determined by:
1. density of nymphal ticks
2. infection prevalence in nymphal ticks
3. human behavior
feeding
mostly deer
Adult
Stage
Eggs
uninfected
Nymphal
Stage
Larvae
uninfected
feeding
feeding
one chance to pick up Borrelia
infection before nymphal stage
humans are at risk
How Does the Host Community Affect Disease Risk
disease risk = lyme disease cases
Increased Disease Risk
Decreased Disease Risk
Amplification
Adding a species to a community
increases the total abundance of
hosts for the pathogen, increasing
the disease risk to the target host
Dilution
Adding 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 Risk
disease risk = lyme disease cases
White footed mice are a preferred host = more ticks
White footed mice are a competent host = more infected ticks
How Does the Host Community Affect Disease Risk
disease risk = lyme disease cases
Increased mammal diversity = more ticks, less ticks, who knows
Increased mammal diversity = less infected ticks
Biodiversity and Disease Risk
Biodiversity and Disease Risk
Tick life cycle (Ixodes pacificus)
Reservoir competence
Hosts differ in their efficiency at acquiring and spreading the Borrelia spirochete
highly competent
dusky-footed woodrat
Neotoma fuscipes
somewhat competent
deer mouse
Peromyscus maniculatus
totally incompetent
western fence lizard
Sceloporus 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 ≈ 30% - 60%
Prevalence of the Borrelia spirochete ≈ 1% - 2%
Does the Forest Community Affect Disease Risk?
Does the Forest Community Affect Disease Risk?
?
?
?
?
What about gypsy moths?
Mast production, or masting, refers to the synchronous,
episodic production of heavy seed crops by a population
of plants
Red oaks (Quercus rubra) mast
every 3-5 years
What are the ecological benefits of masting?
Type II Functional Response
Handling-Mediated Specialization
15
Number of Prey Consumed
h = 0.05
h = 0.5
10
5
0
0
10
20
30
40
50
60
70
Density of Prey Population
80
90
100
Tick life cycle (Ixodes scapularis)
disease risk is maximized 2 years
following mast production