Ixodes scapularis & Lyme disease
spread in Canada: Climate change &
migratory birds
Nick Ogden
What is Lyme disease?
• A disease caused by a spirochaete bacterium Borrelia burgdorferi
• It is a multisystem disease in humans (and animals)
• It is non-fatal but chronic and debilitating if not treated early
• Difficult to diagnose by doctors (clinical presentation and serology)
• ~ 20 000 cases a year in USA despite high level of awareness by the
public & medical practitioners
• It is an infection of wildlife (rodents, birds, mustelids NOT deer) that is
transmitted from one animal to another by ticks
• Ticks are not very choosy about who they bite and bite people by accident
• The geographic range of Lyme disease is tied to that of its tick vectors
Borrelia burgdorferi transmission cycle
Adult ticks
feed on deer
in spring and
autumn
Adult E
Adult
POP
Infected
Tick development
larvae moultand
intoactivity
infected
depend
nymphs
onnext
temperature
spring Eggs
Nymph E
Infected
nymphs infect
rodents or
birdsfeed on
Nymphs
rodents and
birds in spring
Nymph
Larva E
Larva
PEP
Larvaerodents
feed onor
Infected
and
birdsrodents
infect larvae
birds in late
summer
Temperatures are expected to rise dramatically
this century: global warming
What’s in store for Canada?
Temperature rise:
• One of the most seriously affected places on earth
• Mean annual surface temperature rises of ca 4ºC in southern
Ontario/Quebec, up to 10ºC in Nunavut
Change in rainfall:
• Slight increase in rain in the east? Reduced rain in the prairies and
the west
• Change in pattern: heavy rainfall events accompanied by flooding
interspersed by droughts
The greater the mitigation, the less severe the effects, hence Kyoto
• General expectation that the geographic range of arthropods (such as
ticks) will expand north with global warming
Where are the ticks now:
In the USA!
Ixodes pacificus
Ixodes scapularis
In Canada we have only a few reproducing (and selfsustaining) I. scapularis populations
For Lyme to more seriously affect Canada the range of I. scapularis would have to expand
Adult I. scapularis ticks are already submitted to Health Canada by the public
from a very wide geographic range: How come?
We hypothesize that migratory ground-feeding birds are
bringing I. scapularis into, and through, Canada
Northern-migrating
ground-feeding birds
stop-over in tickinfested habitat
Spring migration
coincides with spring
activity period of
Ixodes scapularis
nymphs
Nymphs feed
continuously on birds for
5 days, then drop off into
the habitat
Why study ticks on migratory birds?
• Many nymphal ticks carried by migratory birds survive and
molt into adult ticks, which can bite people and infect them
with Lyme disease.
• Understanding the distribution of these ticks will help alert
health workers and the public as to where the risks are from
these ticks
• Reproducing (established) tick populations carry much risk
more because:
– The ticks are more numerous
– Nymphal ticks are present, which are highly infected, difficult to see
and more likely to infect people than adult ticks
• With global warming we expect that the range of reproducing
populations of ticks to expand
• Where ticks are being dropped in the greatest numbers by
the birds now is where populations are likely to establish
We have developed a simulation model of I. scapularis populations
to identify where temperature conditions are OK for I. scapularis
And forecast for e.g. 2020
Now:
IndexIndex
of tick
at model
equilibrium
of abundance
tick abundance
at model
equilibrium
The range of I. scapularis may continue to expand through this
century
Now
2020s
2050s
2080s
Our research conclusions so far are:
• A northward expansion of the range of Ixodes scapularis, associated
with climate change, is likely because:
– Tick survival increases dramatically
– Ticks are being carried into Canada each year on migratory birds
– The habitat in many areas is suitable for tick survival
• Chaotic weather (heavy rainfall and droughts) may inhibit spread
• Spread into central Provinces may be limited by low rainfall
• Diagnostic expertise by clinicians and laboratories will be crucial in
detecting and monitoring the increased risk of Lyme disease
• We can help by identifying where ticks are landing now, and thus where
tick populations may set up in the future
The role of migratory birds: what we need to know
1.
What bird species are responsible for carrying ticks
into Canada:
–
–
–
2.
What species carry ticks (finding ticks on birds)
How many ticks are carried, on average, by each individual
bird (counting and collecting ticks)
How abundant the birds are (banding records)
Where these birds are going:
–
–
–
3.
From analysis of isotopes in retrices
Banding recoveries
Ecological niche modeling
Where the ticks are going:
–
–
–
Using the above data in simulation models (‘network
analysis’: equivalent to methods used by airlines to estimate
where passengers get on and off planes)
Existing maps of I. scapularis distribution in the USA (CDC)
Range of 5 days flying time for Nymph, 3 days for a Larva
A basic model of tick dispersion by
migratory birds
Depends on phenology of tick
which depends on climate
1971-2000
Proportion of annual number of ticks
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
9
10
11
12
Month
Number of ticks
Questing in
USA woodland
in spring
5 days flight time
Number of ticks
dropping on
location X
in Canada
Numbers of ticks carried per bird
Species: timing of migration
Species: behaviour contacting ticks
Species: migration route
Species: speed of migration
Number of ticks
Questing in
USA woodland
in spring
5 days flight time
Number of ticks
dropping on
location X
in Canada
Bird behaviour and contact with Ixodes scapularis:
A meta-analysis of previous studies
• Data from 15 studies on I. scapularis infestations of
birds (migrating and resident) in the USA and Canada
• Negative binomial regressions with outcome = number
of birds infested and number examined as an offset
• Explanatory variables: foraging behaviour, nesting site,
habitat and ‘taxonomic groupings’. Study ID was a
random effect
• Significant factors +ve association with parasitism:
ground feeding behaviour, ground nesting,
woodland/ecotone habitat and some family groupings
Species selected by meta-analysis
• Corvids: Blue Jay
• Sparrows: Eastern Towhee, Chipping Sparrow, Song
Sparrow, Swamp Sparrow
• Icterids: Brown-headed Cowbird, Common Grackle
• Mimids: Brown Thrasher, Grey Catbird
• Tanagers: Northern Cardinal
• Thrushes: American Robin, Veery, Grey-cheeked, Hermit,
Swainson’s & Wood Thrushes
• Warblers: Worm-eating, Hooded & Canada Warblers,
Ovenbird, Northern Waterthrush, Common Yellowthroat
• Wrens: House Wren
Coincidence of migration with tick questing
Common
American
Yellowthroat
robin
1971-2000
42.5o
40o
Proportion of annual number of ticks
0.6
0.5
0.4
0.3
0.2
0.1
35o
0
1
2
3
4
5
6
7
8
9
10
11
12
Proportion of annual number of ticks
Month
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
Month
8
9
10
11
12
Data from the field: Spring migration 2005 in Canada
• Corvids: Blue Jay 4
• Sparrows: Eastern Towhee 2, Chipping Sparrow 2, Song
Sparrow 13, Swamp Sparrow 7 (WT WC ST Fox GH Lincoln 43)
• Icterids: Brown-headed Cowbird 1, Common Grackle 2
• Mimids: Brown Thrasher 14, Grey Catbird 4
• Tanagers: Northern Cardinal 1
• Thrushes: American Robin 2, Veery 2, Grey-cheeked 2, Hermit
21, Swainson’s 3 & Wood Thrushes
• Warblers: Worm-eating, Hooded & Canada Warblers, Ovenbird
3, Northern Waterthrush, Common Yellowthroat 3
• Wrens: House Wren 9
+ BC chickadee 1, Black & white warbler 1, Black-throated blue warbler 1, Magnolia warbler 1,
Nashville warbler 1, Yellow warbler 1, RC Kinglet 2, Starling 2, RW Blackbird 9
Acknowledgements
The team:
• Bird Studies Canada:
– Audrey Heagy
• Canadian Wildlife Service
– Charles Francis
• Saint-Hyacinthe, QC:
– Alex Thompson (Université de Montréal)
– Michel Bigras-Poulin (Université de Montréal)
• Kingston, ON:
– Chris O’Callaghan (Queens University)
• Toronto, ON:
– Abdel Maarouf (Environment Canada, York University)
• Guelph, ON:
– Dominique Charron (Public Health Agency of Canada)
– Ian Barker (University of Guelph)
• Winnipeg, MB:
– Robbin Lindsay (Public Health Agency of Canada)
Funding by NRC Climate Change Impacts & Adaptations Programme