RICHARD OSTFELD: We live in an age
of emerging infectious disease.
We humans are getting attacked by an increasing
number of brand new infectious diseases.
Where's the next disease likely to break out?
Why?
What are we doing to the environment
that might make a disease more likely to emerge?
To answer those questions, you need
to understand what are the complex interactions
between species in nature that give rise to these emergent
events.
Lyme disease was discovered when a very unusual cluster of cases
of juvenile rheumatoid arthritis occurred in and around Lyme,
Connecticut.
And it took some serious detective work
to figure out that this arthritis was
a late-stage symptom of a disease caused
by a bacterium that's transmitted to people by tick
bites.
And since the late '70s and early '80s,
Lyme disease has grown dramatically,
both in number of cases and in the spatial extent
of the disease itself.
And right now, throughout the North temperate zone,
Lyme disease is the most commonly reported
vector-borne disease.
Vectors are insects or ticks that transmit pathogens
from one host to another.
The ticks that transmit Lyme disease to people
hatch free of infection.
They only take one blood meal, then
they molt into the nymphal stage.
If that host that they felt on was infected,
they can acquire the Lyme bacteria,
and they can then transmit Lyme disease.
But all hosts are certainly not created equal
in terms of disease risks that affect humans.
KATHLEEN LOGIUDICE: We're interested in how
the diversity of the host population
affects the likelihood that ticks are
going to be infected with Lyme.
So that translates into risk to humans.
This is an animal that Kira caught three or four days ago,
and it's been sitting in the rearing facility
eating wonderful food and dropping ticks.
This had, I think, quite a few ticks, didn't it?
KIRA GILMAN: Oh, yeah.
Oh, yeah.
Lots of ticks.
KATHLEEN LOGIUDICE: We can lift it and make some room.
So we need to know how likely each species is
to feed ticks, how many ticks it feeds,
how likely it is to infect the ticks that it feeds,
and then we can sort of deconstruct the host community
and see what would equal high risk to people,
and what would equal low risk to people.
So we've tried to capture everything with fur
and feathers, essentially.
MARY KILLILEA: You got something in this one.
On most days, we get up in the morning, and we go out,
and we check traps that have been set the night before.
JESSE BRUNNER: So this is a female lactating.
MARY KILLILEA: Then any animals that we caught are processed.
Some of them are very good at transmitting the bacteria that
causes Lyme disease, and some are what we
would consider dilution hosts.
JESSE BRUNNER: Yup.
Little mouse.
MARY KILLILEA: Animals that may feed a lot of ticks,
but don't transmit the bacteria.
JESSE BRUNNER: Nice.
Possums tend to feed a lot of ticks.
So they'll get a lot of ticks on them.
Not many of those ticks will actually make it off,
so a lot of them die somewhere in that process.
And then of those that actually make it
through to the next stage, most of them aren't infected.
So they tend to be pretty good dilution hosts.
So a lot of possums in the environment
is actually pretty good for lowering
our risk of Lyme disease.
So we like these little guys.
He's going to play dead.
Bye, guy.
KATHLEEN LOGIUDICE: We determine which host
is the best for spreading Lyme disease by bringing them back
to the lab.
We hold them for about three days.
And during those three days, any larvae
that are naturally infesting the animal will feed and drop off.
KIRA GILMAN: We collect the ticks in here
after they drop off.
So this is our data.
KATHLEEN LOGIUDICE: We search the pan
and collect the ticks that have engorged,
which gives us a couple of pieces of information.
We can figure out how many ticks are feeding
on that animal at any given moment,
and we can eventually test the tick
to determine the percentage of ticks
that end up being infected.
KIRA GILMAN: Right now I'm taking ticks to crush,
and that's how we get the gut material out of the tick
and then isolate the DNA.
Then we run it through a real-time PCR, or polymerase
chain reaction, which is a way of splitting the DNA
and detecting Lyme disease.
RICHARD OSTFELD: What we've found
is that the probability that a tick is going
to acquire an infection when it feeds on a white footed mouse
is about 90%.
Contrast that with the probability
that it's going to get infected if it feeds off
an opossum or a skunk or raccoon,
which they readily bite, and that's down below 10%.
As we fragment the landscape, we chop up continuous forest
into little bits, we lose species.
They disappear.
One of the last creatures is the white footed mouse.
So as we reduce diversity, we're losing
the species that protect us and favoring
the ones that make us sick.
KATHLEEN LOGIUDICE: There's no such thing
as a natural system anymore-- a truly natural system.
As you start reducing forest sizes,
you start making habitats that the really resilient species
can tolerate and some of the other species can.
So what we're studying obviously isn't natural systems.
It's human-impacted natural systems.
And what we're becoming more aware of
is that human impacts can bite us back.