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| PhD comics never fails to have an appropriate image. (http://www.phdcomics.com/store/mojostore.php?_=view&ProductID=12631) |
A while back I said I would discuss what “comes next”
after all the field work. Well, I have started the next big project:
identifying all the ticks I collected from the mice and voles we trapped over
the summer. This is a big project
because I collected over 670 ticks, and these are larvae and nymphs, the
younger life-stages of the tick, and are really small and can only be
identified under a dissecting microscope. It is important to identify all these
ticks to species and life-stage because there are different assumptions about
previous host interactions and possible infections for each group. To catch
everyone up, here is some background before we get too deep into things.
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| This is a diagram of the Ixodes scapularis lifecycle. It is a "cool season" tick because its adults are active in the fall. Most of the ticks in Indaina have all life-stages active in the spring and summer. |
To review, ticks are ectoparasites of vertebrates that have
a 4-stage life cycle: egg, larva, nymph, and adult. These parasites need to
bite a host and obtain a blood meal in order to molt and transition to the next
life-stage. This is somewhat unique because other common vectors like fleas and
mosquitoes only feed on a host directly in the adult stage. This means ticks
can have many interactions with host over their lifetime and have the
opportunity to become infected or pass on infection many times. The larvae
emerge from eggs uninfected, besides obligate bacterial symbionts, so the larva
can pick up a pathogen infection during their first blood meal. When these larvae
molt into nymphs they become infected with whatever bacteria was picked up
during the previous blood meal and can transmit to an uninfected host. This
same thing can happen with adult ticks, but they have had two possible times to
pick up an infection. The adult female ticks then feed to produce eggs, the
males mate with the females during this blood meal and they rarely feed
themselves. Then the females drop of the host and lay thousands of eggs in a “mass”.
The larvae then emerge in the spring and the cycle starts all over again.
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| Mouse with a lot of engorged nymphs on its back. |
The rodent hosts that I am interested can be hosts to the
larval and nymphal stages. In the part of southern Indiana where I conducted
these surveys have three main species of tick, but only two have been found
using rodents as hosts, Ixodes scapularis
(blacklegged deer tick) and Dermacentor
variabilis (American dog tick). The main character that differentiates
larvae from nymphs is that larvae have 6 legs while nymphs have 8 legs (as well
as the adults). Each species has some unique features that I use to discern
between the two.
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| My lab bench |
The tools I use when identifying ticks are a dissecting microscope,
this is a microscope that doesn’t need a specimen to be prepared on a slide. It
used an external light source to illuminate a whole sample. I manipulate the
ticks mainly with a paintbrush. This is a common tool for people who handle
fragile invertebrates in the lab because it can move and stick to the specimen
but won’t accidentally damage it like a normal pair of forceps. There are also “soft”
forceps that are made from a flexible metal that are useful for samples that
are too heavy for the paintbrush to grab on to. I covered my whole bench in
white bench paper. This helps keep my workspace clean, and if any ticks fall or
get dropped they will show up better on the light background. My scope to chair
hight ratio is still off a bit, the chair is too high so I have to hunch to
look into the dissecting scope, which can be a little painful after a long
time. I’m going to have to figure out the best solution to this problem (a
shorter chair or raising up the scope on a platform or something).
Dermacentor is the
most common tick we’ve found on the rodents, from our previous surveys and from
the data I’ve collected from this past year so far. Its mouthparts are somewhat
rounded, the body and legs are a light brown color, and the shield on the back
(dorsal) side comes away from the body in a straight line.
Ixodes seems to be
somewhat less common on these hosts, but still very present. This is of
particular interest to many because this species can carry Borrelia
burgdorferi, the bacteria that causes Lyme disease. These ticks have longer, straighter
mouthparts, are usually darker, blacker in color (the legs in particular,
especially in the larvae), and the shield is rounded all the way around. The
picture I have below compares a nymph from each species. See if you can see
some of the differences I listed.
These little rodents are kind of teeming with parasites, and
ticks are the only ectoparasites I collected from them. Fleas and mites were
fairly common, but are harder to collect because they don’t attach to the host
so they are freely moving through the host’s hair and around the body while you
are trying to grab them with forceps. These parasites are important because
they can also carry pathogens that can infect wildlife and humans (remember theplague post?). Analyzing the blood samples we collected in 2009 showed that many
hosts are infected with Bartonella, a flea-borne pathogen, so fleas may be
really important in this disease community.
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| Flea from a mouse, most likely Orchopeas leucopus. |
Fleas and mites look really different from ticks and each other.
I know fewer details about these creatures, but I do know they look pretty
nasty. Fleas are covered in these little hairs which make them really sticky to
the host fur (and hard to handle with the paintbrush, my usual tool for this). They look like they're "swimming" through the hair when you see them on a host, its pretty crazy.
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| I couldn't get the whole mite in focus at once. The left picture shows its arms in focus, while the right picture shows the little hairs that cover its back. |
I think mites look like little monsters with their front set of legs reaching
above their head. I’m sure many of you think these guys all look like monsters,
but I get so used to looking at ticks I’m not phased by them anymore (what a
strange state to be in, huh?).
If I haven't completely grossed you out with this post, stay tuned for more on parasites!
