The Weird, Wild

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The Weird, Wild World of Parasites
Part 1
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Parasitologists do not usually include bacteria,
fungi, or viruses in what they study, but of
course many bacteria and fungi, and all viruses,
fit into the definition of parasites
organisms that benefit at the expense of other
organisms. It is quite easy to get bogged down
trying to find a universally acceptable
definition of what a parasite is. Lets simplify
things and just proceed according to what is most
commonly studied by parasitologists. The most
typical definition of parasite includes such
organisms as protists, flatworms, nematodes, and
arthropods that live in or on other organisms.
Humans are hosts to over a hundred different
kinds of parasites (not all at the same time!).
Just about any wild (or domestic) animal you
encounter will have parasites. Even most people
that live in quite sanitary environments have at
least some parasites on or in them. We will
emphasize the most common and/or harmful
examples. Note that leeches are omitted as
parasites they are more-or-less temporary
visitors that feed on blood but are not permanent
residents. I guess the same could be said for
ticks and fleas.
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It has been only fairly recently that the
importance of parasites in just about all aspects
of life on Earth has been appreciated by
evolutionary and ecological scientists. This fact
still is not common knowledge among most other
people, including many scientists. An
evolutionary biologist at the University of
Chicago named Leigh Van Valen has proposed an
interesting notion about the significance of
parasites. He calls it the Red Queen Hypothesis
(named for the Red Queen that Alice met in the
Garden of Live Flowers, where the queen said, it
takes all the running you can do, to keep in the
same place.) In this scenario parasites are a
constant driving force in evolution. Sexual
reproduction is thought to be vital in the
on-going struggle between hosts and parasites
the sexual process creates completely new genetic
combinations, some of which confer an advantage
against potential parasites. Of course, the
parasites often can catch up, inasmuch as they
generally reproduce at a much faster pace, and,
they too create new genetic combinations through
sexual reproduction. A fascinating and excellent
book about parasites is Parasite Rex by Carl
Zimmer.
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Katherine Hepburn checks out Humphrey Bogarts
leech collection. (Some say leeches are not
parasites, but they look like it to me.)
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Blood-sucking ticks carry several rickettsial
diseases such as Lyme disease and Rocky Mountain
Spotted Fever. Rickettsias are very small
prokaryotic organisms. Of course, one really
killer bacterial disease, bubonic plague, is
transmitted by fleas from rats.
Lyme disease vector
Rocky Mountain Spotted Fever vector
A gang of fleas having dinner together.
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Protists
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The most deadly parasite for humans has been a
protist (Plasmodium) that causes malaria. More
people have died of malaria than any other cause.
Even now malaria kills 1-2 million people each
year. A fairly close relative, Toxoplasma, is
much less dangerous. In fact, almost everyone has
it living in them without any noticeable effects.
It is only a problem to those with weak immune
systems fetuses and AIDS sufferers. Other
protists shown here include Giardia,
Cryptosporidium (which killed over 200 people in
Milwaukee), and Trypanosoma (African sleeping
sickness, Chagas disease). There are many others
that could have been included. Many parasites,
not just protists, have different parts of their
life cycle in different kinds of hosts. As you go
through the examples, note the different hosts
involved.
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Anopheles mosquito and Plasmodium (causative
agent of malaria) in and among red blood cells.
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Life cycle of Toxoplasma gondii (Toxoplasmosis)
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Toxoplasma is found in many people without any
symptoms exceptions are fetuses and AIDS
sufferers.
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Giardia commonly found in swimming pools lacks
mitochondria and has an amusing appearance.
Causes intense diarrhea and is readily
transmitted within households.
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Cryptosporidium in human intestinal lining. This
protist sickened thousands and killed 200 in
Milwaukee in 1993.
Cryptosporidium merozoites breaking out
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Blood-sucking conenose (Triatoma) - vector for
Chagas disease (Trypanosoma cruzi).
It is now thought that Charles Darwin
contracted this, causing him to be a near invalid
most of his life.
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Life cycle of Trypanosoma spp. cause of African
Trypanosomiasis (sleeping sickness)
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Tsetse fly (Glossina)
Causative agent of African sleeping sickness
(Trypanosoma, sp.)
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Flatworms
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Some flatworms are free-living animals such as
the planaria. However, most flatworms are
parasites such as flukes and the tapeworms.
Most flukes and tapeworms have quite complex life
histories with multiple hosts. In many tapeworms
there is an alternation between herbivore and
carnivore hosts. The herbivore gets the parasite
by consuming plants on which fecal wastes bearing
eggs or other stages have been deposited. Once in
the herbivore host the parasite often forms cysts
within muscles or other tissues it is passed on
when the host is eaten by a carnivore. In the
carnivore the parasite forms the adult tapeworm
that lives in the intestine. If a human ingests
eggs, etc., the result can be devastating,
inasmuch as the parasite can form cysts just
about anywhere heart, lung, brain, bones, etc.
You can see the result on a subsequent slide
showing hydatid cysts of Echinococcus, the dog
tapeworm.
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Life cycle of Fasciola hepatica (Sheep Liver
Fluke)
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Fasciola 2 species of sheep liver fluke
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Fasciola miracidium (from eggs in water
penetrate snail host)
Immature stages of Fasciola
Fasciola cercaria (penetrate vertebrate host in
water)
Fasciola redia (emerge from sporocysts in snail)
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Once a male finds a female he never lets her go
even if she drops dead. (live in large veins in
abdomen)
Cercaria (larva) of Schistosoma
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Taenia solium scolex (how it holds on in the
gut)
Tapeworm cyst in human brain
Taenia solium cysts in human eye (cysts can be
just about anywhere in body)
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Echinococcus dog tapeworm. Actual size is only
3 to 6 mm in length.
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Hydatid cysts (Echinococcus) in a monkey
Each cyst is filled with thousands of small
parasite units.
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Hydatid cyst in human brain (left)
Human kidney with hydatid cysts (arrows)