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Roundworm
parasites affect most species of animals and plants, making them
important agricultural pests. There are also several
species of roundworms that live in humans, some nasty, some
not. Here are a few...
Trichinella
spiralis is also a parasite
that has been around for a while, since it has probably been
responsible for several cultures long-standing dietary laws. Trichinella
can be found around the world, more in temperate zones than the
tropics, mostly in various animals that eat meat, from rats to
bears. Humans most common exposure comes from pork, and pigs
commonly pick it up from eating rats. These worms can live as
juveniles in muscle and other tissues while adults occupy support
tissues and the lymphatic system. A new host becomes infected
by eating tissue containing juveniles. Juveniles become adults
and mate in the new hosts intestines, then females bore out of the
intestines, which can cause a wide range of serious symptoms, settle
someplace and begin to release juveniles, which migrate all over the
body, causing damage as they go, until they mostly coil up in muscle
tissues and "wait" for the host to be eaten by a new host.
Hookworms
infect a variety of mammals, with species in cows, dogs,
cats, and others, as well as humans. Hookworms are fairly
host-specific - worms of non-human hosts cant live long in a
human. You definitely dont want to catch one of the
hookworms specific to humans, though. They have a very
unusual habit for worms that live in the intestine: instead of
living on all of the food around them, they bite through the
intestinal lining and live on blood. Serious infections occur
when bacteria from the intestines get into the surrounding tissues
and/or the blood, and heavy infections can produce enough blood loss
to cause anemia. Its no wonder that one genus is called Necator,
or "killer"! Hookworm eggs pass in feces, and
juveniles live for a while in the soil if its nice and wet.
The worms get into the next host either by latching on and boring
through their skin, or sticking to paws and getting licked
off. If they come in through the skin, they get into the blood
and migrate to the intestines, usually by way of the lungs,
sometimes causing tissue damage as they go. Hookworms for
non-human hosts that penetrate human skin by mistake can wander
under the skin, unable to penetrate further, but the bodys reaction
to them can cause a condition sometimes called
creeping eruption.
Ascaris
is an
impressively large worm, up to 50 centimeters long and
about as thick as a pencil, that lives in human intestines, with
maybe as much as a quarter of the worlds population infected.
They are
taken in as accidentally-swallowed eggs, hatch in the
intestine, and the juveniles bore out, get into the blood, wander
the body (where they can cause problems), emerge in the lungs, grow
there for a while (and possibly cause problems), then migrate up to
be swallowed and get back to the intestines again, where they
mate. Females find males by touch (its dark in an intestine)
and crawl into the males hooked tail for mating; sometimes
they mistake the opening of the ducts from the liver or pancreas for
a males tail and get caught, blocking the flow of digestive
juices. A
heavy infection can produce a
knot of worms that
blocks movement of materials through the intestine. Females
that cant find males have been known to migrate up or down the
canal, reaching the
nose or
anus in some cases - quite a surprise
for the host! Females lay eggs that pass in feces. The
eggs can remain infective in the environment for years, long after
that fecal material has been broken down. Dirty hands in the
mouth explains why children are the most common hosts for these
worms.
Filarial worms are a group of
roundworms that commonly use biting insects to get juveniles from
host to host, then the adults live in the fluid systems - blood or
lymph systems - of the final host. There are several filarial
worms that infect humans, including
Wuchereria,
which can block fluid drainage through the lymph system,
causing gross
swelling of tissues and a form of
elephantiasis.
Onchocerca causes a disease called
river blindness when juvenile
worms enter and gradually
damage the
eyes (the "river" part is due
to the biting fly carriers being tied to rivers for breeding,
restricting the
geographical range of the disease).
Heartworms
are filarial parasites of dogs and cats. The juvenile
worms are carried by mosquitos, and the adults settle in the
chambers and major vessels of the heart. Heartworms do not
generally infect humans.
Dracunculus
medinensis, also called guinea worms, have been known
and written about for centuries (although often called
"serpents" in modern translations), including passages
from Ancient Greek scholars and from the Bible. Adult worms
can be as long as a meter, although they are very thin. As
adults, they live in the tissues under the skin, usually somewhere
at and below the hips, where they may be visible as a white
line. After mating, a female produces huge numbers of eggs
that hatch inside her and begin to migrate out into the surrounding
tissue, often causing an allergic reaction with inflammation and
ulceration of the skin (some ancient texts call them the "fiery
serpents" from their effects on the skin). When the skin
breaks, many many tiny juvenile worms may emerge. An opening
remains in the skin through which the female will continue to
release young. To continue their
life
cycle, the juvenile worms must get into open water and infect a
tiny crustacean; for this reason, worms are most active when
the skin is wet. Ancient treatments, still used in some
places, involve cutting a thin slot in a stick, wetting the skin so
the worm sticks out,
catching the writhing worm in the sticks slot, then winding it slowly out
from under the skin. The medical symbol, the
caduceus,
of a
snake or
snakes wrapped around a pole,
most likely is taken
from one of the few effective devices ancient doctors had, a
worm-removal stick (worm, snake; remember, in ancient classification schemes not
much distinction was made among long wriggly things). Worms infect the next host when
water
containing infected crustaceans is drunk; the juveniles leave
their carriers in the intestine, bore out of the intestine and
migrate to their position under the skin. Humans are affected
both by their allergic reactions to the released juveniles,
infections from bacteria that enter through the broken skin, and
worms that stall in deeper tissues, where they may cause serious
damage. Because there is a fairly simple preventative measure
- physically filtering drinking water -
this parasite is very close to being eliminated.
Pinworms
are an interesting case for a few reasons. First, although
they live inside animals, they may not be parasites, living instead
on cells and materials that would be discarded anyway (this, if
true, makes them commensals, benefiting from another
organism but neither harming nor helping the host). They also
live in the large intestine or colon, which animal
"hitchhikers" almost never do (its not the most
nutrient-rich environment as well as being very oxygen-poor). They are by far the
most common human roundworm in North America and Europe - some
estimates put the infection rate at as much as 75%, and upper class
folks are more likely to be infected, another unusual feature
or these worms. Females ready to lay eggs migrate to the anus,
crawl out, and deposit eggs behind them as they crawl around on the
surface around the opening. The most common symptom of
pinworms is
itching - although these worms are probably not
more common in children, infected children are often are more
likely to show symptoms because they are willing to scratch that area in the
presence of others when it itches. Pinworm eggs are incredibly
tiny - regular vacuum cleaner bags wont generally catch them - and
they float off from the skin surface, even through clothing, as dust
motes and spread everywhere. Picking up eggs, which remain
infective for from one to two weeks, from where they have settled on
a surface and accidentally swallowing them will infect the next
host. This worm is almost impossible to get rid of - even if
you were to be treated, thoroughly cleaned your clothes and left
your house until the eggs there were not viable, you would probably
soon pick more eggs up from somewhere else.
This was only a partial list of roundworms that
live in humans.
There is another roundworm
with a more pleasant pedigree:
Caenorhabditis
elegans is one of the most extensively-studied animals
ever. It, like fruit flies, lab mice, yeast, and mustard
plants, is used as a model organism, which means
scientists study it to answer "Big Questions" about life
in general. C. elegans
(thats how a species is abbreviated) has been used to study many
processes, but probably its most important role has been in the
study of embryo development, especially
HOX genes or homeogenes,
which are responsible for determining the basic layout of an
organism, such as where the front end will be, how the right side
will be different from the left side, how, when, and where organs
will develop, et cetera. C. elegans, like
several roundworm species, has adults with a set number of cells -
in C. elegans the number is 1090 - which means tracking
development from the very first cell and following each new cells
track to the final number is simplified, and can lead to figuring
out the genetics of the developmental process. |
