WORLD OF MICROBIOLOGY AND IMMUNOLOGY Parasites423•
Typically, a parasitic infection does not directly kill a
host, though the drain on the organism’s resources can affect
its growth, reproductive capability, and survival, leading to
premature death. Parasites, and the diseases they cause and
transmit, have been responsible for tremendous human suffer-
ing and loss of life throughout history. Although the majority
of parasitic infections occur within tropical regions and among
low-income populations, most all regions of the world sustain
parasitic species, and all humans are susceptible to infection.
Although many species of viruses, bacteria, and fungi
exhibit parasitic behavior and can be transmitted by parasites,
scientists usually study them separately as infectious diseases.
Types of organisms that are studied by parasitologists include
species of protozoa, helminths or worms, and arthropods.
Protozoa are one-celled organisms that are capable of
carrying out most of the same physiological functions as mul-
ticellular organisms by using highly developed organelles
within their cell. Many of the over 45,000 species of known
protozoa are parasitic. As parasites of humans, this group of
organisms has historically been the cause of more suffering and
death than any other category of disease causing organisms.
Intestinal protozoa are common throughout the world
and particularly in areas where food and water sources are
subject to contamination from animal and human waste.
Typically, protozoa that infect their host through water or food
do so while in an inactive state, called a cyst, where they have
encased themselves in a protective outer membrane, and are
released through the digestive tract of a previous host. Once
inside the host, they develop into a mature form that feeds and
reproduces.
Amebic dysenteryis one of the more common diseases
that often afflicts travelers who visit tropical and sub-tropical
regions. This condition, characterized by diarrhea, vomiting
and weakness, is caused by a protozoan known as Entamoeba
histolytica.Another protozoan that causes severe diarrhea, but
is also found in more temperate regions, is Giardia lamblia.
Among Leeuwenhoek’s discoveries was G. lamblia,which is
a now well-publicized parasite that can infect hikers who
drink untreated water in the back country.
Other types of parasitic protozoa infect the blood or tis-
sues of their hosts. These protozoa are typically transmitted
through another organism, called a vector, which carries the
parasite before it enters the final host. Often the vector is an
invertebrate, such as an insect, that itself feeds on the host and
passes the protozoan on through the bite wound. Some of the
most infamous of these protozoa are members of the genera
Plasmodium,that cause malaria; Trypanosoma, that cause
African sleeping sickness; and Leishmania,which leads to a
number of debilitating and disfiguring diseases.
Helminths are worm-like organisms of which several
classes of parasites are found including nematodes (round-
worms), cestodes (tapeworms), and trematodes (flukes).
Leeches, of the phylum Annelid, are also helminths and con-
sidered as ectoparasitic, attaching themselves to the outside
skin of their hosts. Nematodes, or roundworms, have an esti-
mated 80,000 species that are known to be parasitic. The gen-
eral morphology of these worms is consistent with their name;
they are usually long and cylindrical in shape. One of the mostinfamous nematodes is Trichinella spiralis,a parasite that
lives its larval stage encysted in the muscle tissue of animals,
including swine, and make their way into the intestinal tissue
of humans who happen to digest infected, undercooked pork.
Arthropods are organisms characterized by exoskele-
tons and segmented bodies such as crustaceans, insects, and
arachnids. They are the most diverse and widely distributed
animals on the planet. Many arthropod species serve as carri-
ers of bacterial and viral diseases, as intermediate hosts for
protozoan and helminth parasites, and as parasites themselves.
Certain insect species are the carriers of some of
humanity’s most dreaded diseases, including malaria, typhus,
and plague. As consumers of agricultural crops and parasites
of our livestock, insects are also humankind’s number one
competitor for resources.
Mosquitoes, are the most notorious carriers, or vectors,
of disease and parasites. Female mosquitoes rely on warm-
blooded hosts to serve as a blood meal to nourish their eggs.
During the process of penetrating a host’s skin with their long,
sucking mouth parts, saliva from the mosquito is transferred
into the bite area. Any viral, protozoan, or helminth infections
carried in the biting mosquito can be transferred directly into
the blood stream of its host. Among these are malaria, yellow
fever, W. bancrofti(filariasis and elephantiasis), and D. immi-
tis(heartworm).
Flies also harbor diseases that can be transmitted to
humans and other mammals when they bite to obtain a blood
meal for themselves. For example, black flies can carry river
blindness, sandflies can carry leishmaniasis and kala-azar, and
tsetse flies, found mainly in Africa, carry the trypanosomes
that cause sleeping sickness. Livestock, such as horses and
cattle, can be infected with a variety of botflies and warbles
that can infest and feed on the skin, throat, nasal passages, and
stomachs of their hosts.
Fleas and lice are two of the most common and irritat-
ing parasitic insects of humans and livestock. Lice commonly
live among the hairs of their hosts, feeding on blood. Some
species are carriers of the epidemic inducing typhus fever.
Fleas usually infest birds and mammals, and can feed on
humans when they are transferred from pets or livestock. Fleas
are known to carry a variety of devastating diseases, including
the plague.
Another prominent class of arthropods that contains
parasitic species is the arachnids. Though this group is more
commonly known for spiders and scorpions, its parasitic
members include ticks and mites. Mites are very small arach-
nids that infest both plants and animals. One common type is
chiggers, which live in grasses and, as larva, grab onto pass-
ing animals and attach themselves to the skin, often leading
to irritating rashes or bite wounds. Ticks also live their adult
lives among grasses and short shrubs. They are typically
larger than mites, and it is the adult female that attaches itself
to an animal host for a blood meal. Tick bites themselves can
be painful and irritating. More importantly, ticks can carry a
number of diseases that affect humans. The most common of
these include Rocky Mountain spotted fever, Colorado tick
fever, and the latest occurrence of tick-borne infections,
Lyme disease.womi_P 5/7/03 11:08 AM Page 423