Essentials of Anatomy and Physiology

Plasmodiumspecies are becoming increasingly resist-
ant to the standard antimalarial drugs, which are used
to prevent disease as well as cure it. Work is progress-
ing on several malaria vaccines, and some have shown
promise.
Medications are available that can treat most proto-
zoan infections. Intestinal protozoa, for example, may
be treated with metronidazole or furazolidone.

WORMS (HELMINTHS)
(SEE TABLE 22–7)

Most worms are simple multicellular animals. The
parasitic worms are even simpler than the familiar
earthworm, because they live within hosts and use the
host’s blood or nutrients as food. Many of the parasitic
worms have complex life cycles that involve two or
more different host species.
The flukes are flatworms that are rare in most of
North America but very common in parts of Africa
and Asia. People acquire these species by eating
aquatic plants or raw fish in which the larval worms
have encysted. Within the person, each species lives in
a specific site: the intestine, bile ducts, or even certain
veins. Although rarely fatal, these chronic worm infes-
tations are often debilitating, and the host person is a
source of the eggs of the fluke, which may then infect
others.
Tapeworms are also flatworms (Fig. 22–6). Some
are 10 to 15 feet long, and the fish tapeworm can be as

long as 60 feet. They are as flat as a ribbon, however,

and one could easily be held in the palm of the hand.

The tapeworm holds on to the lining of a host’s small

intestine with the suckers and hooks on its scolex

(front end). The segments, called proglottids, are

produced continuously in most species and absorb

nutrients from the host’s digested food. The only

function of the proglottids is reproduction: Eggs in

one segment are fertilized by sperm from another

segment. Mature proglottids containing fertilized

eggs break off and are excreted in the host’s feces. An

intermediate host such as a cow or pig eats food con-

taminated with human feces, and the eggs hatch

within this animal and grow into larval worms that

encyst in the animal’s muscle tissue. People become

infected by eating poorly cooked beef or pork that

contains cysts.

Parasitic roundworms of people include hook-

worm, pinworm, Ascaris, and Trichinella (see Table

22–7 at the end of this chapter). Medications are avail-

able that can eliminate worm infestations. In endemic

areas, however, reinfestation is quite common.

ARTHROPODS (SEE TABLE 22–8)

Arthropods such as the scabies mite and head lice are

ectoparasitesthat live on the surface of the body.

The infestations they cause are very itchy and uncom-

fortable but not debilitating or life threatening

(Fig. 22–7). Of greater importance are the arthropods

514 An Introduction to Microbiology and Human Disease

F Babesia microti

(x 1200)

F Babesia microti

(x 1000)

E Toxoplasma gondii

(x 1200)

D Plasmodium
(x 500)

A Entamoeba histolytica
(x 800)

B Giardia lamblia

(x 1200)

C Trypanosoma

(x 500)

Red blood

cells

Red blood

cells

Figure 22–5. Protozoa.

QUESTION:In which human

organ system do Entamoeba

andGiardialive?