Download Chapter 23 – Eukaryotic Parasites of Medical Importance I

BIOL 2320
J.L. Marshall, Ph.D.
Chapter 23 – Eukaryotic Parasites of Medical Importance: Protozoa and
Helminths*
*Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for
the exams.
Parasitology covers protozoa and helminths (worms) that live on or in a host body. Parasites are spread to
humans by other humans, animal hosts, and vectors. These notes focus on representative species and genera
and are by no means comprehensive. Chapter figures are especially helpful in following the complex life
cycles of these organisms. Section 5.7 and 5.8 and some figures in the text chapter 5 may be helpful.
23.1 The Parasites of Humans
Parasitology is the study of eukaryotic parasites. Pathogenic protozoa are a diverse group of unicellular,
eukaryotic organisms which cause a variety of diseases (Table 23.1).
23.2 Major Protozoan Pathogens
Some protozoans propagate only as trophozoites, the active feeding stage found in hosts, whereas others
alternate between a trophozoite and a cyst stage (cysts are dormant, stress resistant forms (fig. 5.23)). While
many reproduce asexually, some have more complex life cycles with sexual and asexual phases that can be
carried out in multiple hosts. Table 23.2 lists the more common drugs used for protozoan infections.
Infective Amoebas
Amoebas are motile by means of pseudopodia – cytoplasmic extensions which allow it to crawl across
surfaces.
A. Entamoeba histolytica is the agent of amebiasis, or amebic dysentery. This type of infection is spread
worldwide and affects some 500 million people in the tropics alone. The organism alternates between
trophozoite and cyst stages (fig. 23.1). Cysts released in feces are carried and spread through unsanitary
water and food. See Pathogen Profile #1.
Ingested cysts transform into trophozoites that invade the large intestine causing ulceration and dysentery
(fig. 23.2 and fig. 23.3).
Treatment: metronidazole1, fiodoquinol, and chloroquine are effective in
eliminating the organism.
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Metronidazole (Flagyl™) is the most common treatment for protozoal infections and, unusually, for Clostridium difficile infections
also.
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B. Naegleria fowleri is a common free-living amoeba (fig. 23.4) that only infects humans accidentally. Found
in fresh or brackish (mildly salty) water, ponds, lakes, and occasionally swimming pools and hot tubs
(especially if improperly chlorinated), the amoeba are forced into the nasal passages during swimming. It can
burrow through the nasal mucosa and infect the CNS resulting in acute meningitis. The hemorrhage almost
always proves fatal.
Treatment: the infection proceeds so quickly that usually no treatment is effective.
23.3 The Flagellates (Mastigophorans)
Trichomonads: Trichomonas Species
The principal human parasite of this group is Trichomonas vaginalis, the cause of a common sexually
transmitted disease known as trichomoniasis that infects the vagina, cervix, and urethra (in both sexes)(fig.
23.6). Painful, itchy urethritis with discharge. Approximately 50% carriers are asymptomatic.
Treatment: metronidazole.
Giardia intestinalis and Giardiasis
Giardia lamblia is an intestinal parasite that causes giardiasis (fig. 23.7). G. lamblia’s natural reservoir is
animal intestines, and the source of human infection is cyst-containing fresh water and food. Symptoms of
giardiasis: severe diarrhea (may be chronic in immunocompromised patients), abdominal pain, and flatulence.
Giardiasis is preventable by disinfecting water (boiling or iodine), but chlorine at the concentrations used in
swimming pools does not kill the cysts.
Treatment: responds well to metronidazole or quinacrine.
Hemoflagellates: Vector-Borne Blood Parasites
Hemoflagellates, as the name suggests, are protozoans that occur in blood infections. Mostly tropical
zoonoses spread by insect vectors. Most of these pathogens have complex life cycles with various stages
maturing in insect and human (or other animal) hosts (Table 23.3).
(i) Trypanosoma – have tapering, flagellated cells.
T. brucei – agent of African sleeping sickness (fig. 23.8). Vector: tse-tse fly. The bite of the fly inoculates the
skin; the trypanosome invades the circulatory system where it multiplies causing damage to the spleen, lymph
nodes, and brain. Chronic symptoms include: sleepiness, tremors, paralysis, and coma.
T. cruzi – agent of Chagas disease (endemic to Central and South America) (fig. 23.9). Vector: reduvid (kissing)
bug. Insect feces inoculate a cutaneous portal (cuts & scrapes or the bite site of the insect). Also know to be
transmitted via insect feces in food. Mature trypanosomes invade circulatory system causing chronic
inflammation in organs (esp. the heart and brain) (fig. 23.10). Treatment is problematic; disease is lifethreatening. Possible emerging disease in Texas.
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Can cross the placenta and cause congenital disease.
(ii) Leishmania – have a single, anterior flagellum
Transmitted by blood sucking biting flies, vector: sand flies. Found in equatorial regions. Upon biting the host,
leishmania will reside in macrophages (fig. 23.11). The infection can be local or systemic.
23.4 Apicomplexan Parasites
Very small, intracellular parasites that have complex life cycles and lack motility in their mature stage.
Infectious forms include sporozoites, fecal cysts called oocysts, and tissue cysts. Most diseases are zoonotic
and vector-borne.
Plasmodium: The Agent of Malaria
Plasmodium causes malaria – a life threatening disease. Four species within the genus Plasmodium are of
concern. Humans are a primary host of the asexual phase of the parasite; the female Anopheles mosquito is
the vector/host for the sexual phase. Distributed in a geographic belt around the equator (fig. 23.12), an
estimated 300 to 500 million cases/year of malaria are diagnosed (with at least 2 million deaths of young
people and infants every year). The U.S. sees approximately 1,000-2,000 cases a year, most of which occur in
immigrants. See Pathogen Profile #2.
Sporozoites enter the human circulatory system with mosquito saliva, penetrate liver cells, multiply, and form
hundreds of merozoites. The merozoites infect, multiply in, and lyse red blood cells (fig. 23.13). Figure 23.13
illustrates the life cycle of Plasmodium.
Symptoms: episodes of chills-fever-sweating, anemia, and organ enlargement.
Treatment: chloroquine, quinine, or primaquine
Coccidian Parasites
T. gondii lives naturally in cats that harbor oocysts in their GI tract (fig. 23.15). Toxoplasma gondii causes
toxoplasmosis in humans. Acquired by ingesting raw or rare meats containing tissue cysts or by accidentally
ingesting oocysts from substances contaminated by cat feces. In humans, infection is usually mild and flu-like
EXCEPT in immunodepressed patients (esp. AIDS) where the infection spreads to the CNS, causing brain
damage and death. Can cross the placenta and cause fetal toxoplasmosis wherein the fetus will suffer brain
and heart damage; for this reason:
Pregnant women should NEVER handle cat litter boxes.
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Treatment: pyrimethamine and sulfadiazine (alone or in combination; this is another combination that blocks
folate synthesis).
23.5 A Survey of Helminth Parasites
Parasitic helminths, or worms, are multicellular animals with a wide variety of special adaptations for
parasitizing human hosts including: special mouth parts (hooks), degenerative organ structures (they don’t
waste energy with complex metabolisms when the host will digest food for them), and complex life cycles (fig.
23.17). Adult worms reproduce sexually producing eggs which hatch into larvae. Larvae then mature into
adults in a multi-stage process. In some worms the sexes are separate, and in others they are hermaphroditic
(both sexes in one organism). See Systems Profile 23.2.
See Table 23.4, for major helminth infections of humans and their modes of transmission. (Don’t memorize
the whole thing…it’s just a good resource)
Prevention: Many of the species which are acquired through the consumption of improperly cooked meats
can be prevented by adequate cooking. Water sources can also be purified of contaminating species by
boiling or other treatment (e.g. iodine, chlorination). It is almost always a good recommendation to cook
meats well done.
Treatment:
 Mebendazole is a broad-spectrum antihelminth agent used to treat intestinal roundworm, hookworm,
and trichinosis.
 Niclosamide is specifically used for tapeworm infections
 Pyrantel is for pinworm (Enterobius vermicularis) and hookworm infections. (OTC = Pin-Rid™).
See Table 23.5, for a more comprehensive list of antihelminthic therapeutic agents.
Intestinal Nematodes – Cycle A
Nematodes, or round worms, are filamentous with protective cuticles, circular muscles, complete digestive
tracts, and separate sexes with well-developed reproductive systems.
A. Ascaris lumbricoides (fig. 23.18)
These intestinal nematodes are a very prevalent species indigenous to humans. Eggs ingested with food hatch
into larvae and burrow through the intestine into the circulation. From there they travel to the lungs and
pharynx and are swallowed. Back in the intestine, the adult worms complete the reproductive cycle.
B. Enterobius vermicularis
Also known as pinworm, this is a common childhood infection confined to the intestine. Symptoms include
anal itching as worms exit the body (defecation is not necessary, i.e. they crawl out).
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Intestinal Nematodes – Cycle B
C. Hookworms
Hookworms have characteristic curved ends and hooked mouths (fig. 23.19). Two major species are: Necator
americans (Western hemisphere) and Anclostoma duodenale (Eastern hemisphere). Both species share a
similar life cycle:
Humans shed eggs in feces, which hatch into larvae. Larvae burrow into the skin of the feet and the lower legs
and travel from blood to lungs/pharynx and are swallowed. Adult worms reproduce in the intestine and
complete the cycle.
D. Trichinella spiralis
T. spiralis causes trichinosis, a zoonosis in which humans are dead-end hosts. T. spiralis is acquired from
eating undercooked or raw pork (or bear) containing encysted larvae. Larvae migrate from the intestine to
blood vessels, muscle, heart, and CNS where they enter dormancy (fig. 23.20). Initial symptoms are flu-like:
diarrhea, nausea, abdominal pain, and fever. Life-threatening if the brain and heart are infected. See
Pathogen Profile #3.
Treatment: poor prognosis once the larvae have encysted in muscles.
23.7 Flatworms: The Trematodes and Cestodes
Trematodes, or flukes, are flatworms (platyhelminthes) with leaf-like bodies bearing suckers & hooks on the
mouth and degenerated digestive systems. Most are hermaphroditic.
Blood Flukes: Schistosomes – Cycle D
Blood flukes like Schistosoma species cause schistosomiasis (fig. 23.22). Prevalent in the tropics with an
estimated 100 million active cases at any given time. Adult flukes live in humans and release eggs (via feces or
urine) into water sources. The early larvae develop in the freshwater snail into a second larvae which can
penetrate human skin (people report a tingling sensation when swimming in water heavily contaminated with
this organism). They then migrate to the liver where they mature. Adults migrate to intestine or bladder
(where they can cause bladder obstruction, resulting in blood in the urine) and shed eggs causing chronic
organ enlargement.
Cestodes (Tapeworm) Infections – Cycle C
These flatworms have long, very thin, ribbon-like bodies composed of units called proglottids and a scolex that
grips host intestine. The scolex isn’t a true mouth, rather, a series of hooks that aid in attachment. Each
proglottid is an independent unit capable of absorbing food (which the human host has conveniently digested
for it) and making and releasing eggs (hermaphrodites).
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A. Taenia saginata
T. saginata is the beef tapeworm for which humans are the definitive host (fig. 23.24 b). Animals are infected
by grazing on land contaminated with human feces. Human infection occurs from eating raw beef in which
the larvae have encysted. Larvae attach to the human intestine and mature into adults. An otherwise healthy
adult human can harbor these worms almost asymptomatically for a long time (even years).
B. Taenia solium
T. solium is the pork tapeworm. Eggs consumed in improperly cooked pork will give rise to larvae which go on
to encyst in many organs, including the brain (fig.23.24 and fig. 23.25).
23.8 The Arthropod Vectors of Infectious Disease
Arthropods are invertebrates that include insects and arachnids. These can be ectoparasites that feed on the
blood of the host, and at the same time transmit an infectious agent. See Table 23.6 for a summary of the
arthropod vectors that harbor infectious agents.
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