Foundations in

Microbiology Seventh Edition

Chapter 23

The Parasites of

Medical Importance

Lecture PowerPoint to accompany

Talaro

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23.1 Parasitology

• The study of eukaryotic parasites, protozoa,

and helminths

• Cause 20% of all infectious diseases

• Less prevalent in industrialized countries;

increasingly common in AIDS patients

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23.2 Typical Protozoan Pathogens

• Single-celled, animal-like microbes, most having some form of motility

• Estimated 100,000 species, approximately 25 are important pathogens

• Life cycles vary

– Most propagate by simple asexual cell division of the active feeding cell (trophozoite)

– Many undergo formation of a cyst

– Others have a complex life cycle that includes asexual and sexual phases

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Infective Amebas

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Entamoeba Histolytica and Amebiasis

• Alternates between a large trophozoite, motile by means of pseudopods and a smaller nonmotile cyst

• Trophozoite has a large nucleus and lacks most other organelles

• Humans are the primary hosts

• Ingested

• Carried by 10% of world population

Figure 23.1 Entamoeba histolytica

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Entamoeba Histolytica

• Cysts are swallowed and arrive at the small intestine;

alkaline pH and digestive juices stimulate cysts to

release 4 trophozoites

• Trophozoites attach, multiply, actively move about and

feed

• Asymptomatic in 90% of patients

• Ameba may secrete enzymes that dissolve tissues and

penetrate deeper layers of the mucosa

• Causing dysentery, abdominal pain, fever, diarrhea,

and weight loss

Figure 23.2 Intestinal amebiasis

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Entamoeba Histolytica

• Life-threatening manifestations are: hemorrhage, perforation, appendicitis, and tumorlike growths, amoebomas

• May invade liver and lung

• Severe forms of disease result in 10% fatality rate

• Effective drugs are iodoquinol, metronidazole, and chloroquine

Figure 23.3 Entamoeba histolytica in specimen

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Amebic Infections of the Brain

• Caused by Naegleria fowleri and Acanthamoeba

• Ordinarily inhabit standing water

• Primary acute meningoencephalitis is acquired

through nasal contact with water or traumatic eye

damage

• Infiltration of brain is usually fatal

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An Intestinal Ciliate: Balantidium Coli

• An occupant of the intestines of domestic animals such as pigs and cattle

• Acquired by ingesting cyst-containing food or water

• Trophozoite erodes intestine and elicits intestinal symptoms

• Healthy humans are resistant

• Rarely penetrates intestine or enters blood

• Treatment – tetracycline, iodoquinol, nitrimidazine or metronidazole

Figure 23.5 (a)

Microscopic views of Balantidium coli

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The Flagellates

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Trichomonads: Trichomonas Species

• Small, pear-shaped

• 4 anterior flagella and an undulating membrane

• Exist only in trophozoite form

• 3 infect humans:

– T. vaginalis

– T. tenax

– T. hominis

Figure 23.6 Trichomonads of humans

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Trichomonas Vaginalis

• Causes an STD called trichomoniasis

• Reservoir is human urogenital tract

• 50% of infected are asymptomatic

• Strict parasite, cannot survive long outside of host

• 3 million cases yearly, a top STD

• Female symptoms – foul-smelling, green-to-yellow discharge; vulvitis; cervicitis; urinary frequency and pain

• Male symptoms – urethritis, thin, milky discharge, occasionally prostate infection

• Metronidazole

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Giardia Lamblia and Giardiasis

• Pathogenic flagellate

• Unique symmetrical heart shape with concave ventral surface that acts like a suction cup

• Cysts are small, compact, and multinucleate

• Reservoirs include beavers, cattle, coyotes, cats, and humans

• Cysts can survive for 2 months in environment

• Usually ingested with water and food

• ID 10 to 100 cysts

Figure 23.7 (a)

Identification of trophozoites

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• Cysts enter duodenum, germinate, travel to

jejunum to feed and multiply

• Causes giardiasis – diarrhea, abdominal pain

• Diagnosis is difficult because organism is shed

in feces intermittently

• Treatment: quinacrine or metronidazole

• Agent is killed by boiling, ozone, and iodine

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Hemoflagellates: Vector-Borne Blood

Parasites

• Obligate parasites that live in blood and tissues of human host

• Cause life-threatening and debilitating zoonoses

• Spread in specific tropical regions by blood-sucking insects that serve as intermediate hosts

• Have complicated life cycles and undergo morphological changes

• Categorized according to cellular and infective stages

Hemoflagellates

• Amastigote: the form lacking a free

flagellum

• Promastigote: the stage bearing a single,

free, anterior flagellum

• Epimastigote: the flagellate stage

• Trypomastigote: large, fully formed stage

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Trypanosoma Species and

Tropanosomiasis

• Distinguished by their infective stage; trypomastigote is an elongate, spindle-shaped cell with tapered ends, eel-like motility

• 2 types of trypanosomiasis:

– T. brucei – African sleeping sickness

– T. cruzi – Chagas disease – endemic to Central and South America

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Trypanosoma Brucei and African

Sleeping Sickness

• Spread by tsetse flies

• Harbored by reservoir mammals

• Two variants of disease caused by 2 subspecies:

– T.b. gambiense – Gambian strain; West Africa

– T.b. rhodesiense – Rhodesian strain; East Africa

• Biting of fly inoculates skin with trypomastigotes, which multiplies in blood and damages spleen, lymph nodes, and brain

Figure 23.8

Cycle of the tsetse fly vector

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Trypanosoma Brucei and African

Sleeping Sickness

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• Chronic disease symptoms are sleep

disturbances, tremors, paralysis, and coma

• Trypanosomes are readily demonstrated in

blood, spinal fluid, or lymph nodes

• Treatment before neurological involvement

melarsoprol, eflornithine

• Control involves eliminating tsetse fly

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Trypanosoma Cruzi

• Causes Chagas disease

• Reduviid bug (kissing bug) is the vector

• Infection occurs when bug feces is inoculated into a cutaneous portal

• Local lesion, fever, and swelling of lymph nodes, spleen, and liver

• Heart muscle and large intestine harbor masses of amastigotes

• Chronic inflammation occurs in the organs (especially heart and brain)

• Treatment nifurtimox and benzonidazole

Figure 23.9 American trypanosomiasis

(Chagas disease)

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Figure 23.10 Conditions associated with

Chagas disease

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Leishmania Species and Leishmaniasis

• Leishmaniasis – zoonosis transmitted among

mammalian hosts by female sand flies that require

a blood meal to produce eggs

• Endemic to equatorial regions

• Promastigotes are injected with sand fly bite,

convert to amastigote and multiply; if macrophage

does not migrate the infection is localized;

systemic if macrophage migrates

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• Cutaneous-oriental sore, Baghdad boil –

localized ulcerated sore

• Espundia – skin and mucous membrane

infection of the head; chronic infection

• Systemic (visceral) – high intermittent

fever; weight loss, enlarged spleen, liver,

and lymph nodes

– Kala azar is the most severe and fatal form if

untreated

Figure 23.11 Life cycle and pathology of

Leishmania species

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23.3 Apicomplexan Parasites

• Sporozoans

• Lack locomotor organelles in the trophozoite state

• Alternate between sexual and asexual phases and between different animal hosts

• Most form specialized infective bodies that are transmitted by arthropod vectors, food, water, or other means

– Plasmodium

– Toxoplasma

– Cryptosporidium

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Plasmodium: The Agent of Malaria

• Dominant protozoan disease

• Obligate intracellular sporozoan

• 4 species: P. malariae, P. vivax,

P. falciparum, and P. ovale

• Female Anopheles mosquito is the primary vector;

blood transfusions, mother to fetus

• 300-500 million new cases each year

• 2 million deaths each year

Figure 23.12 The malaria belt

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2 distinct phases of malarial parasite development:

• Asexual phase – human host – Infected female mosquito injects asexual sporozoite

which localizes in liver; it then undergoes

schizogony generating numerous merozoites which

enter circulation in 5-16 days depending on species

– Merozoites attach to and enter red blood cells,

convert to trophozoites and multiply; red cell bursts

releasing merozoites that differentiate into gametes

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• Sexual phase – mosquito host

– Mosquito draws infected RBCs; gametes

fertilize forming diploid cell which forms

sporozoites in stomach

– Sporozoites lodge in salivary glands; available

to infect human host

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Figure 23.13

Plasmodium

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Plasmodium

• Symptoms include episodes of chills-fever-sweating,

anemia, and organ enlargement

• Symptoms occur at 48-72 hour intervals as RBCs

rupture; interval depends on species

• P. falciparum most malignant type; highest death rate

in children

• Diagnosis by presence of trophozoite in RBCs,

symptoms

• Increasing drug resistance

• Therapy is chloroquine, mefloquine

Figure 23.14 Malaria

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Coccidian Parasites

• Zoonotic in domestic animals and birds

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Toxoplasma Gondii and Toxoplasmosis

• Intracellular apicomplexan parasite with extensive distribution

• Lives naturally in cats that harbor oocysts in the GI tract

• Acquired by ingesting raw meats or substances contaminated by cat feces

• Most cases of toxoplasmosis go unnoticed except in fetus and AIDS patients who can suffer brain and heart damage

• Treatment: pyrimethamine and sulfadiazine

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Figure 23.16

The life cycle

and

morphological

forms of

Toxoplasma

gondii

Figure 23.17 Toxoplasmosis in

the brain of an AIDS patient

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Sarcocystis and Sarcocystosis

• Sarcocystis – parasites of cattle, swine, and sheep

• Domestic animals are intermediate hosts; they pick

up infective cysts while grazing on grass

contaminated with human feces

• Humans are infected when the meat is consumed

• Symptoms include diarrhea, nausea, and abdominal

pain

• No specific treatment

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Cryptosporidium: A Newly

Recognized Intestinal Pathogen

• An intestinal pathogen

• Infects a variety of mammals, birds, and reptiles

• Exists in tissue and oocyst phases

• 1990s – 370,000 cases in Milwaukee, WI, due to contaminated water; filtration required for removal

• Ingestion of oocysts gives rise to sporozoites that penetrate intestinal cells

• Causes gastroenteritis, headache, sweating, vomiting, abdominal cramps, diarrhea

• AIDS patients may suffer chronic persistent diarrhea

• No effective drugs

Figure 23.18 Other apicomplexan parasites

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Isospora Belli and Coccidiosis

• Intracellular intestinal parasite with oocyst stage

• Transmitted in fecally contaminated food or drink

• Infection usually asymptomatic or self-limited

• Symptoms include malaise, nausea and vomiting,

diarrhea, fatty stools, abdominal cramping, and

weight loss

• Treat with sulfadiazine and pyrimethamine, when

required

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Cyclospora Cayetanensis and

Cyclosporiasis

• Emerging protozoan pathogen; causes cyclosporiasis

• Oral-fecal transmission; fresh produce and water

• Oocysts enter small intestine and invade the mucosa

• Symptoms of watery diarrhea, stomach cramps,

bloating, fever, muscles aches

• Diagnosis can be complicated

• Treatment: trimethoprim and sulfamethoxazole

Figure 23.19 Cyclospora in a human

fecal sample

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Babesia Species and Babesiosis

• First protozoan found to cause a disease –

redwater fever of cattle

• First protozoan found to be associated with

a vector – tick

• Human babesiosis – relatively rare zoonosis

• Associated with infected rodents

• Infection resembles malaria

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23.4 A Survey of Helminth Parasites

• Adults are large, multicellular animals with specialized tissues and organs

• Adult worms mate and produce fertilized eggs that hatch; larvae then mature in several stages to adults

• The sexes may separate or hermaphroditic

• Adulthood and mating occur in the definitive host

• Larval develop occurs in the intermediate host

• A transport host experiences no parasitic development

• Four basic patterns of life and transmission

Figure 23.20 Four helminth cycles

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Helminths

• Pathology arises from worms feeding on and migrating through tissues, accumulation of worms, and worm products

• Diagnosis based on blood cell count (eosinophilia), serological tests; eggs, larvae, or adult worms in feces; sputum, urine, blood, or tissue biopsies

• Antihelminthic drugs suppress a helminthic metabolic process that differs from the human process, inhibit the worm’s movement, prevent it from holding position, and act locally in the intestine

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Nematode (Roundworm) Infestations

• Most abundant animal groups; 50 species that affect humans

• Elongated, cylindrical worms with protective cuticles, circular muscles, a complete digestive tract, and separate sexes

• Divided into intestinal nematodes, and tissue nematodes

Figure 23.21 Ascaris lumbricoides

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Ascaris Lumbricoides

• A large intestinal roundworm

• Most cases in the U.S. occur in the southeastern states

• Indigenous to humans

• Ascaris spends its larval and adult stages in humans;

release embryonic eggs in feces, and are spread to other

humans; food, drink, or contaminated objects

• Ingested eggs hatch into larvae and burrow through the

intestine into circulation and travel to the lungs and

pharynx and are swallowed

• Adult worms complete cycle in intestines and reproduce

– 200,000 eggs/day

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Ascaris Lumbricoides

• Worms retain motility, do not attach

• Severe inflammatory reactions mark the

migratory route

• Allergic reactions can occur

• Heavy worm loads can retard physical and

mental development

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Trichuris Trichiura and Whipworm

Infection

• Whipworm

• Humans sole host

• Trichuriasis has its highest incidence in the tropics

• Eggs hatch in intestines, larvae attach, penetrate the outer wall and develop into adults

• Females lay 3,000-5,000 eggs daily

• Worms can pierce capillaries, cause localized hemorrhage

• Heavy infestations can cause dysentery, rectal prolapse – can be fatal in children

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Enterobius Vermicularis and

Pinworm Infection

• Pinworm or seatworm

• Enterobiasis most common worm disease of children in temperate zones

• Eggs are picked up from surroundings and swallowed

• After hatching in the small intestine, they develop into adults

• Anal itching occurs when mature females emerge from intestine to release eggs

• Self-inoculation is common

• Tape test

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Hookworms

• Characteristic curved ends and hooked mouths

• Necator americanus and Ancylostoma duodenale

• Humans shed eggs in feces, which hatch into filariform larvae and burrow into the skin of bare feet

• Larvae travel from blood to lungs, proceed up bronchi and throat and are swallowed

• Worms mature and reproduce in small intestine and complete the cycle

• May cause pneumonia, nausea, vomiting, cramps, and bloody diarrhea

• Blood loss is significant – anemia

Figure 23.22 The hookworms

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Strongyloides Stercoralis and

Strongyloidiasis

• Threadworm

• Tiny roundworms completes life cycle in humans or moist soil

• Larvae penetrate skin and migrate to lungs, are swallowed, and complete development in the intestine

• Can reinfect the same host without leaving the body

• Heavy worm loads can cause pneumonitis and eosinophilia, bloody diarrhea, liver enlargement, bowel obstruction, and malabsorption

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Trichinella Spiralis and Trichinellosis

• Life cycle entirely within mammalian host

• Acquired from eating undercooked pork or bear

meat

• Larvae migrate from intestine to blood vessels,

muscle, heart, and brain, where it forms cysts

• First symptoms – flulike, diarrhea

• Second symptoms – muscle and joint pain, shortness

of breath, pronounced eosinophilia

• No cure after larva have encysted

Figure 23.24 The cycle of trichinellosis

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Tissue Nematodes

• Complete their life cycle in human blood, lymphatics, or skin

• Filarial worms; elongate, filamentous bodies, spread by biting arthropods

• Cause chronic, deforming disease

• Wuchereria bancrofti – elephantiasis

• Onchocerca volvulus – river blindness

• Loa loa – eye worm

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Wucherereia Bancrofti and

Bancroftian Filariasis

• Tropical infection spread by mosquitoes

• Vector deposits larvae which move into

lymphatics and develop into adults

• Chronic infection causes blockage of

lymphatic circulation and elephantiasis,

massive swelling in the extremities

Figure 23.25 Bancroftian filariasis

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Onchocerca Volvulus and River

Blindness

• Transmitted by biting black flies

• Larvae develop into adults in subcutaneous tissues

• Adult females migrate via the blood to the eyes, provoking inflammatory reactions

• Coinfection with Wolbachia bacteria causes river blindness

• Treatment: tetracycline and ivermectin

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Loa Loa: The African Eye Worm

• Spread by bite of small flies

• Temperature-sensitive worm migrates

around/under the skin and may enter the eye

• Treatment – pull worm from a small hole in

conjunctiva or diethylcarbamazine

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Trematodes or Flukes

• Flatworms with ovoid leaflike bodies

• Have digestive, excretory, neuromuscular, and

reproductive systems

• Lack circulatory and respiratory systems

• Animals such as snails or fish are usually the

intermediate hosts and humans are the

definitive hosts

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Blood Flukes: Schistosomes

• Schistosomiasis – prominent parasitic disease

• Schistosoma mansoni, S. japonicum,

S. haematobium

• Adult flukes live in humans who release eggs into

water; early larva (miracidium) develops in

freshwater snail into a 2nd larva (cercaria)

• This larva penetrates human skin and moves into the

liver to mature; adults migrate to intestine or bladder

and shed eggs, giving rise to chronic organ

enlargement

Figure 23.26 Stages in the life

cycle of Schistosoma

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Liver and Lung Flukes

• Zoonotic

Liver flukes:

• Opisthorchis (Clonorchis) sinensis – cycles between mammals and snails and fish; humans are infected by eating inadequately cooked fish containing cercariae, larvae crawl into bile duct, mature, and shed eggs into feces; snail are infected

• Fasciola hepatica – cycles between herbivores, snails, and aquatic plants; humans are infected by eating raw aquatic plants; fluke lodges in liver

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Lung fluke:

• Paragonimus westermani – cycles between

carnivorous animals, snails, and

crustaceans; humans infected by eating

undercooked crustaceans; intestinal worms

migrate to lungs

Figure 23.27 Fasciola hepatica

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Cestode (Tapeworm) Infestations

• Flatworms

• Long, very thin, ribbonlike bodies composed of

sacs (proglottids) and a scolex that grips the

intestine

• Each proglottid is an independent unit adapted to

absorbing food and making and releasing eggs

• Taenia saginata

• Taenia solium

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Taenia Saginata

• Beef tapeworm

• Very large, up to 2,000 proglottids

• Humans are the definitive host

• Animals are infected by grazing on land contaminated with human feces

• Infection occurs from eating raw beef in which the larval form has encysted

• In humans, larva attaches to the small intestine and becomes an adult

• Causes few symptoms; vague abdominal pain and nausea; proglottids in stool

Figure 23.28 Tapeworm infestation

in humans

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Taenia Solium

• Pork tapeworm

• Infects humans through ingesting cysts or eggs

• Eggs hatch in intestine, releasing tapeworm larva that migrate to all tissues and encyst

• Most damaging if they lodge in heart muscle, eye, or brain

• May cause seizures, psychiatric disturbances

Figure 23.29 Cysticercosis

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23.5 The Arthropod Vectors of

Infectious Disease

• Arthropods – exoskeleton and jointed legs;

includes arachnids and crustaceans; many

must feed on blood and tissue fluid of host

during life cycle; ectoparasites

• Those of medical importance transmit

infectious microbes in the process of

feeding – biological vectors

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Insects

• Mosquitoes – require an aquatic habitat; females take blood meal transmitting disease: malaria, filariasis, Dengue fever

• Fleas – highly motile, flattened bodies; feed on warm-blooded animals; carry zoonotic diseases: plague, murine typhus

• Lice – small, soft; attach to head and body hair feeding inconspicuously on blood and tissue fluid; release feces that contaminate wound; epidemic typhus, relapsing fever

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Arachnids

• Ticks – cling on vegetation and attach to host on contact; larvae, nymph, and adults get blood meal by piercing skin of host

– Hard or ixodid ticks – small compact, rigid bodies; transmit bacterial, rickettsial, and viral diseases

– Soft or argasid ticks – flexible outer bodies; transmit relapsing fever

Figure 23.30 Arthropod vectors

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