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Page 1: Chapter23 Lecture

Foundations in Microbiology

Sixth Edition

Chapter 23The Parasites of Medical

Importance

Lecture PowerPoint to accompany

Talaro

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Parasitology

• The study of eucaryotic parasites, protozoa and helminths

• Cause 20% of all infectious diseases• Less prevalent in industrialized countries;

increasingly common in AIDS patients

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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

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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

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Entamoeba histolytica

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

• May invade liver and lung• Severe forms of disease result in 10% fatality

rate.• Effective drugs are iodoquinol, metronidazole,

and chloroquine.

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Amebic Infections of the Brain

• Caused by Naegleria fowleri and Acanthamoeba• Ordinarily inhabit standing water• Primary acute meningoencephalitis is acquired

though 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

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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

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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

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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

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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.

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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

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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 is fixed the infection is localized; systemic if macrophage migrates.

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• Cutaneous-oriental sore, Baghdad boil - localized ulcerated sore

• Espunda – 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.

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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

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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 2,000-40,000 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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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, quinine, or primaquine.

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Coccidian Parasites

• Zoonotic in domestic animals and birds

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Toxoplasma gondii and Toxoplasmosis

• Intracelllular 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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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 which give 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

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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

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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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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 are hermaphroditic.• Adults live in the definitive host.• Eggs and larvae may develop in the same host, the

external environment of the intermediate host.• A transport host experiences no parasitic

development.• Four basic patterns of life and transmission

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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

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

• Ascaris lumbricoides, Trichuris trichiura, Enterobius vermicularis, hookworms, Strongyloides stercoralis, Trichinella spiralis, filarial worms

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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, and allow bacteria to leave intestine.• 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 the lower legs.

• 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.

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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 Trichinosis

• 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

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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.

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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.

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Liver and Lung Flukes• ZoonoticLiver flukes:• Opisthorchis (Chlonorchis) sinesis - cycles between

mammals and snails and fish; humans are infected by eating inadequately cooked fish containing cercaria, 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.

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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

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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

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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, zoonoses

• 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

• Flies – tsetse fly, sand fly

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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 ticks – Dermacentor, Ixodes – small

compact, rigid bodies; transmit rickettsial, borrelial, and viral diseases

– soft or argasid ticks – Ornithodoros- flexible outer bodies; transmit relapsing fever

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