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 23.13Plasmodium

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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 23.16The 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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