Chapter Capsule

A. Infectious Amebas

Entamoeba histolytica is the cause of amebiasis, or amebic dysentery, a worldwide human infection that affects approximately 500 million people in the tropics.

1. The protozoan alternates between a trophozoite and a cyst. Cysts released in feces of carriers are spread through unsanitary water and food.

2. Ingested cysts release trophozoites that invade large intestine and cause ulceration and dysentery.

3. Severe cases may occur when the pathogen penetrates into deeper layers; effective drugs are iodoquinol, metronidazole, and chloroquine.

4. Amebic brain infections are caused by Naegleria fowleri and Acanthamoeba, free-living inhabitants of natural waters. Primary acute meningoencephalitis is acquired through nasal contact with water or traumatic eye damage. Infiltration of brain is usually fatal.

B. Infectious Ciliates

The only important ciliate pathogen is Balantidium coli, an occupant of the intestines of domestic animals such as pigs and cattle. It is acquired by humans when cyst-containing food or water is ingested. The trophozoite erodes the intestine and elicits intestinal symptoms.

C. Infectious Flagellates

1. Trichomonas is an animal parasite or commensal. The principal human pathogen is T. vaginalis, the cause of a very common STD known as trichomoniasis that infects the vagina, cervix, and urethra. Both sex partners can be treated with metronidazole.

2. Giardia lamblia is an intestinal flagellate that causes giardiasis. Its natural reservoir is animal intestines, and the source of infection is cyst-contaminated fresh water and food. Symptoms are like those of Entamoeba, with severe diarrhea that may be chronic in immunodeficient patients. It is preventable by disinfecting water and treatable with quinacrine or metronidazole.

3. Hemoflagellates are protozoans that occur in blood during infection. They cause mostly tropical zoonoses spread by insect vectors. The pathogens have complex life cycles, with various mastigote phases that develop in the insect and human hosts.

a. Trypanosoma have tapering, flagellated cells. Two major types of trypanosomiasis are geographically isolated and have different blood-feeding vectors endemic to regions where vectors live.

b. T. brucei causes African sleeping sickness, spread by tsetse flies and harbored by reservoir mammalian hosts. The biting fly inoculates skin with the trypanosome, which multiplies in the blood, and damages the spleen, lymph nodes, and brain. Chronic disease symptoms are sleepiness, tremors, paralysis, and coma.

c. T. cruzi causes Chagas disease, a disease endemic to Latin America. Its cycle is similar to T. brucei, except that the reduviid (kissing) bug is the vector. Infection occurs when bug feces are inoculated into a cutaneous portal. Chronic inflammation occurs in the organs (especially heart and brain).

d. Leishmania contains geographically separate species that cause leishmaniasis, a zoonosis of wild animals that is transmitted by the bite of phlebotomine (sand) flies. Infected macrophages carry the pathogen into the skin and bloodstream, giving rise to fever, enlarged organs, and anemia. Kala azar is the most severe and fatal form.

D. Apicomplexans are tiny obligate 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.

1. Plasmodium is the cause of malaria; human is primary host for the asexual phase of parasite; female Anopheles mosquito is vector/host for the sexual phase.

a. Malaria is distributed primarily in a belt around the equator, with an estimated 300 to 500 million cases/year worldwide.

b. Infective forms for humans (sporozoites) enter blood with mosquito saliva, penetrate liver cells, multiply, and form hundreds of merozoites. These multiply in and lyse red blood cells.

c. Symptoms include episodes of chills-fever-sweating, anemia, and organ enlargement. Therapy is chloroquine, quinine, or primaquine.

2. Toxoplasma gondii lives naturally in cats that harbor oocysts in the GI tract.

a. Toxoplasmosis is acquired by ingesting raw or rare meats containing tissue cysts or by accidently ingesting oocysts from substances contaminated by cat feces.

b. In humans, infection is usually mild and flulike. Immunodepressed (AIDS) patients or fetuses (fetal toxoplasmosis) suffer brain and heart damage.

3. Cryptosporidium is a vertebrate pathogen that exists in both tissue and oocyst phases. Most cases of cryptosporidiosis are caused by handling animals with infections or by drinking contaminated water. Infection causes enteric symptoms.

4. Isospora is an uncommon human intestinal parasite transmitted through fecal contamination that causes coccidiosis.

5. Cyclospora causes a diarrheal illness when its oocysts are ingested in fecally contaminated produce and water.

A. The parasitic helminths, or worms, are multicellular animals with specialized mouthparts and adaptations such as reduction of organs, protective cuticles, and complex life cycles.

B. Adult worms mate and produce fertile eggs that hatch into larvae that mature in several stages to adults. In some worms, the sexes are separate; others are hermaphroditic.

1. Adults live in definitive host.

2. Eggs and larvae may develop in the same host, external environment, or intermediate host.

3. Types of transmission involve egg-laden feces in soil and water that are infectious when ingested; eggs that go through larval phase in the environment and infect by penetrating skin; ingestion of animal flesh or plants containing encysted larval worms; or the inoculation of the parasite by an insect vector that is the intermediate host.

4. Helminth pathology arises from worms feeding on and migrating through tissues and accumulation of worms and worm products.

5. Useful antihelminthic drugs control worms by paralyzing their muscles and causing them to be shed or interfering with metabolism and killing them.

6. Other control measures are improved sanitation of food and water, protective clothing, cooking meat adequately, and controlling vectors.

C. Nematodes

Roundworms are filamentous with protective cuticles, circular muscles, a complete digestive tract, and separate sexes with well-developed reproductive organs.

1. Intestinal nematodes: Ascaris lumbricoides is a very prevalent species indigenous to humans. Eggs ingested with foods hatch into larvae and burrow through the intestine into the circulation. From there, they travel to the lungs and pharynx and are swallowed. In the intestine, the adult worms complete the reproductive cycle.

2. Trichuris trichiura, the whipworm, is a small parasite transmitted like Ascaris but restricted to the intestine throughout development.

3. Enterobius vermicularis, the pinworm, is a common childhood infection confined to the intestine. Eggs are picked up from surroundings and swallowed. After hatching, they develop into adults, and the females release eggs that cause anal itching; self-inoculation is common.

4. Hookworms have characteristic curved ends and hooked mouths. The two major species, Necator americanus (Western Hemisphere) and Ancylostoma duodenale (Eastern Hemisphere), share a similar life cycle, transmission, and pathogenesis. Humans shed eggs in feces, which hatch into larvae and burrow into the skin of lower legs. Larvae travel from blood to lungs and are swallowed. Adult worms reproduce in the intestine and complete the cycle.

5. Strongyloides stercoralis, the threadworm, is a tiny nematode that completes its life cycle in humans or in moist soil. Larvae emerging from soil-borne eggs penetrate the skin and migrate to the lungs, are swallowed, and complete development in the intestine. The parasite can reinfect the same host without leaving the body.

6. Trichinella spiralis causes trichinosis, a zoonosis in which humans are a dead-end host. Disease is acquired from eating undercooked or raw pork or bear meat containing encysted larvae. Larvae migrate from the intestine to blood vessels, muscle, heart, and brain, where they enter dormancy.

7. Filarial worms are long, threadlike worms with tiny larvae (microfilariae) that circulate in blood and reside in various organs; are spread by various biting insects.

a. Wuchereria bancrofti causes bancroftian filariasis, a tropical infestation spread by various species of mosquitoes. The vector deposits the larvae which move into lymphatics and develop. Adult females shed microscopic worms into blood. Chronic infection causes blockage of lymphatic circulation and elephantiasis, which manifests as massive swelling in the extremities.

b. Onchocerca volvulus causes river blindness, an African disease spread by small, river-associated insects that feed on blood. Inoculated larvae migrate to skin to mature and form nodules. Chronic infections lead to eye involvement and inflammation that can destroy the cornea.

c. Loa loa, the African eye worm, is spread by the bite of small flies. The worm migrates around under the skin and may enter the eye.

D. Trematodes, or flukes, are flatworms with leaflike bodies bearing suckers; most are hermaphroditic.

1. Blood flukes and Schistosoma species. Schistosomiasis is a prevalent tropical disease. Adult flukes live in humans and release eggs into water. The early larva, or miracidium, develops in the freshwater snail into a second larva, or 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.

2. Zoonotic flukes include the Chinese liver fluke Opisthorchis sinensis, which cycles between mammals and snails and fish. Humans are infected when eating fish containing larvae, which invade the liver. Fasciola hepatica cycles between herbivores, snails, and aquatic plants. Humans are infected by eating raw aquatic plants; fluke lodges in liver.

E. Cestodes or Tapeworms

These flatworms have long, very thin, ribbonlike bodies (strobila) composed of sacs (proglottids) and a scolex that grips the host intestine. Each proglottid is an independent unit adapted to absorbing food and making and releasing eggs.

1. Taenia saginata is the beef tapeworm for which 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 (cysticercus) has encysted. The larva attaches to the small intestine and becomes an adult tapeworm.

2. T. solium is the pork tapeworm that infects humans through cysticerci or by ingesting eggs in food or drink (cysticercosis). The larvae hatch and encyst in many organs and can do damage.