Human Parasites

 

Parasitic diseases in humans are caused by protozoans and helminthes. The cysts and trophozoite forms of protozoans are most often found in feces but are also found in other tissues or excreta. The helminthes are most often detected by the presence of their eggs in feces; but these, too, can be found in tissues or other excreta.

 

This page will present pictures of ten parasites that cause disease in humans.

 

Protozoans

 

Entamoeba histolytica

Entamoeba histolytica cyst

 

Entamoeba histolytica is, possibly, the most pathogenic amoeba for humans. Humans are the primary host for this pathogen. It is spread via the anal to oral route. It is usually acquired from contaminated water, or from foodstuffs contaminated by untreated sewage.

 

Entamoeba histolytica causes amoebic dysentery, characterized by bloody stools and diarrhea accompanied by abdominal pain. Some of those infected remain asymptomatic. Occasionally, the organism digests its way through the intestinal wall and invades other organs and the viscera. This form of the disease may be fatal.

 

The cysts are shed in the feces of infected persons. The cysts may be isolated and detected in the clinical laboratory from feces.

 

Iodoquinol and chloroquine are often used to treat amoebic dysentery.

 

 

Balantinium coli

Balantinium coli cyst

 

Balantinium coli is a giant ciliate that causes diarrhea in humans. It burrows into the intestinal mucosa and rarely invades tissue further. The most common source of this organism as an agent of human disease is from pig’s fecal material. However, it may be spread from person to person by fomites and contaminated water.

 

Oral tetracycline usually controls the infection.

 

 

Giardia lamblia

Giardia lamblia cysts

 

Giardia lamblia is a pathogenic flagellate of humans. It causes diarrhea and abdominal pain, along with a chronic fatigue syndrome that is otherwise asymptomatic and difficult to diagnose. The cysts are the most common means of spread of this organism. Giardia lamblia cysts have been spread via contaminated water and fomites.

 

Treatment requires the use of metronidazole or quinacrine. This treatment has been very successful.

 

 

Trypanosoma brucei

Trypanosoma brucei gambiense

Trypanosoma brucei is the hemoflagellate that causes sleeping sickness. It is spread by the bite of the tsetse fly, which transfers the organism from alternate host such as the cow. The disease is endemic to Africa; two geographically isolated strains are known.

 

Treatment for this disease includes using suramin or pentamidine; both are fairly toxic.

 

 

Plasmodium sp.

Plasmodium falciparum

Note the ring forms present.

Malaria is caused by species of the Plasmodium genus. Plasmodium sp. are sporozoan obligate intracellular parasites of liver and red blood cells. All Plasmodium sp. are spread by the bite of the Anopheles mosquito and occasionally by blood or blood-contaminated needles. Both the human liver and red blood cells and the salivary gland of the mosquito are required for completion of the life cycle of this pathogen. Diagnosis is done by screening thick blood smears stained with Wright’s Stain. The picture above is from a thin blood smear stained in the same manner.

 

Chloroquine is the usual treatment, but resistant strains are known. Prevention of spread of malaria must include avoidance of mosquito bites. Draining standing water can prevent breeding. Insecticides have been useful in limiting the geographical areas affected by malaria.

 

 

Helminthes

 

Ascaris lumbricoides

 

Ascaris lumbricoides egg

Ascaris lumbricoides is a parasitic nematode (roundworm). A. lumbricoides invades the gastrointestinal tract after consumption of its eggs in contaminated food or drink or from fomites. A. lumbricoides migrates from the intestines to the lungs via the bloodstream. It is then swallowed and returned to the small intestine, where it reproduces. A high parasite load can cause nutritional deficiencies, especially in those consuming marginal diets.

 

Diagnosis is most often made by detection of the eggs in a fecal specimen. However, whole specimens may be obtained from the mouth or nares after the worms have migrated to these sites. Whole specimens have also been obtained surgically from the intestines.

 

Piperazine and Pyrantel are often used for treatment of A. lumbricoides infestations.

 

 

Enterobius vermicularis

Enterobius vermicularis egg

 

Enterobius vermicularis, the pinworm, is a common infestation of children. The worm is spread by consumption of the egg that contaminates food and fomites. The worm reproduces in the intestinal tract, with adult female laying her eggs on the anus. Egg laying occurs while the host sleeps, often during the very early morning hours. The larvae may hatch there and migrate into the colon, increasing the parasite load. However, the presence of the female and her eggs usually makes the anus itch. This may disrupt the sleep, but often results in the transfer of eggs from the anus to the fingertips as scratching occurs. The contaminated fingertips serve as the route for reinfestation and contamination of more food or fomites.

 

Clinical diagnosis of the infestation usually involves retrieving the eggs from the anus by using cellophane tape in the early morning hours. The tape is transferred to a microscope slide for observation.

 

Piperazine and Pyrantel are often used for treatment of pinworm infestations.

 

 

Trichuris trichiuria

Trichuris trichiuria egg

 

Trichuris trichiuria, the whipworm, causes infestation after consumption of eggs contaminating foodstuffs. It reproduces in the intestinal tract and the eggs are found in the feces. Heavy parasite loads may result in dysentery in the host.

 

Treatment of Trichuris trichiuria infestation often employs Pyrantel and Mebendazole.

 

Fasciola hepatica

Fasciola hepatica female

 

Fasciola hepatica egg

 

Fasciola hepatica, the liver fluke, has a complex life cycle, requiring water snails as intermediate hosts. Humans and other mammals acquire the organism by eating cyst-contaminated water plants. Watercress is a common source of the parasite for humans. The cysts release immature flukes that migrate to the liver and gallbladder. A high load of the parasite may obstruct the biliary tract.

 

Praziquantel may be used to treat this infestation.

 

 

Taenia sp.

Taenia saginata, the beef tapeworm, eggs

 

Taenia sp., the tapeworms or cestodes, are consummate examples of parasitism. Their bodies are reduced to mostly reproductive organs. The "head" of the worm, the scolex, holds on to the intestinal wall. Behind the "head" are proglottids, which are mainly composed of ovaries and testes. The most mature proglottids are found near the "tail" of the flattened worm; these release eggs. Larvae may migrate to other tissues and form cysts. They may interfere with the function of the affected organ if the parasite load is high.

 

Humans acquire the beef tapeworms by consuming undercooked beef contaminated by encysted Taenia saginata larvae, but not by consumption of their eggs. Pork tapeworm infestation, caused by Taenia solium, may be acquired by either route.

 

Diagnosis may occur when the host observes proglottids on their feces. The eggs can be observed when isolated from the feces.

 

Niclosamide treatment destroys the tapeworm proglottids and is used for treatment.

 

 

This page was produced by Barbara Krumhardt, Ph.D., Science Group Leader, Biology Instructor, Urban Campus, Des Moines Area Community College, Des Moines, Iowa, USA 50314

Back to Urban Campus Biology Department