Case Based Pediatrics For Medical Students and Residents
Department of Pediatrics, University of Hawaii John A. Burns School of Medicine
Chapter VI.31. Protozoans and Parasites
Loren G. Yamamoto, MD, MPH, MBA
October 2002

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A 2 year old boy from in a war refugee camp is treated for dehydration secondary to vomiting and foul diarrhea. After rehydration, the staff suspects that he may have Hirschsprung's disease based on his history and growth pattern. He is flown to your hospital for surgical evaluation. He is obviously malnourished and underweight, but he improves clinically. After eating lunch, the boy vomits and the nurses note the presence of several large worms in the vomitus. They later note that he has a visible worm in his stool. A stool sample is sent for ova and parasite analysis which identifies Giardia cysts and ascaris eggs. He is treated with albendazole which covers both the Giardia and ascaris.

Protozoans and parasites are less common in the United States. Many of them require fecal-oral transmission or vectors (insects and rodents). Proper public health measures which are taken for granted in the U.S. are responsible for minimizing our exposure to these infections. These measures include regulations and laws ensuring clean water, agriculture and livestock, proper food handling, sanitation (sewage and trash disposal), vector control (insects and rodents), and housing. American lifestyle factors including good nutrition, footwear and skin care also reduce the risk of infection.

Protozoans are eukaryotic large single celled organisms. Parasites are organisms which live in or on another organism, which would include viral and bacterial infections as well. However, the general understanding when one refers to parasites, is that these include "worms" such as roundworms and flatworms. Since many of these organisms are gastrointestinal, common presentation patterns include gastroenteritis, failure to thrive, weight loss, edema, and or abdominal discomfort. The immune system has a difficult time eradicating protozoans and parasites. Eosinophilia is often associated with parasites, but eosinophilia is only present in the presence of invasive worms.

Protozoans can be classified into ameba, flagellates and sporozoans. Pneumocystis used to be considered a protozoan, but it now classified as an opportunistic fungi.

Ameba include Entamoeba histolytica often called amebiasis, which classically causes bloody diarrhea (amebic dysentery), but it may also present with non-specific gastroenteritis, edema, failure to thrive or an amebic liver abscess. The diagnosis can be made by finding organisms in the stool or by serology. Naegleria fowleri is a rare and fatal infection. Naegleria live in hot springs, so most infections are acquired by swimming or diving in hot springs. Naegleria enter the body and infect the brain rapidly resulting in encephalitis, coma and death (often despite treatment).

Flagellates include giardia, trichomonas, trypanosomes and leishmania. Giardia lamblia causes diarrhea, malabsorption and abdominal discomfort. Giardia organisms are generally not found in stool samples, and Giardia cysts are only sometimes found in the stools. Giardia organisms were most commonly identified by having the patient swallow a gelatin capsule on a string which is then retrieved and analyzed. Giardia serology has now replaced these other methods of identifying giardiasis.

Trichomonas vaginalis (trich for short) is a sexually transmitted motile protozoan which causes vaginitis. Clinically, a pruritic vaginal discharge results, with or without dysuria. Motile protozoans are seen on a microscopy wet mount of the vaginal discharge. Males are often asymptomatic.

Trypanosomiasis can be of the African or the South American type. African sleeping sickness is caused by Tryp. brucei rhodesiense and Tryp. brucei gambiense, which are both spread by the biting tsetse fly vector. Symptoms of infection include fever, headache, malaise, rash, lymphadenopathy, etc. Spontaneous remission or coma and death may follow. American trypanosomiasis (also known as Chagas disease) is cause by Tryp. cruzi which is spread by the reduviid bug. Cardiomyopathy may classically result, but other presentations include malaise, fever, lymphadenopathy, encephalitis and death.

Leishmania infections are transmitted by sandfly vectors. Leishmania donovani causes visceral leishmaniasis (also known as kala-azar) which results in fever, malaise, lymphadenopathy, organomegaly, anemia and weight loss. Leishmania tropica cause cutaneous leishmaniasis. Leishmania braziliensis causes mucocutaneous leishmaniasis.

Sporozoans include cryptosporidium, plasmodium and toxoplasma. Cryptosporidium may be a contaminant in some water systems. It is resistant to chlorination, but many home filtration systems will filter out the cysts. Infection with cryptosporidium results in self limited diarrhea in healthy persons (no treatment required), but in immunocompromised individuals, it may cause fever, chronic diarrhea, weight loss, and death.

Malaria is caused by the various Plasmodium species (ovale, vivax, falciparum, malariae). Malaria is a complex infection which is described in a separate chapter. It is transmitted by anopheles mosquito vectors and it can be prophylaxed and treated to a limited extent with chloroquine and primaquine.

Toxoplasma gondii is harbored by cats. Infection can be acquired by exposure to cat feces (by touch or through contaminated food). Most healthy adults have an asymptomatic or a non-specific illness similar to infectious mononucleosis. Infection in pregnant mothers during early gestation puts the fetus at risk of congenital toxoplasmosis which may result in brain injury, microcephaly and chorioretinitis. Immunocompromised patients may develop chorioretinitis and focal brain lesions.

Trematodes (also called flukes) are unsegmented flatworms which contain the groups fasciola, clonorchis, paragonimus, and schistosoma. Intestinal flukes include fasciolopsis buski found in Asia. Clonorchis sinensis (the Asian liver fluke) is acquired by eating raw or undercooked freshwater fish. Fasciola hepatica (another liver fluke) is found in sheep raising areas and infection is acquired by eating organisms which have contaminated agriculture. Paragonimus westermani is a lung fluke found in Asia and Africa. It is acquired by eating contaminated shellfish. Schistosomes are classified as blood flukes because they spread through the circulation. Schistosoma species include mansoni, japonicum, and haematobium. These organisms can penetrate the skin directly when exposed to contaminated water. S. mansoni and S. japonicum cause intestinal symptoms. S. haematobium causes bladder symptoms including bladder carcinoma.

Cestodes (also called tapeworms) are segmented flatworms. Taenia solium is the pork tapeworm and Taenia saginata is the beef tapeworm. Humans acquire these when ingesting undercooked pork and beef that are contaminated with larval forms known as cysticerci. In most instances, intestinal infection with large tapeworms result. In some instances, cysticerci invade the eye or brain resulting in focal brain lesions which often leads to seizures known as neurocysticercosis. This is very common in countries where livestock raising conditions are not well regulated and undercooked pork is frequently consumed. The southern U.S. regions encounter frequent cases of neurocysticercosis in Mexican immigrants and visitors. The fish tapeworm, Diphyllobothrium latum, is found in undercooked freshwater fish. This organism has largely been eliminated from previously endemic areas in Minnesota (the land of lakes) and the Great Lakes. Raw salmon ingestions may be a source of D. latum, because even though salmon are salt water fish, they may acquire the infection when they return to freshwater.

Nematodes are roundworm groups which can be remembered by the mnemonic NEMATODES (with 2 As and 3 Ts). These stand for Necator, Enterobius, Mosquito borne (Wucheria and Brugia), Ascaris, Ancylostoma, Trichuris, Trichinella, Toxocara, Onchocerca, Dracunculus, Eye worm (Loa loa), and Strongyloides. They infect humans via ingestion (EATT: Enterobius, Ascaris, Trichuris, Trichinella) or by skin penetration (SAN: Strongyloides, Ancylostoma, Necator).

Necator americanus and Ancylostoma duodenale are hookworms which are endemic in warm moist climates. N. americanus is endemic in the southeast U.S. Larvae in the soil attach (hook) between the toes and penetrate the skin into the bloodstream. They migrate into the lungs causing an eosinophilic pneumonia. Organisms are coughed up and swallowed where they enter the GI tract. They attach (hook) to the intestinal wall where they cause a chronic infection.

Enterobius vermicularis (pinworms) are the most common parasite in the U.S. Many children are said to have these and they are usually asymptomatic. It is likely that spread occurs among young children with poor oral hygiene habits in group settings (e.g., preschool). These worms are about 0.7 cm long and they resemble moving pieces of white thread. They are usually found in the perianal area. These worms reside in the rectum. They exit the anus and lay eggs at night when the child is asleep. The worms then reenter the anus. Some advocate obtaining a perianal sample by applying sticky tape or an adhesive plastic paddle to the anus several hours after the child has fallen asleep. Alternatively, visual inspection at night will often identify the worms. In girls, the worms may enter the vagina instead of the anus. This will cause dysuria or vaginal pruritus. This is commonly seen in emergency departments at night. If the parent brings the child in early, the moving worms are usually still visible in the perineal area. If the parent brings the child in late, dead degenerating worms may be visible in the perineum. Otherwise, a chief complaint of sudden onset of dysuria or vaginal discomfort in the absence of urinalysis evidence of a UTI, is suggestive of pinworms. These can be eradicated with a single dose of mebendazole. This can be repeated in two weeks to reduce the likelihood of reinfection.

Ascaris lumbricoides is probably the most common parasitic worm in the world. If you see a large intestinal worm, odds are this is what it is. They are large and pink resembling smooth earthworms. Ascaris worms survive and grow in the intestine. Infection may be asymptomatic or they may grow large enough to cause a bowel obstruction. Newly hatched larvae may migrate throughout the body into the liver, heart and lungs. Eosinophilic pneumonia is caused by migrating ascaris larvae as they invade the lungs.

Mosquito borne nematodes include Wucheria bancrofti and Brugia malayi which both cause elephantiasis. These are difficult to treat. These infestations are referred to as filariasis, which also includes Loa loa (an African eye worm, which is spread by the Chrysops fly vector) and Onchocercal volvulus (also known as river blindness, which is spread by the Simulium blackfly).

Trichuris trichiura (whipworm) is found in tropical regions. The infestation can be asymptomatic or the usual GI chronic infestation symptoms. Trichinella spiralis is the pork roundworm, which is acquired by eating poorly cooked contaminated pork. The larvae initially cause GI infestation symptoms. As maturing worms lay new eggs, these larvae hatch and migrate into the bloodstream where they lodge in muscle tissue forming calcifications which cause fever and myalgia. Toxocara canis and Toxocara cati are dog and cat parasites which may be asymptomatic or cause visceral larvae migrans.

Dracunculus medinensis is known as the Guinea worm. Larvae in tiny aquatic crustaceans are ingested in contaminated water. Over time, adult worms grow subcutaneously (nearly one meter long), which are classically removed through the skin by gradually pulling them (rolling them onto a stick).

Strongyloides stercoralis (threadworm) is another parasite that enters the body through exposed feet stepping over contaminated soil.

Commonly used antiparasitic drugs include albendazole, pyrantel pamoate, mebendazole and metronidazole. It should be noted that this chapter is written for general background information only. Therapeutic decisions should be based on more comprehensive information describing each infection type. Some parasitic infections do not benefit from treatment.

Albendazole is a relatively new drug which covers a broad range of parasitic infections. Mebendazole (Vermox) and pyrantel pamoate are older drugs which are also highly efficacious. Metronidazole (Flagyl) and tinidazole have activity against many protozoans. Many parasites respond to albendazole and pyrantel pamoate. Diethylcarbamazine is used for filariasis parasites (and dog heartworms). Praziquantel works for most flatworms (flukes and tapeworms).

Summary of Protozoans and Parasites (Treatment agents in parentheses. Alb=albendazole, Pyr=pyrantel pamoate, Meb=mebendazole, Metr=metronidazole) [Route of infection in brackets]. [FO] = fecal-oral transmission. The purpose of this summary is to provide a general educational overview. Note that treatment alternatives, doses and durations are not provided here. Please use a current authoritative reference for clinical therapeutic decisions.

I. Protozoans
. . . . A. Ameba
. . . . . . .. . 1. Entamoeba histolytica (Metr) [FO]
. . . . . . .. . 2. Naegleria fowleri (amphotericin) [hot springs]
. . . . B. Flagellates
. . . . . . .. . 1. Giardia lamblia (Metr) [FO, contaminated water]
. . . . . . .. . 2. Trichomonas vaginalis (Metr) [sexually transmitted]
. . . . . . .. . 3. Trypanosomes
. . . . . . .. . . . . . a. Tryp cruzi (nifurtimox or benznidazole) [reduviid bug vector]
. . . . . . .. . . . . . b. T. rhodesiense, T. gambiense (pentamidine) [tsetse fly vector]
. . . . . . .. . 4. Leishmania (stibogluconate) [sandfly vector]
. . . . C. Sporozoans
. . . . . . .. . 1. Cryptosporidium (paromomycin, amphotericin) [drinking water]
. . . . . . .. . 2. Plasmodium-Malaria (complex treatment) [anopheles mosquito vector]
. . . . . . .. . 3. Toxoplasma (pyrimethamine) [cat feces, contaminated food]

II. Trematodes (flukes)
. . . . A. Intestinal: Fasciolopsis buski (praziquantel) [FO]
. . . . B. Liver
. . . . . . .. . 1. Clonorchis sinensis (praziquantel) [raw freshwater fish],
. . . . . . .. . 2. Fasciola hepatic (triclabendazole) [sheep-raising area, contaminated agriculture]
. . . . C. Lung: Paragonimus westermani (praziquantel) [shellfish]
. . . . D. Blood: Schistosoma (praziquantel) [skin penetration in contaminated water]

III. Cestodes (tapeworms)
. . . . A. Beef: Taenia saginata (praziquantel, albendazole) [ingesting contaminated raw beef]
. . . . B. Pork: Taenia solium (praziquantel, albendazole) [ingesting contaminated raw pork]
. . . . C. Fish: Diphyllobothrium latum (praziquantel) [ingesting infected raw freshwater fish]

IV. Nematodes (roundworms): NEMA(2)T(3)ODES
. . . . A. Necator (albendazole) [FO, skin]
. . . . B. Enterobius (mebendazole) [FO]
. . . . C. Mosquito borne
. . . . . . .. . 1. Wucheria bancrofti (diethylcarbamazine) [mosquito vector]
. . . . . . .. . 2. Brugia malayi (diethylcarbamazine) [mosquito vector]
. . . . D. Ascaris (pyrantel pamoate, albendazole) [FO]
. . . . E. Ancylostoma (albendazole) [FO, skin]
. . . . F. Trichuris (mebendazole) [FO]
. . . . G. Trichinella (mebendazole) [poorly cooked pork]
. . . . H. Toxocara (albendazole) [dog and cat feces]
. . . . I. Onchocerca: river blindness (ivermectin) [Simulium blackfly vector]
. . . . J. Dracunculus: Guinea worm (metronidazole) [contaminated water]
. . . . K. Eye worm: Loa loa (diethylcarbamazine) [Chrysops fly]
. . . . L. Strongyloides: threadworm (ivermectin) [FO, skin]


1. Name two parasites which are associated with the ingestion of uncooked freshwater fish?

2. What is the most common parasitic worm in American children?

3. Name two parasites associated with the ingestion of poorly cooked pork?

4. Name two motile (flagellated) protozoans infections commonly found in the U.S.

5. Name two types of hookworms.

6. Name four protozoans and two parasites transmitted by mosquito vectors.

7. Name 3 or 4 protozoans and parasites that are transmitted by biting flies.

8. Name 2 or 3 protozoans and parasites that invade the brain.


1. Hawley LB. High-Yield Microbiology and Infectious Disease. 2000, Baltimore, Lippincott Williams & Wilkins.

2. Drugs for Treatment of Parasitic Infections. In: Pickering LK, et al (eds). 2000 Red Book: Report of the Diseases, 24th edition. 2000, Elk Grove Village, IL: American Academy of Pediatrics, pp. 694-717.

Answers to questions

1. Diphyllobothrium latum (fish tapeworm), Clonorchis sinensis (Asian liver fluke),

2. Pinworms (Enterobius vermicularis).

3. Trichinella spiralis, Taenia solium.

4. Trichomonas vaginalis, Giardia lamblia.

5. Ancylostoma duodenale, Necator americanus

6. Malaria (Plasmodium vivax, falciparum, haematobium, malariae), filariasis (Wucheria bancrofti, Brugia malayi).

7. Tryp cruzi [reduviid bug vector, which is not really a fly, but it is a biting bug], T. rhodesiense and T. gambiense [tetse fly vector], leishmania [sandfly vector], Onchocerca [Simulium blackfly vector], loa loa eye worm [Chrysops fly].

8. Taenia solium (neurocysticercosis), Naegleria fowleri, Toxoplasmosis, Loa loa (eye).

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