Parasitic Infections


Essentials of Diagnosis Blood flukes: demonstration of eggs in feces, urine, or rectal biopsy Liver flukes: demonstration of eggs in feces Intestinal flukes: demonstration of adult worms or eggs in feces Lung flukes: demonstration of eggs in sputum or feces All trematode species that are parasitic for humans are digenetic. Sexual reproduction in the adult fluke is followed by asexual multiplication in the larval stage. Most species of adult trematodes have an oral and ventral sucker. The life cycles of trematodes that are important human pathogens are similar among all five major species. Eggs are excreted in the feces of the host, hatch in fresh water, and require a primary host […]


Human infections caused by cestodes, or tapeworms, may occur within the lumen of the bowel, where adult cestodes attach themselves to the host intestine (Box 1). Alternatively, human infection may be the result of dissemination of cestodes from the bowel to involve extraintestinal sites, often by larval forms of the parasite. The life cycle of cestodes is determined by definitive hosts, in whom the mature adult worm lives, and intermediate hosts, which harbor the larval forms of the parasite. Humans are a definitive host for six cestodes: Diphyllobothrium latum, Taenia solium, Taenia saginata, Hymenolepis diminuta, Hymenolepis nana, and Dipylidium caninum. In addition, humans may be intermediate hosts for Echinococcus granulosis and […]

Echinococcal Infection

Essentials of Diagnosis Radiographic finding of cyst Positive echinococcal serology Aspiration of cyst revealing echinococcal sand or hooks Typical histologic appearance of cyst wall General Considerations The normal life cycle of Echinococcus species does not involve humans. Human disease occurs when humans become an accidental intermediate host for the parasite, and tissue invasion is followed by the formation of cysts (hydatid cysts). The definitive hosts for echinococcal species are canines (usually dogs), in whom the adult worms live. There may be several hundred worms within a host, and the worms are small, usually 3-6 mm long. The scolex is attached to the dog intestine, and to each scolex is attached a […]

Dipylidium Caninum Infection

Essentials of Diagnosis Motile proglottids 23 by 8 mm. Proglottids have genital pores at either end and contain egg clusters. Eggs occur in compartmented clusters (diagnostic). Scolex has 4-7 rows of hooklets and 4 suckers. General Considerations D caninum is distributed worldwide and is associated with wild and domesticated cats and dogs. The life cycle is similar to that of H diminuta, with an obligatory arthropod intermediate host. The adult worm lives in dogs, cats, or humans, and gravid proglottids are released from the adult worm either singly or in short chains. Eggs are passed in the stool, and ingestion of eggs by the intermediate host results in the development of […]

Taenia Saginata Infection

Essentials of Diagnosis Stool examination reveals spheroidal yellow-brown eggs (31-43 mm). Motile proglottids that appear singly in stool. Mature proglottids are square. Scolex has no hooklets and four suckers. Gravid proglottid has 15-20 lateral branches. General Considerations T saginata infection is commonly associated with the ingestion of undercooked beef. This is distinguished from infection with T solium because human infection with the larval form (as in cysticercosis) is extremely rare with T saginata infection. T saginata infection is common in areas of the world with intensive cattle breeding, such as central Asia and central and eastern Africa. Alternative intermediate hosts for T saginata include llamas, buffalo, and giraffes. The life cycle […]

Cysticercosis (Cysticercus Cellulosea Infection)

Essentials of Diagnosis Surgical excision of involved tissue, with microscopic identification of parasite. Frequently calcified cysts present on x-ray or computed tomography (CT) scans. Positive serology indicating previous exposure to T solium. Fine-needle aspiration of cysts (characteristic cytomorphology). General Considerations Cysticercosis is caused by invasion of tissue by the larval forms of T solium, which have been referred to as Cysticercus cellulosea, although the name is not taxonomically correct and introduces confusion. Within a host infected by the adult T solium, eggs or proglottids are passed in the stool. Once eggs or proglottids are ingested by either pigs (intermediate hosts) or humans (definitive and intermediate hosts), eggs hatch in the gastric […]

Taenia Solium Infection

Essentials of Diagnosis Spheroidal yellow-brown eggs (31-43 um). Scolex has hooklets and four suckers. Proglottids usually appear as short chains. Mature proglottids are square and nonmotile. Gravid proglottid has 7-13 lateral branches on each side of uterus. General Considerations T solium infection occurs worldwide; endemic areas include Mexico, South and Central America, Africa, Southeast Asia, India, and the Philippines. T solium infection is commonly linked to the ingestion of undercooked pork, although other animals may harbor the larval form of the parasite. Infection may be intestinal, which is typically asymptomatic, or extraintestinal (called cysticercosis, see below), which is caused by larval forms of T solium within the tissues of the human […]

Diphyllobothrium Latum Infection

Essentials of Diagnosis Stool examination reveals ovoid, yellow-brown eggs (60-75 um by 40-50 um). Chains of proglottids (up to 50 cm long) may be passed in stool. Proglottids are wider than long (3 by 11 mm). Scolex has no hooklets and two grooves (bothria). Gravid proglottid contains rosette-shaped central uterus. General Considerations D latum is found worldwide, and infection is acquired by ingestion of contaminated raw or improperly cooked freshwater fish. Because of enthusiasm for raw or undercooked fish, Siberia, Europe, Canada, Alaska, and Japan are endemic regions for D latum infection. Once the D latum cyst has been ingested, the worm matures within the human intestine and begins to produce […]


Essentials of Diagnosis Intestinal nematodes: demonstration of characteristic eggs or parasites in stool. Bloodstream nematodes (filariasis): clinical diagnosis can be made; fresh blood smear may be confirmatory. Tissue nematodes: clinical diagnosis can be made; skin snip or other tissue examination may show organism. General Considerations Nematodes (roundworms) are nonsegmented, tapered, bilaterally symmetrical, cylindrical organisms that have complete digestive tracts and reproduce sexually. Although > 500,000 species of nematodes have been described, only a small number are commonly encountered as human parasites. Most nematodes have complex life cycles, sometimes involving several larval forms and intermediate hosts or free-living stages. The pathogenic nematodes may be categorized as primarily intestinal or extraintestinal tissue parasites […]

Tissue Nematode Infections

LYMPHATIC FILARIASIS Lymphatic filariasis is a bloodstream and lymphatic infection caused by the filarial nematodes Wuchereria bancrofti, Brugia malayi, and Brugia timori. The disease is endemic in the tropics and subtropics of both hemispheres. A mosquito serves as an intermediate host and vector; the peak blood parasitemia and optimum time of the day or night for obtaining blood smears differ in various parts of the world, corresponding to the feeding pattern of the local mosquito vectors. After deposition by mosquitoes of infectious microfilariae into humans during a blood meal, 6-12 mo are required before adult worms mature and begin producing numerous circulating microfilariae to continue the life cycle. Symptoms of acute […]