Informational Tables

5.1 Helminths: key characteristicsa

Helminth

Diagnostic stage

Comments

Nematodes (roundworms)

Ascaris lumbricoides

Egg: both fertilized (oval to round with thick, mammilated-tuberculated shell) and unfertilized (tends to be more oval-elongate, with bumpy shell exaggerated) can be found in stool.
Adult worms: 10–12 in. (25.4 – 30.5 cm), found in stool. Rarely (in severe infections), migrating larvae can be found in sputum, generally during the initial stage of pulmonary migration.

Unfertilized eggs will not float in flotation concentration method; adult worms tend to migrate when irritated (anesthesia, high fever), so check patients from areas of endemicity for infection prior to elective surgery. Both fertilized and unfertilized eggs can be decorticate (missing the bumpy shell).

Trichuris trichiura (whip-worm)

Egg: barrel shaped, with two clear, polar plugs; adult worm rarely seen (may be seen during colonoscopy); quantitate eggs (rare, few, etc.), since light infections may not be treated.

Dual infections with Ascaris may be seen (both infections acquired from egg ingestion in contaminated soil); in severe infections, rectal prolapse may occur in children or bloody diarrhea can be mistaken for amebiasis (bloody diarrhea usually not seen in the United States).

Enterobius vermicularis (pinworm)

Egg: football shaped, with one flattened side
Adult worm: about 3/8 in. (0.9 – 1 cm) long, white with pointed tail; female migrates from anus and deposits eggs on perianal skin. Clear cellulose tapes and/or paddles recommended; the routine stool exam is not an option.

May cause symptoms (itching) in some patients; test of choice is tape preparation; three consecutive (or three different days) negative tapes necessary to rule out infection at 90%; symptomatic patient often treated without actual confirmation of infection; eggs become infective within a few hours.

Ancylostoma duodenale, Necator americanus

Egg: eggs of both identical; oval, with broadly rounded ends, thin shell, clear space between shell and developing embryo (8- to 16-ball stage); overlap in geographic areas

May cause symptoms in some patients (blood loss anemia on differential smear in heavy infections). If stool remains unpreserved for several hours or days, eggs may continue to develop and hatch; rhabditiform larvae may resemble those of Strongyloides stercoralis.

Strongyloides stercoralis

Rhabditiform larvae (noninfective) usually found in stool (short buccal cavity or capsule with large, genital primordial packet of cells [“short and sexy”]); in very heavy infections, filariform larvae occasionally found in sputum (particularly in cases of hyperinfection and autoinfection) and/or filariform (infective) larvae found in stool (slit in tail)

May see unexplained peripheral eosinophilia, abdominal pain, unexplained episodes of sepsis and/or meningitis, pneumonia (migrating larvae) in immunocompromised patient. Potential for internal autoinfection can maintain low-level infections for many years (patient will be asymptomatic, with elevated eosinophilia); hyperinfection can occur in compromised patient (leading to disseminated strongyloidiasis and death); agar plate culture is the most sensitive diagnostic method; many infections are low level, and larvae are difficult to recover.

Ancylostoma braziliensis (dog-cat hookworm)

Humans are accidental hosts; larvae wander through outer layer of skin, creating tracks (severe itching, eosinophilia); no practical microbiological diagnostic tests

Cause of cutaneous larva migrans; typical setup for infection: dogs and cats defecate in sandbox, and hookworm eggs hatch and penetrate human skin in contact with infected sand or soil (e.g., in the case of children playing in a sandbox)

Toxocara cati or Toxocara canis (dog-cat ascarid)

Humans are accidental hosts; ingestion of dog or cat ascarid eggs from contaminated soil; larvae wander through deep tissues (including eye); can be mistaken for cancer of the eye (retinoblastoma); serology helpful for confirmation; eosinophilia

Cause of visceral larva migrans, neural larva migrans, and ocular larva migrans; requests for laboratory services often originate in ophthalmology clinic; although a larva may rarely be seen on ophthalmologic exam, diagnosis is generally based on serologic results.

Cestodes (tapeworms)

Taenia saginata (beef tapeworm)

Scolex (four suckers, no hooklets), gravid proglottids (>12 primary branches on single side) are diagnostic; eggs indicate Taenia spp. only (thick, striated shell containing six-hooked embryo or oncosphere); worm usually approx. 12 ft long

Adult worm can cause symptoms in some individuals; acquired from ingestion of raw or poorly cooked beef; usually only single worm/patient; individual proglottids may migrate from anus; proglottids can be injected with India ink to show uterine branches for identification. Clearing agents are also recommended.

Taenia solium (pork tapeworm)

Scolex (four suckers with hooklets), gravid proglottids (<12 primary branches on single side) are diagnostic; eggs indicate Taenia spp. only (thick, striated shell, containing six-hooked embryo or oncosphere), worm usually approx. 12 ft long

Adult worm can cause GI complaints in some individuals; cysticercosis (accidental ingestion of eggs) can cause severe symptoms in the CNS; infection with the adult worm is acquired from ingestion of raw or poorly cooked pork; usually only single worm/ patient; occasionally two or three proglottids (hooked together) are passed; proglottids can be injected with India ink to show uterine branches for identification; cysticerci are normally small and contained within enclosing membrane; occasionally they may develop as racemose type where the worm tissue proliferates like a tumor in a brain ventricle

Diphyllobothriumb latum^b (broad fish tapeworm)

Scolex (lateral sucking grooves), gravid proglottid (wider than long, reproductive structures in center “rosette”); eggs operculated

Can cause GI complaints in some individuals; acquired from ingestion of raw or poorly cooked freshwater fish; life cycle has two intermediate hosts (copepod, fish); worm may be 30 ft long; associated with vitamin B12 deficiency in genetically susceptible groups (Scandinavians)

Hymenolepis nana (dwarf tapeworm)

Adult worm not normally seen; egg round to oval, thin shell, containing six-hooked embryo or oncosphere with polar filaments lying between embryo and egg shell

Can cause GI complaints in some individuals; acquired from ingestion of eggs (only life cycle where the intermediate host, the grain beetle, can be bypassed); ingestion of infected grain beetles less common source of infection; life cycle of egg to larval form to adult can be completed in human; most common tapeworm in the world

Hymenolepis diminuta (rat tapeworm)

Adult worm not normally seen; egg round to oval, thin shell, containing six-hooked embryo or oncosphere with no polar filaments lying between embryo and egg shell

Uncommon; egg can be confused with that of H. nana; eggs may be submitted in proficiency testing specimens and must be differentiated from those of H. nana.

Echinococcus granulosus

Adult worm found only in the carnivore (dog); hydatid cysts develop (primarily in liver) when humans accidentally ingest eggs from dog tapeworms; cyst contains daughter cysts and many scolices; examine fluid aspirated from cyst at surgery

Humans are accidental intermediate hosts; normal life cycle is in sheep and other herbivores (intermediate hosts) and dogs (definitive hosts), with hydatid cysts developing in liver, lungs, etc., of sheep. Human may be unaware of infection unless fluid leaks from cyst (can trigger an anaphylactic reaction) or pain is felt from cyst location.

Echinococcus multilocularis

Adult worm found only in carnivores (fox, wolf); hydatid cysts develop (primarily in liver) when humans accidentally ingest eggs from carnivore tapeworms; cysts grow into surrounding tissues like an infiltrating malignancy with no outer limiting membrane.

Humans are accidental intermediate hosts; prognosis poor; surgical removal of tapeworm tissue very difficult; found in Canada, Alaska, and, less frequently, in the northern United States, although it is becoming more common in the United States, where the geographic range is moving further south

Trematodes (flukes)

Fasciolopsis buski (giant intestinal fluke)

Eggs found in stool; very large and operculated (morphology like that of Fasciola hepatica eggs)

Symptoms depend on worm burden; acquired from ingestion of plant material (water chestnuts) on which metacercariae have encysted; worms hermaphroditic

Fasciola hepatica (sheep liver fluke)

Eggs found in stool; cannot be differentiated from those of F. buski

Symptoms depend on worm burden; acquired from ingestion of plant material (watercress) on which metacercariae have encysted; worms hermaphroditic

Opisthorchis spp. Clonorchis spp. (Chinese liver fluke)

Eggs found in stool; very small (>35 μm); operculated, with shoulders into which operculum fits

Symptoms depend on worm burden; acquired from ingestion of raw fish; eggs can be missed unless 400 × power is used for examination; eggs can resemble those of Metagonimus yokogawai and Heterophyes heterophyes (small intestinal flukes); worms hermaphroditic; associated with cholangiocarcinoma

Paragonimus westermani, other
Paragonimus spp. (lung flukes)

Eggs coughed up in sputum (brownish “iron filings” = egg packets); can be recovered in sputum or stool (if swallowed); operculated, with shoulders into which operculum fits; although the eggs may be seen in the sputum, rust hemoptysis causes the “orange/brown” color identified as “iron filings”

Symptoms depend on worm burden and egg deposition; acquired from ingestion of raw crustaceans; eggs can be confused with those of D. latum; infections seen in Asia; infections with Paragonimus mexicanus found in Central and South America; Paragonimus kellicotti in the United States; worms are hermaphroditic but often cross-fertilize with another worm if present

Schistosoma mansoni (blood fluke)

Eggs recovered in stool (large lateral spine); collect specimens with no preservatives (to indicate egg viability); worms in venules of large intestine; egg viability studies are not commonly performed for any of the schistosome species; if eggs are found, the patient is treated (particularly in non-endemic settings).

Acquired from skin penetration of single cercariae from freshwater snail; pathology caused by body’s immune response to eggs in tissues; adult worms in mesenteric venules cause no problems; adult worms are separate sexes

Schistosoma haematobium (blood fluke)

Eggs recovered in urine (large terminal spine); collect specimens with no preservatives (to indicate egg viability); worms in venules of bladder

Acquired from skin penetration of single cercariae from freshwater snail; pathology as with S. mansoni; collect 24-h and spot urine samples with no preservatives; chronic infection associated bladder cancer (squamous cell carcinoma); adult worms are separate sexes

Schistosoma japonicum (blood fluke)

Eggs recovered in stool (very small lateral spine); collect specimens with no preservatives (to indicate egg viability); worms in mesenteric venules of small intestine

Acquired from skin penetration of multiple cercariae from freshwater snail; pathology as with S. mansoni; infection usually most severe of the three because of original loading infective dose of cercariae from freshwater snail (multiple cercariae stick together); pathology associated with egg production, which is greatest in S. japonicum infections
NOTE: APPLIES TO ALL CASES OF SUSPECT SCHISTOSOMIASIS Occasionally some of the adult worms may be found in other than their normal body sites (venules of large or small intestine or bladder). Consequently, eggs may be found in urine rather than stool and vice versa. Any patient suspected of having schistosomiasis should submit both urine and stool for diagnostic examination.