TABLE A3.
Key characteristics of helminthsa
| Helminth(s) | Diagnostic stageb | Comments |
|---|---|---|
| Nematodes (roundworms) | ||
| Ascaris lumbricoides | Eggs are both fertilized (oval to round with a thick, mammillated/tuberculated shell) and unfertilized (tend to be more oval or elongate, with an exaggeratedly bumpy shell); eggs can be found in stool; adult worms are 10–12 in. and found in stool; rarely (in severe infections), migrating larvae can be found in sputum | Unfertilized eggs do not float by the flotation concn method; adult worms tend to migrate when irritated (by anesthesia or high fever); hence, patients from areas of endemic infection should be checked for infection prior to elective surgery |
| Trichuris trichiura (whipworm) | Eggs are barrel shaped with 2 clear, polar plugs; adult worms are rarely seen; eggs should be quantitated (rare, few, etc.), since light infections may not be treated | Dual infections with A. lumbricoides may be seen (both infections are acquired from ingestion of eggs from contaminated soil); in severe infections, rectal prolapse may occur in children, or bloody diarrhea can be mistaken for amoebiasis (these clinical manifestations are usually not seen in the United States) |
| Enterobius vermicularis (pinworm) | Eggs are football shaped with one flattened side; adult worms are about 3/8 in. long and white, with a pointed tail; females migrate from the anus and deposit eggs on the perianal skin | Causes itching in some patients; the test of choice is cellulose tape prepn; 4–6 consecutive daily tapes are necessary to rule out infection; symptomatic patients are often treated without actual confirmation of infection; eggs become infective within a few hours |
| Ancylostoma duodenale (Old World hookworm), Necator americanus (New World hookworm | Eggs of these two species are identical; they are oval with broadly rounded ends, a thin shell, and a clear space between the shell and developing embryo (8 to 16 cell stages); adult worms are rarely seen in clinical specimens | Causes blood loss anemia on differential smears in patients with heavy infections; if stool remains unpreserved for several hours or days, the eggs may continue to develop and hatch; rhabditiform larvae may resemble those of Strongyloides stercoralis |
| Strongyloides stercoralis | Rhabditiform larvae (noninfective) are usually found in the stool; they have a short buccal cavity or capsule with large genital primordial packet of cells (“short and sexy”); in very heavy infections, larvae are occasionally found in sputum; filariform (infective) larvae can be found in stool (with a slit in the tail) | May cause unexplained eosinophilia, abdominal pain, unexplained episodes of sepsis and/or meningitis, and pneumonia (migrating larvae) in compromised patients; the potential for internal autoinfection can maintain low-level infections for many years (a patient is asymptomatic or has eosinophilia); hyperinfection can occur in compromised patients (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 or cat hookworm | Humans are accidental hosts; larvae wander through the outer layer of the skin, creating tracks (causing severe itching and eosinophilia); no practical microbiological diagnostic tests exist | Cause of cutaneous larva migrans; the typical setup for infection is when dogs and cats defecate in sand boxes and hookworm eggs hatch and penetrate human skin when in contact with infected sand or soil (children playing in sand box) |
| Toxocara cati and T. canis (cat and dog ascarid | Humans are accidental hosts; infection is by ingestion of dog or cat ascarid eggs in contaminated soil; larvae wander through deep tissues (including the eye); can be mistaken for cancer of the eye; serologic tests are helpful for confirmation; infection causes eosinophilia | Cause of visceral larva migrans and ocular larva migrans; requests for laboratory services often originate in an ophthalmology clinic; serology may be helpful |
| Cestodes (tapeworms) | ||
| Taenia saginata (beef tapeworm) | A scolex (4 suckers, no hooklets) and gravid proglottid (with 12 branches on a single side) are diagnostic; eggs indicate Taenia spp. only (thick, striated shell containing a 6-hooked embryo or oncosphere); worm usually about 12–15 ft long | Adult worms cause symptoms in some individuals; infection occurs via ingestion of raw or poorly cooked beef; there is usually only a single worm per patient; individual proglottids may crawl from the anus; India ink can be injected into proglottids to visualize the uterine branches for identification |
| Taenia solium (pork tapeworm) | A scolex (4 suckers with hooklets) and gravid proglottid (with 12 branches on a single side) are diagnostic; eggs indicate Taenia spp. only (with a thick, striated shell containing a 6-hooked embryo or oncosphere); the worm is usually about 6–20 ft long | Adult worms cause gastrointestinal complaints in some individuals; cysticercosis (accidental ingestion of eggs) can cause severe central nervous system symptoms; infection is via ingestion of raw or poorly cooked pork; there is usually only a single worm per patient; occasionally 2 or 3 proglottids (hooked together) are passed; India ink can be injected into proglottids to visualize the uterine branches for identification; cysticerci are normally small and contained within an enclosing membrane; they occasionally develop as the “racemose” type, in which the worm tissue grows in the body like a metastatic cancer |
| Diphyllobothrium latum (broad fish tapeworm) | A scolex (lateral sucking groove) is present; the gravid proglottid is wider than long, with reproductive structures in the center “rosette”; eggs are operculated | Causes gastrointestinal complaints in some individuals; infection is via ingestion of raw or poorly cooked freshwater fish; the life cycle has 2 intermediate hosts (copepod and fish); worms may reach 30 ft long; the illness is associated with vitamin B12 deficiency in genetically susceptible groups (e.g., Scandinavians) |
| Hymenolepis nana (dwarf tapeworm) | Adult worms are not normally seen; eggs are round to oval and have a thin shell containing a 6-hooked embryo or oncosphere with polar filaments lying between the embryo and egg shell | Causes gastrointestinal complaints in some individuals; infection is via ingestion of eggs (the only life cycle in which the intermediate host [grain beetle] can be bypassed); the life cycle of the egg to the larval form to the adult can be completed in humans; this may be the most common tapeworm in the world |
| Hymenolepis diminuta (rat tapeworm) | Adult worms are not normally seen; eggs are round to oval and have a thin shell containing a 6-hooked embryo or oncosphere without polar filaments lying between the embryo and egg shell | Uncommon; eggs can be confused with H. nana eggs; eggs are submitted in proficiency testing specimens and must be differentiated from those of H. nana |
| Echinococcus granulosus | Adult worms are found only in carnivores (dog); hydatid cysts develop (primarily in the liver) when humans accidentally ingest eggs of the dog tapeworms; cysts contain daughter cysts and many scolices; a laboratory should examine fluid aspirated from a cyst at surgery | Humans are accidental intermediate hosts; the normal life cycle is from a dog to a sheep, with the hydatid cysts developing in the liver, lung, etc., of the sheep; human hosts may be unaware of their infection unless fluid leaks from the cyst (can trigger an anaphylactic reaction) or pain is felt at the cyst location |
| Echinococcus multilocularis | Adult worms are found only in carnivores (fox or wolf); hydatid cysts develop (primarily in the liver) when humans accidentally ingest eggs of the carnivore tapeworms; cysts grow like a metastatic cancer, with no limiting membrane | Humans are accidental intermediate hosts; prognosis is poor; surgical removal of the tapeworm tissue is very difficult; this organism is found in Canada, Alaska, and, less frequently, in the northern United States but is becoming more common in the United States, where the geographic range is moving further south |
| Trematodes (flukes) | ||
| Fasciolopsis buski (giant intestinal fluke) | Eggs are found in stool; they are very large and operculated (their morphology is like that of F. hepatica eggs) | Symptoms depend on worm burden; the organism is acquired from ingestion of plant material on which metacercariae have encysted (e.g., water chestnuts); worms are hermaphroditic |
| Fasciola hepatica (sheep liver fluke) | Eggs are found in stool; cannot be differentiated from those of F. buski | Symptoms depend on worm burden; the organism is acquired from ingestion of plant material on which metacercariae have encysted (e.g., watercress); worms are hermaphroditic |
| Clonorchis (Opisthorchis) sinensis (Chinese liver fluke) | Eggs are found in stool; very small (35 μm); they are operculated with shoulders, into which the operculum fits | Symptoms depend on worm burden; the organism is acquired from ingestion of raw fish; eggs can be missed unless a 40× objective is used for examination; eggs can resemble those of Metagonimus yokogawai and Heterophyes heterophyes (small intestinal flukes); worms are hermaphroditic |
| Paragonimus spp. (lung fluke) | Eggs are coughed up in sputum (brownish “iron filings” are egg packets); can be recovered in sputum or stool (if swallowed); are operculated with shoulders, into which the operculum fits | Symptoms depend on worm burden and egg deposition; infection is acquired from ingestion of raw crabs; eggs can be confused with those of D. latum; infections seen in the Orient (infections with Paragonimus mexicanus are found in Central and South America); Paragonimus kellicotti infections (rare) are seen in the United States; worms are hermaphroditic but often cross-fertilize with another worm if present |
| Schistosoma mansoni (blood fluke) | Eggs are recovered in stool (large lateral spine); specimens should be collected with no preservatives (to allow demonstration of egg viability); worms occur in veins of the large intestine | Acquired from skin penetration by a single cercaria from a freshwater snail; pathological findings are caused by the host immune response to the presence of eggs in tissues; adult worms in veins cause no problems; adult worms are separate sexes |
| Schistosoma haematobium (blood fluke) | Eggs are recovered in urine (large terminal spine); specimens should be collected with no preservatives (to allow demonstration of egg viability); worms occur in veins of the bladder | Acquired from skin penetration by a single cercaria from a freshwater snail; pathological findings are as with S. mansoni; 24-h and spot urine samples should be collected; chronic infection has an association with bladder cancer; adult worms are separate sexes |
| Schistosoma japonicum (blood fluke) | Eggs are recovered in stool (very small lateral spine); specimens should be collected with no preservatives (to allow demonstration of egg viability); worms occur in veins of the small intestine | Acquired from skin penetration by multiple cercariae from a freshwater snail; pathological findings are as with S. mansoni; infection is usually the most severe of the 3 Schistosoma species because of the original loading dose of infective cercariae from a freshwater snail (multiple cercariae stick together); symptoms are associated with egg production, which is greatest in S. japonicum infections |
a
Modified from reference 10.
b
1 in. = 2.54 cm; 1 ft = 30.48 cm.