Human enterobiasis or pinworm infection is caused by the intestinal nematode,
Enterobius vermicularis. It is the most common helminthic infection worldwide. The
highest rate of infection occurs in school-aged children (5–10 years),
institutionalized persons, and household members or caretakers of an infected
individual. Unlike other helminth infections, all socioeconomic groups are affected.
Transmission occurs via the fecal–oral route with finger sucking considered a source
of infection for children. If one family member is infected, all members of the family
Humans are the only host of E. vermicularis. The entire life cycle occurs in the
gastrointestinal system for approximately 4 weeks. Infected eggs are ingested from
are ingested they hatch in the small intestine (duodenum), larvae migrates to the large
intestine where they molt twice before becoming adult worms. Copulation occurs in
the ileum, where the male typically dies and passes in the stool. The gravid female
settles into the large intestine and caecum. The female body fills with approximately
11,000 eggs and makes nocturnal migrations to the rectum and anus where oxygen is
needed for the maturation of eggs. Eggs are then deposited in the area around the anus
skin of the perianal area, evidence of E. vermicularis eggs are found under the microscope.
What signs and symptoms does N.D. exhibit to confirm a diagnosis of pinworm?
Many infected with pinworms are asymptomatic or present with mild symptoms;
the most common symptom being nocturnal perianal pruritus and scratching because
of the deposit of eggs to the perianal skin. The intense scratching may lead to
secondary bacterial infections and sleep disturbances. Patients with major
infestations may present with abdominal pain, anorexia, or insomnia. Enterobiasis
infections rarely cause serious disease, but it may result in dysuria, appendicitis, or
N.D. is presenting with perianal itching, inability to sleep because of intense
itching and a positive “tape test” showing E. vermicularis eggs indicative of a
DpDx-Laboratory Identification of Parasitic Diseases of Public Concern. Enterobiasis Image Gallery.
https://www.cdc.gov/dpdx/enterobiasis/index.html. Accessed July 31, 2017.
CASE 81-9, QUESTION 2: How should N.D.’s infection be managed? Should his parents be treated?
There are three options for the treatment of pinworm: albendazole (Albenza),
pyrantel pamoate (PinX––available without a prescription), and mebendazole (not
commercially available in the United States). Albendazole 400 mg as a single dose
or pyrantael pamoate 11 mg/kg (maximum 1 g) are viable options for N.D.; a second
dose must be repeated in 2 weeks. Mebendazole is currently not commercially
available in the United States, but it is available internationally. Mebendazole is
given as a single 100 mg dose and repeated in 2 weeks.
Albendazole and mebendazole are broad-spectrum antihelmites that selectively
interferes with the microtubules in the intestinal cells of nematodes and glycogen
depletion leading to their death.
68,69 Albendazole is considered the drug of choice in
the treatment of enterobiasis with a 90% cure rate. Both albendazole and
mebendazole are well tolerated with abdominal pain as the most common adverse
effect reported. Pyrantel pamoate is a depolarizing neuromuscular agent that acts by
paralyzing adult worms, resulting in their expulsion in the stool before eggs are
69 The most common adverse effects are nausea and vomiting. The liquid
formulation must be shaken well before use and may be mixed with fruit juice or
All members of N.D.’s family needs to be treated simultaneously. Other
preventative methods include proper hand hygiene, particularly after toilet use and
before meals, trimming of fingernails, and frequent washing of bed linens and
undergarments using hot water.
Intestinal cestodes (tapeworms) are segmented worms; adults primarily live in the
gastrointestinal tract, but larvae can live in any organ.
that affect humans: Taenia saginata (beef tapeworm), Taenia solium (pork
tapeworm), Diphyllobothrium latum (broad or fish tapeworm), Taenia asiatica
(Asian tapeworm), and Hymenolepis nana (dwarf tapeworm). Humans become
infected by eating raw or undercooked meat (pork or beef) or fish (Diphyllobothrium
A tapeworm attaches to the mucosa of the intestines by a scolex, the anterior
portion of the tapeworm “head of the tapeworm.”
70–72 The crown of the scolex is
referred to as the rostellum; it may be armed with hooklets or unarmed (no hooklets).
Proglottids or segments form behind a short neck. As each proglottid matures, they
form a chain called a strobili, giving the tapeworm its ribbon-like appearance. The
length of the strobili varies by species (T. saginata may have over 2,000 proglottids).
Mature proglottids contain eggs that are indistinguishable between species. A
definitive diagnosis is made by examining the scolex, proglottids, and eggs.
Tapeworms obtain nutrients through their skin directly from the host, they do not
invade the tissue mucosa or acquire blood; therefore, symptoms are often absent or
71 Typical symptoms involve the gastrointestinal tract, mild
eosinophilia, and elevated IgE levels may be detected. T. solium infections may lead
to cysticercosis; neurocysticercosis, the most serious form, involves the central
nervous system. It is important, therefore, to seek treatment with any tapeworm
Humans are the only definitive host for T. saginata and T. solium. Cattle or pigs are
infected by ingesting vegetation contaminated with eggs or gravid proglottids passed
in feces. Eggs hatch and invade the intestinal wall and migrate to the muscles where
they develop into cysticerci. Humans become infected by eating raw or undercooked
meat. In humans, the cysticercus develop in the small intestines to an adult worm over
a period of 2 months and can survive for years. Adult worms produce proglottids,
which mature and become gravid, detach from the worm, and migrate to the anus or
It is estimated that the prevalence of Taenia infections is approximately 170 to 200
71 T. solium occurs worldwide where pigs are raised, but it
is most prevalent in Latin America, sub-Saharan Africa, China, India, and Southeast
70–71 T. saginata can occur anywhere raw or undercooked beef is eaten. Most
prevalent areas of occurrence are Eastern Europe, Russia, East Africa, Latin
America, Indonesia, and China.
73 Rural and poor socioeconomic areas in developing
countries with poor hygiene are at greatest risk.
74 Most cases of neurocysticercosis in
industrial countries are primarily seen in refugees, immigrant populations, or
travelers from endemic countries.
Cysticercosis is caused by the larval cysts of the T. solium tapeworm. The larvae
migrate to muscle tissue, brain, subcutaneous tissue, or the eye. When the infection
develops in the central nervous system it is called neurocysticercosis and is
responsible for one-third of acquired epilepsy cases worldwide.
symptoms vary by location of the cyst. Clinical manifestations of cysts in the muscle
may be absent other than a lump under the skin. Cysts that deposit in the eye may
cause floaters in the eye or blurry vision.
neurocysticercosis depends on how many cysts are found and the location; symptoms
include seizures, focal neurologic deficits, cognitive decline, and increased
Diagnosis of neurocysticercosis is made by clinical presentation and radiologic
imaging such as magnetic resonance imaging (MRI) or CT scans.
be customized for each patient, depending on the size, location, and stage of the
cysticerci. Treatment includes use of antihelminthic agents, such as praziquantel
and/or albendazole. Coadministration of corticosteroids and antiepileptic drugs are
often used to manage seizures and the inflammatory response because of the
cysticidal activity of praziquantel therapy. Severe cases may require surgery.
Taenia Saginata and Taenia Solium
QUESTION 1: L.D. is a 28-year-old missionary who lived 12 months in Ethiopia. He has returned to the
separate days, and a cellophane tape test of the perianal area revealed presence of T. saginata.
Are L.D.’s symptoms consistent with Taenia infection? How can infections be differentiated?
Patients presenting with tapeworm infections may present with a range of
symptoms. Many present with no symptoms, but if symptoms are present then they are
often mild. Symptoms include abdominal pain, cramps, flatulence, constipation,
nausea, vomiting, headache, vitamin deficiency, or weight loss. More severe
symptoms include appendicitis and pancreatitis. Patients may report the voiding of
segments from the anus and in the feces. Some patients may present with moderate
eosinophilia and elevated IgE because of the weak immunogenic activity of the adult
The eggs of T. saginata and T. solium are morphologically indistinguishable. A
definitive diagnosis of species can be made by examining the scolex (in rare cases)
or more likely mature gravid proglottids. Eggs and proglottids are only detected in
the feces 2 to 3 months after the infection has been established. The CDC
recommends three separate stool samples on different days to establish a diagnosis.
T. solium scolex is characterized by a scolex with four large suckers and rostellum
with two rows of hooks (see Fig. 81-6).
72,81,82 T. saginata has a scolex with four
suckers but no rostellum or hooks (see Fig. 81-7).
81 Biomolecular assays are more
accurate than microscopic stool examinations to distinguish between species but are
only available in a research laboratory setting.
https://www.cdc.gov/dpdx/taeniasis/index.html. Accessed July 31, 2017.)
https://www.cdc.gov/dpdx/taeniasis/index.html. Accessed July 31, 2017.)
CASE 81-10, QUESTION 2: How should L.D. be treated and how should it be evaluated?
The drug of choice for both T. saginata and T. solium infections is praziquantel
(Biltricide) 5 to 10 mg/kg as a single dose.
83 Praziquantel works by killing the adult
worm but does not destroy the eggs. Typical adverse effects include malaise,
headache, dizziness, abdominal discomfort, and rarely urticaria. L.D. should be
advised to swallow the tablet whole with water and a meal. The bitter taste of the
tablet may cause L.D. to gag or vomit, so it is advisable to swallow the tablet
promptly. Praziquantel cannot be chewed, but it may be halved or quartered. It is
contraindicated to administer praziquantel with strong Cytochrome P450 (CYP450)
inducers, such as rifampin, since adequate blood levels of praziquantel may not be
achieved. L.D. should also be advised to not drive or operate heavy machinery on the
day of treatment and the following day. Praziquantel has a high cure rate (up to 98%)
in T. saginata and T. solium infections.
An alternative regimen is niclosamide 2 g as a single oral dose for adults and 50
mg/kg in children. It is not available for human use in the United States. For those
infections resistant to praziquantel and niclosamide, nitazoxanide has been used with
success in T. saginata infections.
71 With any treatment it is recommended to examine
stool samples for eggs 1 and 3 months after treatment to confirm eradication of the
Recovery of the scolex for species identification may be necessary; purging of the
71 There are two options: administer electrolyte–polyethylene
glycol prior to therapy or administer castor oil or magnesium sulfate solution within
2 hours of taking the anthelminthic drug. The tapeworm and its fragments should pass
within 6 to 12 hours. Patients should be instructed to collect all feces for up to 72
hours to collect all fragments.
Pediculosis (lice infections) can be caused by Pediculus humanus capitis (head lice)
(Fig. 81-8) , Pediculus humanus (body lice), or Pthirus pubis (crab lice). Lice
infections remain a major problem throughout the world, and infestations can be
present in all socioeconomic groups.
It is estimated that treatment costs for head
lice may exceed $1 billion in the Unites States.
Figure 81-8 Pediculus humanus capitis (head lice).
Head and body lice have similar life cycles, but their habitat preferences distinguish
the two varieties. The life cycle of lice has three stages: egg, nymph, and adult. After
fertilization, the female lays eggs (up to 10 a day), which attaches tightly to hair or
seams of clothing. The eggs (nits) (see Fig. 81-9) hatch in 7 to 10 days and produce
nymphs, which go through three molts to become adults. Louse feed by injecting a
small amount of their saliva into the host and withdraw small amounts of blood;
without these meals the louse will die. The lice penetrate the host skin via stylets
within their head and attach themselves by a circlet of teeth on their proboscis. Crab
lice are usually found in pubic hair, less frequently on facial hair, eyebrows,
eyelashes, and axillary hairs.
The highest incidence of head lice is seen among schoolchildren aged 3 to 12 years
and in household members. It is less common in African Americans because of
different hair texture. The incidence of head lice infestation in the United States has
been estimated to be as much as 12 million cases.
90 Body lice infest clothing by
laying their eggs on the fabric. Head and crab lice infest hair by laying their eggs at
the base of hair fibers. Hair and body lice are transferred between hosts by personal
and clothing contact, whereas crab lice are transmitted by sexual contact.
Figure 81-9 Nit or egg of lice on hair shaft.
says he has “lice all over his body.”
Why are the symptoms in W.L. consistent with lice infection?
The most common complaint of patients with head and body lice is pruritus of the
scalp, ears, neck, and other body parts. Chronic infestations may result in
hyperpigmentation and thickening of the skin. These changes are often seen in the
waist, groin, and upper thigh areas (“vagabond’s disease”). In severe infestations,
intractable itching and scratching can result in folliculitis, hemorrhagic macules or
papules, or secondary bacterial infections.
In contrast, schoolchildren who are
exposed frequently to head lice may have only minor pruritus affecting the scalp,
CASE 81-11, QUESTION 2: How should W.L. be treated for lice infection of the head, body, and genital
Concurrent treatment for the pustular bacterial lesions and lice infestation should
be initiated in W.L. First-line treatment of head lice in both adults and children is
permethrin 1% (Nix) rinse. Alternatives are 0.5% malathion (Ovide) lotion and
pyrethrin 0.33% plus 4% piperonyl butoxide (Rid). Recent studies indicate that there
may be differences in efficacy among the topical agents. Resistance of head lice to
Ovide lotion (0.5% malathion) has been documented in the United Kingdom. The
formula used in the United States contains terpineol, dipentene, and pine needle oil,
possibly responsible for delaying resistance in the United States.
approved as a second-line therapy because of its neurologic toxicities, and it should
be used only when other alternatives have failed (see Table 81-4).
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