Figure 81-2 Entamoeba histolytica.
Amebiasis is an infection caused by the intestinal protozoan parasite Entamoeba
histolytica (Fig. 81-2), resulting in diarrhea or amebic dysentery. Extraintestinal
infections may occur when the parasite spreads to other organs, the most common
being the liver, resulting in hepatic abscesses.
28,29 This parasitic infection is
distributed globally, affecting more than 50 million people with over 100,000 deaths
30 Those most affected are people living in, traveling to, or recent
immigrants from endemic areas where there are suboptimal sanitary conditions,
particularly in tropical or subtropical developing countries. Transmission occurs by
the fecal–oral route via direct person-to-person contact or by consumption of
31 Risk factors include poor sanitary conditions, anal
sexual exposure, and household contact with an infected person. Most individuals are
infected with E. dispar (80%) or E. moshkovskii, which are antigenically different
strains from the pathogenic E. histolytica (10%) and do not cause symptomatic
32,33 Amebiasis can be asymptomatic, or it may present as colitis or
dysentery. Extraintestinal lesions include abscesses in the liver, lungs, skin, and,
Infection occurs by ingestion of mature cysts present in fecally contaminated water or
food. Excystation occurs in the lumen of the small intestines where eight trophozoites
are released from one cyst and migrate to the large intestine (Fig. 81-3).
Trophozoites multiply by binary fission. In noninvasive infections, trophozoites
remain in the intestinal lumen. In invasive infections, trophozoites invade the
intestinal mucosa producing ulcerations, resulting in amebic dysentery.
Trophozoites may enter the bloodstream traveling to the liver, and in rare instances to
the brain, lungs, and genitals forming abscesses. Both trophozoites and cysts are
passed in the feces of an infected individual. Trophozoites do not survive outside the
host body, if ingested they will be destroyed by gastric juices. The cyst form can
survive days to weeks in the external environment, only killed by temperatures in
excess of 55°C or hyperchlorinated water.
Figure 81-3 Entamoeba histolytica trophozoites.
150/90 mm Hg, heart rate 90 bpm, temperature 37.6°C. He has no signs of jaundice or lymphadenopathy.
findings does B.W. have that supports the diagnosis of amebic colitis?
Patients presenting with acute amebic colitis may present with bloody or watery
diarrhea, abdominal pain, constipation, tenesmus, fever, rectal bleeding (especially
in children), nausea, and anorexia.
36–39 On physical examination patients may present
with elevated temperature, tachycardia, and hypertension in cases of severe colitis.
Intestinal amebic dysentery may be suspected in patients from endemic areas with
bloody or watery diarrhea, abdominal pain, and fever. Diarrhea lasting more than 10
days should be evaluated for intestinal parasites.
40 Almost all patients presenting
with amebic colitis will have heme-positive stools. Microscopic examination of
stool samples can only identify the presence of trophozoites or cysts but cannot
differentiate between pathogenic or nonpathogenic strains of the Entamoeba species.
Three stool samples from different days need to be collected, because cysts may be
missed if only one sample is tested. Serology tests cannot determine between a
current or past infection. There are various antigenic assay detection kits
commercially available with varying specificity and sensitivity to differentiate the
type of species found in the stool. B.W. is a recent immigrant from an endemic
country and is presenting with symptoms consistent with amebic colitis. Antigenic
examination of the stool confirmed B.W. has amebiasis, with the presence of E.
CASE 81-4, QUESTION 2: What medications are available to treat B.W.’s symptomatic infection? What
medication regimen is preferred?
Treatment is necessary if E. histolytica has been specifically identified. Treatment
is not necessary with E. dispar infections.
36–38 Treatment involves the elimination of
both trophozoites and cysts from the intestinal lumen. There are two classes of drugs
to treat B.W.’s amebiasis: tissue amebicides and intraluminal amebicides. Those
presenting with mild-to moderate intestinal symptoms or severe intestinal symptoms
or extraintestinal disease should be treated with a tissue amebicide followed by a
course of intraluminal amebicide therapy.
41 B.W.’s symptomatic amebic colitis needs
to be treated with both a tissue amebicide and intraluminal agent. Tissue amebicides
include nitroimidazoles, nitazoxanide (Alinia), and chloroquine. The drug of choice
with a 90% cure rate are the 5-nitroimidazoles, metronidazole (Flagyl), and
29,39 Nitazoxanide shows promise, but the data are limited;
chloroquine is ineffective in intestinal illnesses. Metronidazole will kill the
trophozoites in the intestines and the tissues, but it does not eradicate the cysts from
the intestine. To eradicate colonization, therapy should be followed with an
intraluminal amebicide, such as paromomycin, diloxanide furoate (Furamide––not
approved in the United States or Canada), or iodquinol (Yodoxin).
Drug Therapy of Various Parasitic Infections
Drug of Choice Dosage Adverse Effects Warnings
Asymptomatic including cyst passers
Iodoquinol Adults: 650 mg PO TID ×
Symptomatic or invasive intestinal infections
Metronidazole Adults: 750 mg PO TID ×
Tinidazole Adults: 2 g PO once daily
Followed by luminal amebicide:see above for dosing
Metronidazole Adults: 750 mg PO/IV
Tinidazole Adults: 2 g PO once daily
Followed by luminal amebicide agent to eliminate intestinal colonization—see above for dosing
Albendazole Adults and children: 400
Pyrantel pamoate Adults and children: 11
Albendazole Adults and children: 400
Diethylcarbamazine Adults: Day 1, 50 mg PO;
Albendazole 400 mg PO once daily for Nausea, vomiting, Avoid during pregnancy
10 days headache, and abnormal
Praziquantel Adults and children: 40
Metronidazole Adults: 250 mg PO TID
Tinidazole Adults: 2 g PO single dose
Nitazoxanide Adults and children: >12
Albendazole 400 mg PO daily × 5 days Nausea, vomiting,
Mebendazole 200 mg PO TID for 5
Mebendazole Adults and children: 100
Albendazole 400 g PO single dose Nausea, vomiting, Avoid during pregnancy
1% Permethrin (Nix) See Table 81-4 for
Ivermectin Adults and children: 200
Sodium stibogluconate Adults: 20 mg /kg IV or
abnormalities (ST- and Twave changes)
Liposomal Amphotericin B Immunocompetent: 3
Ivermectin Adults: 200 mcg/kg PO;
Lindane (Kwell) Apply topically once Not recommended in
Crotamiton 10% (Eurax) Apply topically to whole
Localskin irritation Do not apply to raw,
Praziquantel Adults and children: 5–10
Albendazole Adults: 400 mg BID × 8–
Metronidazole Adults: 2 g PO × 1 day or
cOff-label use for tapeworms. Dose for Hymenolepis nana is 25 mg/kg single dose.
eEchinococcus granulosus and Echinococcus multilocularis.
BID, twice daily; IM, intramuscularly; IV, intravenously; PO, orally; TID, three times daily.
B.W.’s amebic colitis should be treated with metronidazole 750 mg orally or IV 3
34 Alternative treatment with tinidazole 2 g once daily for
3 days is often better tolerated because of its shorter duration of therapy. Common
adverse effects of metronidazole are nausea, metallic taste, and abdominal
discomfort. Patients should be warned of a possible disulfiram reaction with alcohol
consumption during and up to 72 hours after the completion of therapy.
be followed by an intraluminal amebicide; the drug of choice is paromomycin 25 to
35 mg/kg/day orally in three divided doses for 5 to 10 days, second-line agents
include diloxanide or iodoquinol
(see Table 81-3). The most common adverse
effects of paromomycin are abdominal pain/cramping, nausea, and diarrhea.
Paromomycin and metronidazole should not be taken simultaneously.
In those with a confirmed diagnosis of amebiasis, one study showed the addition of
the probiotic Saccharomyces boulardii with metronidazole reduced the duration of
bloody diarrhea and aided in the clearance of cysts when compared to metronidazole
B.W. must be monitored for the improvement of symptoms of diarrhea and
abdominal pain. Follow-up should continue for 3 months after treatment. Three
separate stool samples should be examined for cysts.
M.A. is an asymptomatic cyst passer. Cyst passers often do not develop invasive
infections and sometimes can clear the infection spontaneously.
nonpathogenic cyst E. dispar does not require treatment, but all asymptomatic
patients with positive test results of E. histolytica must be treated to reduce the
CASE 81-5, QUESTION 2: What medications should be used to treat M.A.?
Intraluminal amebicides are the drug of choice in the treatment of asymptomatic
amebiasis. Paromomycin is the drug of choice, 25 to 35 mg/kg/day in three divided
doses for 5 to 10 days with meals. Diloxanide furoate (not commercially available in
the United States) or iodoquinol may also be recommended, but paromomycin has a
higher efficacy rate when compared to diloxanide.
45 M.A.’s stools must be tested
monthly for 3 months to assure eradication of the infection.
QUESTION 1: M.M. is a 26-year-old man presenting to the ED complaining of upper right quadrant
Extraintestinal infections occur in less than 1%, of which liver abscesses are the
most common. The right hepatic lobe is the most frequently affected area because of
the portal circulatory system of the colon.
38 M.M.’s clinical presentation and
laboratory findings are consistent of a hepatic abscess. Only diarrhea is reported in
approximately 50% of those infected.
38 The drug of choice for amebic liver abscess
is metronidazole for 5 to 10 days followed with a luminal agent such as
37 Aspiration or drainage of the abscess is not recommended unless
there is a failure to respond to therapy after 4 to 5 days, evidence of a secondary
infection or impending rupture into the pericardium. After the completion of therapy,
M.M. should be monitored for regression of the abscess with ultrasounds, and lesions
may take up to 12 months to resolve. Complications of hepatic abscesses may include
perforation of the abdominal cavity, septic shock, or superinfections. Infections
outside of the intestines or liver are extremely rare (<0.1%), dissemination of the
amebae to the brain or skin are almost always associated with amebic liver abscess.
Because of the rarity of these infections, there are no definitive guidelines for
are negative for the presence of liver abscesses. A diagnosis of intestinal amebiasis is made.
How would you manage S.W.’s infection? What are your concerns regarding her current condition?
S.W. needs to be treated for her intestinal amebiasis. The treatment of choice is
metronidazole, but she is in her first trimester of pregnancy. Nitroimidazoles such as
metronidazole and tinidazole should be avoided during the first trimester of
pregnancy. Metronidazole readily crosses the placenta, and effects on the
development of the fetus are unknown.
42 S.W. should be treated with paromomycin
25 to 35 mg/kg/day in three divided doses for 5 to 10 days. Paromomycin is an
aminoglycoside that is poorly absorbed in the bowel with an excellent safety profile.
The most common adverse effects are nausea, vomiting, and abdominal cramps.
S.W.’s stools must be examined for 3 months to assure resolution of the infection.
If S.W.’s infection does not resolve or progresses to fulminant colitis or amebic
liver abscess, she will safely be beyond her first trimester and should be treated
appropriately with a tissue amebicide such as metronidazole followed by a luminal
as paromomycin, or second-line agents such as iodoquinol or diloxamide furoate.
If iodoquinol is chosen, the most common adverse effects are headache, nausea, and
vomiting. Optic nerve damage is a concern with elderly patients and should be
avoided. Reported cases of peripheral neuropathy have occurred in doses in excess
of the recommended dose. Should diloxamide furoate be prescribed the most
common adverse effect is flatulence, but it is not commercially available in the
Giardia is a globally endemic illness caused by the protozoa Giardia lamblia.
highest number of cases are typically seen in developing countries with inadequate
sanitary conditions, untreated water, or contaminated food. Although primarily seen
in Asia, Africa, or South America, it is the most frequent parasitic intestinal disease
34,47 Those at greatest risk live in rural areas with poor sanitation,
low socioeconomic conditions, and have contact with infected individuals or travel
internationally. Giardia is most prevalent in children and immunocompromised
individuals. Presentation of illness varies between individuals from self-limiting
diarrhea to more severe symptoms of chronic diarrhea, abdominal cramps, loose pale
greasy stools that float, fatigue, weight loss and malabsorption of fat, lactose,
, but many present with no symptoms.
may lead to mucosal damage in the small intestines.
Infection occurs with the ingestion of cysts in contaminated water, food, or by the
fecal–oral route. In the small intestine, excystation releases trophozoites (each cyst
46 Trophozoites replicate in the lumen of the proximal
small bowel where they can be free or attached to the mucosa by a ventral sucking
disk. Encystation occurs as the parasites transit toward the colon and are excreted in
the feces. The cysts are hardy, surviving in environmental elements such as cold
water and chlorination for weeks (Fig. 81-4).
QUESTION 1: P.C., a 23-year-old woman who returned 1 week ago from a mission trip from Zambia,
. Three separate stool sample were positive for G. lamblia cysts with antigenic tests
confirming the presence of G. lamblia.
What clinical findings does P.C. have that support the diagnosis of Giardia?
Figure 81-4 Giardia lamblia cyst.
Patients with Giardia may be asymptomatic, present with self-limiting diarrhea or
with chronic diarrhea may suffer from dehydration and malabsorption of fat, lactose,
47,50 The typical incubation period is 6 to 15 days after
ingestion of the cysts. Infections may occur with as little as 10 cysts.
of a giardiasis diagnosis is confirmed by signs and symptoms and the presence of
cysts or trophozoites in stool samples. One stool sample may miss the presence of
cysts, so it is recommended to test multiple stool samples (at least three). Antigenic
testing of stool samples either ELISA or direct fluorescent antigen tests are more
sensitive than traditional wet mount microscopy and should be performed if a patient
presents with symptoms consistent with giardia but have multiple negative results for
cysts. A differential diagnosis ruling out bacterial (Salmonella, Shigella,
Campylobacter) and viral (rotavirus, norovirus) causes should be made because the
presentation may be similar. Giardia differs from these infections by its length of
illness (7–10 days before first presentation) and evidence of weight loss.
P.C.’s history of missionary work in Zambia is considered high risk. She may have
consumed contaminated food or water, particularly if she swallowed lake water. Her
symptoms are consistent with chronic giardia: foul-smelling greasy stools, periods of
diarrhea and constipation, fatigue, and abdominal cramps. Some patients will present
with flatulence as well. Her laboratory results are consistent with malabsorption:
fecal fat content is greater than 12 g (normal fecal fat content is <7 g)
greater than 7 is indicative of fat malabsorption resulting in steatorrhea.
infections may also present with anemia and low vitamin B12
lamblia cysts identified in the stool after three tests and confirmed with antigenic
testing. A definitive diagnosis of Giardia can be made.
CASE 81-8, QUESTION 2: How should P.C. be treated?
Treatment of choice for P.C.’s giardiasis is a nitroimidazole. Metronidazole 250
mg orally 3 times a day for 5 to 7 days is the preferred option with efficacy rates
53 Tinidazole 2 g as a single dose may be better tolerated because
of the shorter duration of therapy with similar rate of efficacy. Alternative treatment
options include nitazoxanide (Alinia) 500 mg orally twice a day for 3 days,
paromomycin 500 mg 3 times daily for 10 days (drug of choice during the first
trimester of pregnancy), albendazole (Albenza) 400 mg orally once daily for 5 days.
55 are effective options but are not commercially
available in the United States.
P.C. should be counseled to practice good hygiene, washing hands with soap and
water for at least 20 seconds, to control
56 Symptoms of diarrhea typically subside in 1 to 2 days
and completely resolve in 10 days. Symptoms of malabsorption may take up to 4 to 8
If treatment failure occurs, another course of therapy is necessary.
Resistant strains have been treated with success with either higher doses or a longer
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