Page 112 - JSOM Spring 2018

An Ongoing Series

Giardiasis

Mark W. Burnett, MD

Introduction
Giardiasis is the most common type of intestinal parasitic in- of infecting others can be excreted for weeks to months by
fection of humans in the United States and is a frequent cause those with untreated symptomatic or asymptomatic infections.
of gastroenteritis worldwide. In the United States, more than Persistence of symptoms after completion of antibiotic therapy
20,000 cases are reported annually, but this is likely an un- can be seen in up to one-half of patients who develop postin-
derestimate, because more than half of infections in humans fectious lactose intolerance, which can take weeks to resolve.
are asymptomatic. Worldwide, it is estimated that giardiasis Symptoms may also persist in the immunosuppressed, in those
causes symptomatic infections in more than one-quarter bil- who do not fully complete treatment, or in those who are in-
lion people annually, especially in areas where personal hy- fected with a drug-resistant parasite.
giene is compromised.
Diagnosis
Background and Clinical Presentation
Enzyme immunoassay or direct fluorescent antibody assays
Giardiasis is caused by the protozoan parasite Giardia in- are the standard tests for diagnosis of giardiasis in the United
testinalis, which was previously known as G. lamblia or G. States, but they may not be available in the developing world.
duodenalis. Eight separate genotypes (A–H) exist, but only Microscopic discovery of cysts or trophozoites in the stool can
genotypes A and B cause infections in humans. Humans are be challenging if the number of infective organisms is small or
the primary reservoir of infection of other humans, but dogs, the laboratory personnel are inexperienced. Multiple samples
cats, beavers, cattle, and a multitude of other animals can be may be required to make the diagnosis, or empiric treatment
the source in urban, rural, or wilderness environments. Risk considered in a deployed environment.
of infection increases in those who travel to developing world
regions where the infection is common and in those who drink Treatment
contaminated tap water or untreated freshwater from lakes
or streams. Giardiasis can also occur by eating contaminated Metronidazole (Flagyl) has long been the most common treat-
food, by being in close contact with those who are infected, ment; the course is daily for 5–10 days. Alcohol consumption
and by sexual contact. while being treated with metronidazole may cause adverse ef-
fects. Tinidazole treatment consists of a one-time dose. Nita-
Infection occurs through the ingestion of Giardia cysts; as zoxanide is a 3-day course of treatment.
few as 10 cysts may cause a symptomatic infection. The cysts
hatch, form trophozoites that multiply in the small intestine, Vaccination
and then encyst in the large intestine. An infected person can
shed between 1 billion and 10 billion cysts daily. As soon as the No vaccination currently exists for the prevention of Giardia
cysts have been excreted from the body in stool, they are infec- infections.
tious. It has been estimated that up to one-third of those who
reside in the developing world have been infected at least once Importance in a Deployed Setting
in their lifetime. Cysts can survive for weeks to months after
being excreted and can be resistant to chemical disinfection. Giardiasis is a threat to troops in the field in training areas
in the United States and commonly causes debilitating diar-
Symptoms develop 1 to 3 weeks after ingestion of the infectious rhea in those who reside or work in the developing world. It
cysts. The classic presentation of giardiasis is frequent foul- should be considered for those who are afebrile and suffering
smelling, watery stools; flatulence; and abdominal distention. from profuse, foul-smelling, watery diarrhea that does not
Fever and blood in the stools are rare. Left untreated, symp- contain blood. Prevention is key and includes using approved
tomatic disease can be protracted and debilitating, with sig- water sources, boiled water, or water filtration systems to pre-
nificant weight loss and loss of the desire to eat. Cysts capable vent illness.

COL Burnett is currently chief of pediatric infectious diseases at Tripler Army Medical Center, Honolulu, HI, and is the pediatric subspecialties
consultant to the US Army Surgeon General. He is board certified in pediatrics and pediatric infectious diseases.

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