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Giardiasis Surveillance
United States, 1992–1997
Centers for Disease Control and Prevention (CDC) Continuing Medical Education for U.S. Physicians and Nurses
August 11, 2000
The MMWR series of publications is published by the Epidemiology Program Office,Centers for Disease Control and Prevention (CDC), U.S. Department of Health andHuman Services, Atlanta, GA 30333.
Centers for Disease Control and Prevention.CDC Surveillance Sum- maries, August 11, 2000. MMWR 2000;49(No. SS-7).
[Author(s)]. [Title of particular article]. In: CDC Surveillance Sum- maries, August 11, 2000. MMWR 2000;49(No. SS-7):[inclusive pagenumbers].
Centers for Disease Control and Prevention . Jeffrey P. Koplan, M.D., M.P.H.
The production of this report as an MMWR serial publication was coordinated in Epidemiology Program Office . Barbara R. Holloway, M.P.H.
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Vol. 49 / No. SS-7
August 11, 2000
Reports Published in CDC Surveillance Summaries Since January 1, 1990
Most Recent Report
State-Specific Prevalence of Selected Health Contribution of Birth Defects to Infant Mortality Family Planning Services at Title X Clinics *Abbreviations
Agency for Toxic Substances and Disease Registry National Center for Chronic Disease Prevention and Health Promotion National Center for Environmental Health and Injury Control National Center for HIV, STD, and TB Prevention National Center for Injury Prevention and Control National Institute for Occupational Safety and Health Vol. 49 / No. SS-7
Reports Published in CDC Surveillance Summaries Since January 1, 1990 — Continued
Most Recent Report
Infant Mortality (see also National Infant Mortality; Neisseria gonorrhoeae, Antimicrobial Resistance in NCPS August 11, 2000
Vol. 49 / No. SS-7
Giardiasis Surveillance — United States, 1992–1997
1Epidemic Intelligence Service, Epidemiology Program Office, CDC 2Division of Parasitic Diseases, National Center for Infectious Diseases, CDC Abstract
Problem/Condition: Giardia intestinalis, the organism that causes the gastrointestinal
illness giardiasis, is the most commonly diagnosed intestinal parasite in public health
laboratories in the United States. In 1992, the Council of State and Territorial
Epidemiologists assigned giardiasis an event code that enabled states to begin
voluntarily reporting surveillance data on giardiasis to CDC.
Reporting Period: This report includes data that were reported from January 1992
through December 1997.
Description of the System: The National Giardiasis Surveillance System includes data
about reported cases of giardiasis from participating states. Because most states were
already collecting data on occurrence of giardiasis, the assignment of an event code to
giardiasis has allowed voluntary reporting of these data to CDC via the National
Electronic Telecommunications System for Surveillance.
Results: Since 1992, the number of states reporting cases of giardiasis to CDC has risen
from 23 to 43. The annual number of giardiasis cases reported has ranged from 12,793
in 1992 to 27,778 in 1996. In 1997, cases per 100,000 state population ranged from 0.9
to 42.3, with 10 states reporting >20.0 cases per 100,000 population and a national
average of 9.5 cases per 100,000 population. In 1997, New York State, including New
York City, reported the highest number of cases (3,673, or 20.3 cases per 100,000
population), accounting for 14.5% of cases nationally; however, Vermont reported the
highest incidence rate in 1997 (42.3 cases per 100,000 population). Both states have
active surveillance systems in place for giardiasis. Cases have an approximately equal
sex distribution. Nationally, rates were the highest among children aged 0–5 years,
followed closely by persons aged 31–40 years. In these two age groups, most cases
were reported during late summer and early fall — an indication that transmission
occurred during the summer.
Interpretation: This report documents the first nationwide look at epidemiologic
parameters and disease burden estimates for giardiasis in the United States.
Transmission occurs in all major geographic areas of the country. The seasonal peak
in age-specific case reports coincides with the summer recreational water season and
might reflect the heavy use by young children of communal swimming venues (e.g.,
lakes, rivers, swimming pools, and water parks) — a finding consistent with Giardia’s
low infectious dose, the high prevalence of diaper-aged children in swimming venues,
the extended periods of cyst shedding that can occur, and Giardia’s environmental
resistance. Estimates based on state surveillance data indicate that as many as 2.5
million cases of giardiasis occur annually in the United States.
August 11, 2000
Public Health Action: Giardiasis surveillance provides data to educate public health
practitioners and health-care providers about the scope and magnitude of giardiasis in
the United States. These data can be used to establish research priorities and to plan
future prevention efforts.
Giardiasis is the gastrointestinal illness caused by the flagellated protozoan Giardia intestinalis, also known as G. lamblia or G. duodenalis. Giardia is the most commonlydiagnosed intestinal parasite in public health laboratories in the United States (1–4 ).
Giardia was the most frequently identified etiologic agent of outbreaks associated withdrinking water in the United States for the years 1976–1994 (5 ).
Giardia is spread from person to person and from animals to humans through fecal- oral transmission, has an incubation period of 3–25 days (median, 7–10 days), and hasa two-stage life cycle — trophozoite and cyst. The life cycle begins with ingested cysts,which release trophozoites (10–20 µm x 5–15 µm) in the duodenum. These trophozo-ites attach to the surface of the intestinal epithelium by using a ventral sucking disk andthen reproduce by binary fission. The trigger for encystment is unclear, but the processresults in the inactive, environmentally resistant form of Giardia — a cyst (11–14 µm x7–10 µm) that is excreted in feces (6 ).
Giardiasis occurs when cysts are ingested through person-to-person transmission or ingestion of fecally contaminated food or water. The infectious dose is low: humanscan be infected with as few as 10 cysts (6,7 ). Persons at greatest risk of exposure toinfection are children in day care, their close contacts, men who have sex with men,backpackers and campers (via ingestion of unfiltered, untreated drinking water), travel-ers to disease-endemic areas, and persons drinking water from shallow wells (8–11 ).
Giardia is found worldwide and infects domestic and wild animals (e.g., cats, dogs, cattle, deer, and beavers) (6,7 ). Giardiasis usually occurs sporadically, although out-breaks do occur. Waterborne outbreaks, associated with ingestion of both drinking andrecreational water (e.g., lakes, rivers, or swimming pools) (5 ), and foodborne outbreaks(12,13 ) are well documented as are person-to-person outbreaks among men who havesex with men (8 ) and among children and staff in day care centers (14 ). The relativecontribution of waterborne, foodborne, and person-to-person transmission to sporadicgiardiasis is unknown.
Clinically, Giardia produces a broad spectrum of gastrointestinal symptoms, in- cluding one or more of the following symptoms: diarrhea, flatulence, bloating, weightloss, abdominal cramping, nausea, malabsorption, foul-smelling stools, steatorrhea,fatigue, anorexia, and chills. Although the hallmark of giardiasis is diarrhea, asymp-tomatic infections can occur (15,16 ), especially in children and in persons with priorinfections (6 ). Giardia cysts can be excreted in the stool intermittently for weeks ormonths, resulting in a protracted period of communicability (14,17,18 ).
Because infections can be asymptomatic or characterized by mild signs and symp- toms, giardiasis is often regarded as a benign gastrointestinal illness, although chronicor debilitating giardiasis has been reported (15 ). From 1979 through 1988, an esti-mated 4,600 hospitalizations per year in the United States resulted from severe giardia-sis and its complications, resulting in an average of 23,238 days per year in the hospital Vol. 49 / No. SS-7
and a mean annual incidence of 2.0 hospitalizations per 100,000 persons (15 ). Volumedepletion was reported for 33% of case-patients, and 19% of hospitalized children aged<5 years had a codiagnosis of failure to thrive (15 ).
Many effective treatment alternatives are available for patients with symptomatic giardiasis (Table 1). Metronidazole is the treatment most often prescribed in the UnitedStates. Furazolidone is a less effective treatment option, but it is the only drug ap-proved by the U.S. Food and Drug Administration (FDA) for treatment of giardiasis inthe United States. Because furazolidone is available in liquid form, it is often used totreat children (6 ). Quinacrine, an effective and inexpensive treatment option, is notavailable from any U.S. manufacturer, although several compounding pharmacies havemade it available. Tinidazole is widely used throughout the world; however, it is notapproved for use in this country. Albendazole has been reported to be as effective asmetronidazole with fewer side effects among children aged 2–12 years (19 ). Paromo-mycin, a nonabsorbed aminoglycoside, is less effective than other agents but is usedfor treatment among pregnant women because of potential teratogenic effects of theother agents (20 ). A combination of metronidazole and quinacrine has been used totreat refractory cases (21 ).
TABLE 1. Drugs for treatment of giardiasis*
(listed alphabetically)

Adults (nonpregnant)
100 mg by mouth 4 times a day for 7–10 days 250 mg by mouth 3 times a day for 5–7 days 500 mg (30mg/kg/day) by mouth 3 times a day for 7 days Pregnant Women
500 mg (30 mg/kg/day) by mouth 3–4 times a day for 7 days Children
6–8 mg/kg/day by mouth divided 3–4 times a day for 7–10 days 15 mg/kg/day by mouth divided 3 times a day for 5 days 30 mg/kg/day by mouth divided 3 times a day for 7 days 6 mg/kg/day divided 3 times a day for 5 days (maximum = 300 mg/day) Refractory Cases
750 mg by mouth 3 times a day for 14 days 100 mg by mouth 3 times a day for 14 days * Sources: Ortega YR, Adam RD. Giardia: overview and update. Clin Infect Dis 1997;25:545–50; Taylor GD, Wenman WM, Tyrrell DLJ. Combined metronidazole and quinacrine hydrochloride therapy for chronic giardiasis. CMAJ1987;136:1179–80; Sanford JP, Gilbert DN, Moellering RC, Sande MA. The Sanford guide to antimicrobial therapy,27th edition. Vienna, VA: Antimicrobial Therapy, Inc., 1997; and Anonymous. Drugs for parasitic infections. MedLett Drugs Ther 1998;40:1–12.
† Not commercially available in the United States.
August 11, 2000
Despite the public health importance of giardiasis, reliable data on national inci- dence and prevalence in the United States are not available. Estimates have been ex-trapolated from published data collected by states with active giardiasis surveillance,laboratory surveys, and waterborne-disease outbreak reports. Giardiasis is the mostfrequently reported diarrheal disease in northern New England (10 ), and from 1983through 1986, it was the most common reportable disease in Vermont (22 ). Giardiaalso was the most prevalent protozoan parasite in Arkansas during 1997 (23 ) as wellas the most commonly reported enteric pathogen in Wisconsin during the years1983–1986 (24 ).
The prevalence of Giardia in stool specimens submitted for examination ranges from 2% to 5% in industrialized countries and from 20% to 30% in developing coun-tries, and it can be as high as 35% among children attending day care centers in theUnited States in a nonoutbreak setting (4,6 ). Before the tightening of water treatmentstandards, an estimated 25% of endemic cases of giardiasis in the United States werewaterborne (7,9 ). Furthermore, giardiasis was one of the two leading recognized eti-ologies in the 129 water-associated disease outbreaks that occurred during the years1991–1994 in the United States (7 ).
Although giardiasis reporting is required by 43 states, it is not a nationally notifiable disease (25,26 ). In 1992, the public health importance of the disease prompted theCouncil of State and Territorial Epidemiologists to assign an event code (code 11570) togiardiasis to allow states to voluntarily transmit their reported giardiasis data to CDCvia the National Electronic Telecommunications System for Surveillance (NETSS) (27 ).
State health departments collect data on giardiasis case-patients from both health-careproviders and laboratories, and the data are subsequently transmitted with notifiabledisease information to CDC via NETSS.
The purpose of the National Giardiasis Surveillance System is to estimate the dis- ease burden in the United States and to monitor the demographic parameters (sex,age, race, ethnicity), seasonality, and geographic variation of giardiasis. This reportsummarizes national surveillance data on giardiasis for the years 1992–1997.
The diagnosis of giardiasis is made through examination of stool specimens that are typically collected and preserved in 10% formalin, although fresh stool may also beexamined. Light microscopy can be used to visualize the parasite via wet mount, stain-ing (trichrome or iron hematoxylin), or the direct fluorescent antibody detection method(monoclonal antibodies). In addition, enzyme linked immunosorbent assay (ELISA)-based kits have been used to detect Giardia-specific antigen in stool (4,6 ).
Approximately 85% of infections can be diagnosed with a single stool specimen.
Sensitivity increases with the number of stool specimens examined, so that three speci-mens collected every other day during a 5-day period will detect approximately 90% ofinfections (28 ). In the rare occasions when infections are suspected but multiple stooltests are negative, duodenal fluid can be sampled for Giardia trophozoites. Three pro-cedures have commonly been used: the string test (Entero-test), endoscopy with upperintestinal aspiration, and endoscopy with upper intestinal biopsy (6 ). No serologic testis commercially available (4,6 ).
The National Giardiasis Surveillance System collects data on persons who have either symptomatic or asymptomatic giardiasis, seek health care, have a positive diag- Vol. 49 / No. SS-7
nostic test result, and are reported to both the state and CDC (29 ). This report repre-sents the first time that national surveillance information on giardiasis has been pub-lished and includes reports from the 43 participating states.
Since the inception of the National Giardiasis Surveillance System in 1992, the num- ber of states voluntarily reporting cases and the number of states reporting >100 casesper year increased through 1994 and subsequently plateaued. In 1992, only 23 statesreported giardiasis cases to CDC, with only 17 states reporting >100 cases. By 1997, 43states reported giardiasis cases, with 40 of those states reporting >100 cases each.
In 1992, a total of 12,793 cases of giardiasis were reported. The number of reported cases eventually doubled by 1994. Between 1994 and 1997, case reporting plateaued at25,389–27,778 cases reported per year, with sex distribution being approximately equal(Figure 1). In 1997, New York State, including New York City, reported the highest num-ber of cases (3,673), accounting for 14.5% of cases nationally.
FIGURE 1. Giardiasis case reports, by sex* — United States, 1992–1997
ts 25,000
po 20,000
Re 15,000

Year of Report
*1.0%–2.3% of case reports have no sex identification.
In 1997, cases per 100,000 population ranged from 0.9 to 42.3 in reporting states.
Vermont reported the highest incidence rate (42.3 cases per 100,000 population) (Table2). Ten states reported >20.0 cases per 100,000 population (Alaska, Colorado, Minne-sota, Nebraska, New Hampshire, New York, North Dakota, Oregon, Vermont, and Wis-consin). Seven states reported no cases to CDC in 1997 (California, Connecticut, Ken-tucky, New Jersey, North Carolina, South Carolina, and Texas) (Table 2).
A bimodal age distribution in giardiasis rates was observed. The highest rates of giardiasis occurred among children aged 0–5 years, followed closely by persons aged31–40 years (Figure 2).
August 11, 2000
TABLE 2. Giardiasis case reports — United States, 1992–1997*
No. case reports
% of total
in 1997†
in 1997†



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