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Reporting Obligations

Individuals who have or may have giardiasis shall be reported to the local Health Unit.

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Aetiologic Agent

Giardiasis is caused by the protozoa, Giardia lamblia (also known as G. intestinalis or G. duodenalis). The organism is found in two forms, a pear-shaped trophozoite and an ovoid cyst. The trophozoite is relatively fragile, and dies when excreted from the body. The cyst, which is environmentally hardy, is the infective form.

Clinical Presentation

Symptoms may include acute, self-limiting diarrhea, chronic diarrhea, steatorrhea, abdominal cramps, bloating, frequent loose and pale greasy stools, fatigue, and weight loss. There is usually no extra-intestinal invasion, but reactive arthritis and, in severe giardiasis, damage to duodenal and jejunal mucosal cells may occur. Persons may also be asymptomatic.

Modes of Transmission

Person-to-person transmission occurs by hand-to-mouth transfer of cysts from the feces of an infected individual, especially in institutions and day care settings; this is probably the principal mode of spread. Anal intercourse also facilitates transmission.

Incubation Period

Usually 3–25 days or longer; median 7–10 days.

Period of Communicability

Duration of cyst excretion is variable but can range from weeks to months. Giardiasis is communicable for as long as the infected person excretes cysts.

Risk Factors/Susceptibility

Asymptomatic carrier rate is high; infection is frequently self-limited. Persons with HIV infection may have more serious and prolonged giardiasis.

Diagnosis and Laboratory Testing

Diagnosis is made by microscopic examination of clinical specimens (e.g., stool, duodenal fluid, small bowel biopsy) for G. lamblia cysts or trophozoites, or by Giardia immunoassays for G. lamblia antigen. Three specimens taken 2–3 days apart will identify 80–90% of infections.

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Treatment and Case Management

Treatment recommendations are under the direction of the attending health care provider.

Inform patient that symptomatic cases will be excluded from conducting activities in high-risk settings such as the food industry, healthcare or daycare until symptom free for 24 hours, OR symptom free for 48 hours after discontinuing use of anti-diarrheal medication.

People with diarrhea caused by Giardia species should not use recreational water venues such as swimming pools, lakes and rivers while symptomatic. Children who had diarrhea attributable to Giardia and who are incontinent should avoid recreational water activities for 1 week after symptoms resolve.

Provide education about the illness and how to prevent the spread of infection (proper hand hygiene after toileting or diapering and before handling food; avoid consumption of surface water without prior treatment; regular testing of private water samples).

Patient Information

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Additional Resources

Heymann, D.L. Control of Communicable Disease Manual (19th Ed.). Washington, American Public Health Association, 2008


Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2022.