Nearly half of infected individuals are asymptomatic. For those with symptoms, the onset is typically 1 to 2 weeks after infection. Symptoms may include abdominal pain, nausea, flatulence, and large volume watery, foul-smelling, greasy stools. Children may present with abdominal pain with minimal diarrhea. Due to a large number of stools, infected individuals will often be dehydrated.[12] Less commonly, patients may also present with fever. Cases involving skin lesions and joint pain due to reactive arthritis have also been reported. Symptoms typically self-resolve within four weeks of onset. In chronic infections, individuals may present with weight loss as well as signs or symptoms of vitamin deficiency.

The CDC recommends that providers consider giardiasis in the differential diagnosis of all individuals with more than three days of diarrhea. Patients should be asked about risk factors such as recent international or wilderness travel, contact with unsanitary water, daycare work, and sexual practices.[13] Women of reproductive age should be screened for pregnancy, as this will affect treatment options.[14]

Physical examination is most often benign and often only reveals mild dehydration. Fevers are not common but may occur. Mild but diffuse abdominal tenderness to palpation may be present, and borborygmi may be apparent on auscultation. Providers should assess for possible skin manifestations such as hives or granuloma annulare.[15]

Stool antigen detection assays and nucleic acid amplification tests (NAAT) are available and are typically quicker and more sensitive and specific than microscopy.[16] Giardia may be difficult to detect with microscopy as the protozoa are only intermittently shed. The sensitivity of microscopy can be increased by collecting three stool samples on different days.[17] Standard ova and parasite laboratory testing does not always include giardia testing, so the CDC recommends providers specifically request testing for giardia when submitting stool samples. As the differential for giardiasis includes other parasitic diseases, microscopy should be performed even when antigen or NAAT tests are obtained.

The majority of presenting patients will be non-toxic and may only require oral rehydration for initial fluid resuscitation. In more severe cases, intravenous (IV) fluids may be needed.[18][19]

Metronidazole is the first-line treatment for giardiasis.[20] Typical dosing is 250 to 500 mg 3 times a day for 5 to 10 days, though studies indicate once-daily dosing may be as effective. Metronidazole should be used with caution in pregnant women, especially in the first trimester, due to concerns for cleft lip. Also, patients should be counseled on avoiding alcohol due to the disulfiram effect (flushing, headaches, and nausea). Metronidazole can also be safely used in children in a typical dose of 30 mg/kg to 50 mg/kg per day divided into three doses.

Other possible regimens include tinidazole, nitazoxanide, mebendazole, albendazole, and paromomycin. Paromomycin is poorly systemically absorbed and may be considered if giardiasis needs to be treated in a pregnant patient in her first trimester.[21]

Data appears to be conflicting about the most effective treatment. A systematic review found that albendazole may be as effective as metronidazole with fewer side effects.[22] If patients continue to have symptoms despite therapy, a medication from another class should be used.

The differential diagnosis for giardiasis includes:

• Travelers' diarrhea,
• Lactose intolerance,
• Inflammatory bowel disease[23],
• Cryptosporidiosis,
• Tropical sprue,
• Irritable bowel syndrome[24],
• Strongyloidiasis, and
• Viral gastroenteritis

The CDC recommends testing all individuals with diarrhea lasting more than three days for giardiasis.

Giardiasis can lead to complications, including irritable bowel syndrome, chronic fatigue syndrome, food allergies, and even reactive arthritis.[25] Patients presenting with these conditions should be screened for possible giardia exposure.

Giardia cysts are resistant to chlorination. Iodine can be used for disinfection but may take up to 8 hours before the water is safe for drinking. Filters are also available. Travelers should ensure filters meet the National Safety Foundation (NSF) Standard 53 or NSF Standard 58 ratings for oocyst or cyst reduction. Boiling water for 10 minutes also kills the cysts.[26]

Giardiasis is a relatively common enteric infection caused by the protozoa Giardia duodenalis. The majority of patients present with watery diarrhea and abdominal cramps. Emergency department providers, nurse practitioners, and primary caregivers need to know that hydration is the key in all patients. Only those with severe symptoms need anti-helminthic medications. If there is any doubt about the diagnosis or management, an infectious disease expert should be consulted. Following treatment, symptoms subside rapidly, but a few patients may develop sensitivity to dairy products for a few weeks or months.