Giardiasis is an enteric infection caused by the protozoa Giardia duodenalis. A common disease in low-resource settings, it often presents with flatulence and watery diarrhea. In the United States, the disease is most often seen in international travelers, wilderness travelers, and daycare workers. Though often asymptomatic, patients may have severe enough symptoms to result in dehydration and weight loss. Treatment with a nitroimidazole or antihelminthic medication is often rapidly curative.
Giardiasis is caused by the protozoa G. duodenalis (also referred to as Giardia lamblia and Giardia intestinalis). Infected animals excrete cysts into fresh water where they are immediately infective and can exist for weeks to months. Seven genetic assemblages (A-F) have been identified, with only genotypes A and B being known to infect humans. 
Ingesting cysts through contaminated water or person-to-person direct contact causes human infection. The cysts undergo excystation within the intestinal system and subsequently release trophozoites. Trophozoites appear as pear-shaped flagellated protozoa with 2 nuclei.
Giardiasis is the most common enteric protozoal infection worldwide, affecting nearly 2% of adults and 8% of children in developed countries. Also, estimates show that nearly 33% of the population in developing countries have been infected with giardiasis. The prevalence within the United States is estimated to be roughly 1.2 million, with the majority of cases not identified due to the carrier being asymptomatic. The Centers for Disease Control and Prevention (CDC) reported there were 15,223 cases submitted in 2012. The largest reported affected demographic was children 0 to 4 years of age, with the largest percentage of cases being reported from the northwest United States. Peak incidence occurs in late summer and early fall.
Protozoa are spread through the fecal-oral route, most commonly through the ingestion of contaminated water or food. It can also be spread through person-to-person or, less commonly, animal-to-person. An infected individual can shed nearly 10^8 to 10^10 cysts per day while the infectious dose can be as low as 10. Individuals with sub-clinical disease can serve as carriers and may infect others. Within the United States, populations at risk include international travelers, wilderness travelers, daycare workers, men who have sex with men, and professions with contact with human waste.
The cause of symptoms in giardiasis is poorly understood. Trophozoites have a ventral disk that they use to adhere to the intestinal epithelium. Researchers theorize that the protozoa disrupt small intestine epithelial cell junctions as well as brush border enzymes. Affected patients can demonstrate altered gastrointestinal motility. The protozoa release thiol proteinases and lectins that have a cytopathic effect. The combination of these effects increases permeability and decreases the ability to process saccharides.
A biopsy is seldom warranted in the evaluation of suspected giardiasis. When obtained in the evaluation of chronic diarrhea, however, histopathology will reveal normal to sub-total villous atrophy, with the degree of atrophy correlating with the severity of the disease. Repeat biopsy following treatment and improvement of symptoms will typically reveal resolution of the villous atrophy.
Nearly half of infected individuals are asymptomatic. For those with symptoms, 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. 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 4 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 clinicians consider giardiasis in the differential diagnosis of all individuals with more than 3 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. Women of reproductive age should be screened for pregnancy as this will affect treatment options.
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.
Stool antigen detection assays and nucleic acid amplification tests (NAAT) are available and are typically quicker and more sensitive and specific than microscopy. 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. 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.
Metronidazole is the first-line treatment for giardiasis. 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. In addition, patients should be counseled on avoiding alcohol due to disulfiram effect (flushing, headaches, and nausea). Metronidazole can also be safely used in children in a typical dose of 30 to 50 mg/kg per day divided into 3 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.
Data appears to be conflicting in regards to the most effective treatment. A systematic review found that albendazole may be as effective as metronidazole with fewer side effects. 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, cryptosporidiosis, tropical sprue, and irritable bowel syndrome. 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. 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 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.
Giardiasis is a relatively common enteric infection caused by the protozoa Giardia duodenalis. The majority of patients present with a watery diarrhea and abdominal cramps. Emergency department physicians, nurse practitioners and primary caregivers need to know that hydration is the key in all patients. Only those with severe symptoms need an 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.