Giardiasis overview

# Overview
Giardiasis is a common enteric infectio caused by Giardia lamblia, a flagellated protozoan parasite. It is a worldwide infection that is common in settings of poor sanitation. Giardiasis may be classified based on the duration of clinical manifestations into either acute (2-4 weeks) or chronic (more than 4 weeks). Common risk factors in the development of giardiasis include recent history of hiking and camping, immunosuppression, young age (especially < 5 years of age), exposure to infected individuals, drinking unsafe water, recent sexual history with unprotected anal or oral-anal contact, and recent travel to developing countries. Giardia is usually transmitted via the fecal-oral route through personal contact and contaminated water and food.  Following transmission of Giardia, patients may remain asymptomatic for 1-3 weeks. Early symptoms typically include acute, profuse, watery diarrhea, bloating, and abdominal cramping, which are usually self-limited. If left untreated, giardiasis may persist in a small proportion of patients and subsequently results in chronic giardiasis, which typically manifests with chronic diarrhea, anorexia, weight loss, malaise, and failure to thrive. Complications may be related to either severe dehydration (e.g. acute kidney injury), malabsorption (e.g. vitamin B12 deficiency), post-infectious diseases (e.g. reactive arthritis, chronic fatigue syndrome), or rarely, spread of Giardia to extraluminal sites (e.g. involvement of the gallbladder, pancreas, or eyes). The prognosis is generally excellent. Recurrence of the disease is particularly common among children even with optimal treatment. The diagnosis of giardiasis requires the detection of Giardia parasites in at least 1 out of 3 stool samples upon ova and parasite examination. Multiple stool collections (i.e., three stool specimens collected on separate days) should be performed for ova and parasite examination (O & P) to increase test sensitivity since Giardia cysts are excreted intermittently (detects approximately 80% of cases). Patients diagnosed with giardiasis require antimicrobial therapy. Medical therapy for giardiasis includes either metronidazole, albendazole or quinacrine. Furazolidone or nitazoxanide may be used in pediatric patients. Patients must be monitored for the persistence of symptoms following adequate therapy (suggestive of treatment failure) or the re-development of symptoms (recurrence). There is no vaccine against giardiasis. Prophylaxis against giardiasis is not recommended. Hygiene practices (such as hand washing, drinking safe water) may help reduce the risk of Giardia transmission.

# Historical Perspective
The trophozoite form of Giardia was first observed in 1681 by Antoni van Leeuwenhoek in his own diarrheal stools. Giardia was initially named Cercomonas intestinalis by Lambl in 1859. It was then renamed Giardia lamblia by Stiles in 1915 in honor of Professor A. Giard of Paris and Dr. F. Lambl of Prague.

# Classification
Giardiasis may be classified based on the duration of clinical manifestations into either acute (2-4 weeks) or chronic (more than 4 weeks).

# Pathophysiology
Giardia is usually transmitted via the fecal-oral route through personal contact and contaminated water and food. Giardia is a zoonotic infection that may also transmitted from animals to humans. Major reservoir hosts include beavers, dogs, cats, horses, and cattle. Following transmission, Giardia colonizes the human intestine and attaches to the epithelium by a ventral adhesive disc. The mechanism of pathogenesis of Giardia is thought to include increased pro-apoptotic processes, subsequent loss of intestinal epithelial barrier, hypersecretion of electrolytes, and increased exposure to luminal antigens to subepithelial host immune cells. It is thought lymphocyte activation, particularly CD8+ T-cells, results in local inflammation, as well as diffuse shortening of microvilli (without villous atrophy). Dysfunctional microvilli are then unable to absorb luminal nutrients, resulting in the development and worsening of clinical manifestations of giardiasis.

# Causes
Giardiasis is caused by Giardia lamblia (synonymous with Lamblia intestinalis and Giardia duodenalis), a flagellated protozoan parasite.

# Differential Diagnosis
Giardiasis must be differentiated from other causes of abdominal pain, bloating, acute or chronic diarrhea, and weight loss, such as other infectious causes of gastroenteritis, including bacterial, viral, fungal, and parasitic pathogens, in addition to non-infectious causes, including acute pancreatitis, appendicitis, bowel obstruction, diverticulitis, drug reaction, hyperthyroidism, inflammatory bowel disease, celiac disease, lactose intolerance, Whipple disease, tropical sprue, and lymphoma.

# Epidemiology and Demographics
Giardiasis is a worldwide infection. It is the most common cause of parasitic diarrhea with a prevalence that may be as high as 20% to 40% in settings of poor sanitation. In the USA, the incidence of giardiasis is thought to be decreasing from 20 to 25 cases per 100,000 individuals between 1990 and 1998 to approximately 4 to 5 cases per 100,000 individuals in 2012. Children, particularly < 5 years of age, are more frequently affected with giardiasis than adults. There is no gender or racial predilection for the development of giardiasis.

# Risk Factors
Risk factors in the development of giardiasis include recent history of hiking and camping, immunosuppression, young age (especially < 5 years of age), exposure to infected individuals, drinking unsafe water, recent sexual history with unprotected anal or oral-anal contact, and recent travel to developing countries.

# Natural History, Complications and Prognosis
Following transmission of Giardia, patients may remain asymptomatic for 1-3 weeks. Early symptoms typically include acute, watery diarrhea, bloating, and abdominal cramping, which are usually self-limited. If left untreated, giardiasis may persist in a small proportion of patients and subsequently results in chronic giardiasis. Complications may be related to either severe dehydration (e.g. acute kidney injury), malabsorption (e.g. vitamin B12 deficiency), post-infectious diseases (e.g. reactive arthritis, chronic fatigue syndrome), or spread of Giardia to extraluminal sites (e.g. involvement of the gallbladder, pancreas, or eyes). The prognosis is generally excellent. Recurrence of disease is common among children even with optimal treatment.

# Diagnosis

## History and Symptoms
Symptoms of acute giardiasis include watery diarrhea, steatorrhea, bloating,abdominal pain, indigestion, flatulence, and vomiting.  Symptoms of chronic giardiasis include anorexia, weight loss, malaise, and failure to thrive. Less common symptoms of giardiasis include low-grade, intermittent fever, symptoms of allergic reaction, and neurologic symptoms (neurasthenia, sleep disorder, depression).

## Physical Examination
Physical examination among patients with giardiasis is usually unremarkable. In the acute phase, patients with giardiasis often appear sick-looking. In the chronic phase, patients with giardiasis often appear malnourished with significant weight loss. Physical examination findings may include low-grade fever, dry mucus membranes (dehydration), and abdominal distention and tenderness.

## Laboratory Findings
The diagnosis of giardiasis requires the detection of Giardia in at least 1 out of 3 stool samples upon ova and parasite examination. Multiple stool collections (i.e., three stool specimens collected on separate days) should be performed for ova and parasite examination (O & P) to increase test sensitivity since Giardia cysts are excreted intermittently (detection of approximately 80% of cases). Blood samples are usually unremarkable. Fecal immunoassays may also be used for the diagnosis of Giardia if stool collections were negative and giardiasis is still suspected. Only molecular testing, such as polymerase chain reaction techniques, can be used to identify the subtypes of Giardia. Blood samples are usually unremarkable.

## Other Diagnostic Studies
No other tests are required for the diagnosis of giardiasis. Other diagnostic studies may include the entero-test (string test), D-xylose absorption test for patients with giardiasis-related vitamin B12 deficiency, and esophagogastroduodenoscopy with small bowel biopsy for patients who are suspected to have giardiasis despite negative lab findings or patients who remain symptomatic after adequate therapy.

# Treatment

## Medical Therapy
Patients diagnosed with giardiasis require antimicrobial therapy. Medical therapy for giardiasis includes either metronidazole, albendazole or quinacrine. Furazolidone or nitazoxanide may be used in pediatric patients. Patients must be monitored for the persistence of symptoms following adequate therapy (suggestive of treatment failure) or the re-development of symptoms (recurrence).

## Prevention
There is no vaccine against giardiasis. Prophylaxis against giardiasis is not recommended. Hygiene practices (such as hand washing, drinking safe water) may help reduce the risk of Giardia transmission.