01Approach to the Critical Patient/attcp42

Immediate priorities

›Dehydration and electrolyte threats

›Hemodynamic instability

›SBP < 90 mmHg or MAP < 65 mmHg

›Tachycardia with orthostasis

›Severe dehydration criteria

›Dry mucous membranes, sunken eyes, reduced skin turgor

›Inability to tolerate oral rehydration

›If severe dehydration, initiate IV fluid resuscitation

›Normal saline or lactated Ringer's solution

›Reassess hemodynamic response every 30 minutes

›High-risk host recognition

›Immunocompromised state

›HIV/AIDS with low CD4 count

›Common variable immunodeficiency or IgA deficiency

›Pediatric severe cases

›Failure to thrive or growth stunting

›Signs of malnutrition

›Escalate to admission if unable to maintain PO intake or ongoing hemodynamic compromise

Monitoring and targets

›Vital sign monitoring

›Continuous pulse oximetry if clinically ill

›Dehydration-related tachycardia trend

›Orthostatic vital sign assessment

›Weight monitoring

›Acute weight loss as severity marker

›Chronic weight loss up to 11 kg reported in severe cases

›Rehydration targets

02History/hx81

Presentation pattern

›Core gastrointestinal syndrome

›Diarrhea character

›Loose, watery diarrhea early in illness

›Greasy, foul-smelling, fatty stools over time (steatorrhea)

›Nonbloody stools typical; blood or mucus suggests alternative diagnosis

›Associated symptoms

›Bloating and excessive flatulence

›Abdominal cramping

›Nausea and anorexia

›Weight loss

›Timeline and incubation

›Incubation period 7 days average (range 1 to 2 weeks)

›Onset after recreational water or travel exposure

›Symptom onset can help narrow exposure source

›Disease course

›Acute phase 2 to 6 weeks if untreated in immunocompetent hosts

›Chronic giardiasis waxes and wanes for months if untreated

Exposure and risk history

›Water and travel exposures

›Untreated surface water (camping, backpacking)

›Lakes, streams, and unfiltered well water

›Shallow well water depth < 7.5 m

›Travel to endemic areas

›Regions with poor sanitation infrastructure

›Developing countries with contaminated water supplies

›Recreational water exposure

›Swimming pools, lakes, rivers

›Giardia cysts resistant to chlorination

›Person-to-person and fecal-oral exposures

›Daycare attendance or employment

›Most commonly reported age group children < 5 years

›High-risk setting for institutional outbreaks

›Oral-anal sexual contact

›Documented route of transmission

›Sexual history relevant

›Household contacts with similar symptoms

›Person-to-person transmission common

›Family cluster outbreaks

›Host risk factors

›Immunocompromised states

›HIV infection or AIDS

›IgA deficiency or hypogammaglobulinemia

›Common variable immunodeficiency

›Occupation

›Food handlers require treatment even if asymptomatic

›Childcare workers at elevated risk

›Animal exposure

›Beavers and domestic animals as potential reservoirs

›Pets including dogs and cats

Alarm features and atypical presentations

›Red flag features

›Significant weight loss

›Chronic giardiasis can cause up to 11 kg weight loss

›Failure to thrive in children

›Signs of malabsorption

›Vitamin D deficiency

›Iron deficiency anemia

›Biliary or pancreatic involvement (rare)

›Right upper quadrant pain

›Jaundice

›Atypical presentations

›Asymptomatic carriage

›Common in endemic regions

›Detected incidentally or through contact tracing

›Constipation-predominant presentation

›Alternating constipation and diarrhea

›Chronic fatigue mimicking IBS

›Extraintestinal manifestations

›Urticaria (uncommon but reported)

›Arthralgias and reactive arthritis

Past medical history

›Relevant prior conditions

›Prior giardiasis or parasitic infections

›Reinfection common with ongoing exposure

›Immune status affects severity and duration

›Immunodeficiency states

›HIV, common variable immunodeficiency, IgA deficiency

›Transplant or malignancy on immunosuppressive therapy

›History of IBS or functional GI disorders

›May mask or mimic chronic giardiasis

›Low threshold to test for Giardia in these patients

›Prior GI surgery affecting motility

›Altered anatomy may affect drug delivery

›Higher risk of persistent infection

03Physical Exam/exam50

Vitals and hydration assessment

›Hemodynamic stability

›Blood pressure and pulse

›Tachycardia and orthostasis suggest moderate-severe dehydration

›Fever typically low-grade or absent; high fever suggests alternative diagnosis

›Orthostatic vital signs

›SBP drop >= 20 mmHg or HR rise >= 20 bpm on standing

›Positive orthostatics indicate volume depletion

›Hydration status

›Mucous membranes and skin turgor

›Dry mucous membranes

›Reduced skin turgor

›Capillary refill

›> 2 seconds suggests dehydration

›Peripheral perfusion assessment

›Urine output history

›Oliguria as dehydration marker

›Dark concentrated urine

Abdominal exam

›Abdominal findings

›Diffuse mild tenderness

›Periumbilical or diffuse cramping pain

›No peritoneal signs (if present, consider alternative diagnosis)

›Bowel sounds

›Hyperactive bowel sounds

›Borborygmi with bloating

›Abdominal distension

›Gaseous distension from malabsorption

04Differential Diagnosis/ddx48

Parasitic and protozoal infections

›Intestinal parasites

›Cryptosporidiosis

›ICD-10 A07.2

›Watery diarrhea with similar water/travel exposure

›Distinguished by acid-fast staining; more severe in immunocompromised

›Cyclosporiasis

›ICD-10 A07.4

›Prolonged watery diarrhea often foodborne (fresh produce)

›Diagnosed by modified acid-fast stain or PCR

›Amebiasis (Entamoeba histolytica)

›ICD-10 A06.9

›Bloody or dysenteric stools distinguish from Giardia

›Liver abscess possible complication

›Giardiasis

›ICD-10 A07.1

›SNOMED CT 47893000

Bacterial and viral enteritis

›Infectious diarrhea mimics

›Viral gastroenteritis

›Typically self-limited < 7 days

›No steatorrhea; no fatty stools

›Bacterial enteritis

›Salmonella, Campylobacter, Shigella

›More acute onset, fever, bloody stools more common

›Stool culture positive

›Clostridium difficile colitis

›Recent antibiotic use

05Laboratory Tests/lt45

Stool diagnostic tests

›Stool Giardia antigen (EIA)

›Sensitivity 87 to 100%, specificity approximately 100%

›Rapid and cost-effective first-line test

›Single stool sample sufficient

›Limitations

›May remain positive post-eradication

›Do not use to confirm cure; use symptom resolution

›Stool multiplex PCR panel

›Sensitivity 92 to 100%, specificity 96.9 to 100%

›Detects multiple GI pathogens simultaneously

›Preferred when multiple parasites or bacterial pathogens possible

›Preferred over antigen for test of cure

›Antigen assays may remain falsely positive post-treatment

›PCR more reliable for eradication confirmation

›Stool microscopy with direct fluorescent antibody (DFA)

›Preferred diagnostic test per AAFP guidelines

›Single sample detects approximately 50% of cases

›Three samples detect up to 90% of cases

›Ova and parasites examination

›Do not order if diarrhea < 7 days in immunocompetent without travel history (IDSA Choosing Wisely)

›Reserve for appropriate clinical context

Serum and metabolic labs

›Electrolytes and renal function

›Basic metabolic panel

›Hypokalemia from diarrheal losses

›Hyponatremia in severe dehydration

›Creatinine and BUN

06Diagnostic Tests/dt54

Scoring Systems

›Giardiasis severity assessment

›No validated scoring system exists specifically for giardiasis severity

›Severity stratification based on clinical criteria

›Mild: ambulatory, tolerating PO, no dehydration

›Clinical severity categories

›Mild-moderate: ambulatory diarrhea, manageable symptoms

›Severe: significant dehydration, weight loss, malabsorption, unable to maintain PO

›Pediatric nutritional assessment tools

›WHO growth standards for failure to thrive

›Waterlow criteria for malnutrition classification

MRI

›MRI role in giardiasis

›Not routinely indicated for intestinal giardiasis

›No established MRI protocol for diagnosis

›Stool testing is diagnostic mainstay

›MRI abdomen/pelvis for complications

›Suspected biliary involvement (cholangiopathy in immunocompromised)

›Rare: biliary or pancreatic duct dilatation in severe cases

›MRI enterography

›May be considered for chronic cases mimicking Crohn disease

›Mucosal edema in small intestine not specific for Giardia

CT

›CT abdomen indications

›Not first-line for uncomplicated giardiasis

›Radiation exposure without additional diagnostic yield for routine cases

›Stool testing preferred

›CT indications in selected cases

07Disposition/dispo30

Discharge criteria

›Safe for discharge

›Clinical criteria

›Mild-moderate symptoms tolerable as outpatient

›Tolerating oral rehydration

›No high-risk features

›No significant dehydration after ED assessment

›No concerning electrolyte abnormalities

›Appropriate follow-up arranged

›Primary care within 5 to 7 days

›Test of cure not required if symptoms resolve

Admission criteria

›Inpatient admission indications

›Severe dehydration not responsive to oral rehydration in ED

›Inability to tolerate PO despite antiemetics

›Persistent hemodynamic instability

›Electrolyte abnormalities

›Significant hypokalemia or hyponatremia

›Requires IV correction and monitoring

›Immunocompromised host

›Severe or prolonged course in HIV, transplant, or immunodeficiency

›Higher risk of complications and treatment failure

›Pediatric admission indications

›Severe dehydration with failure to maintain PO

›Failure to thrive or malnutrition requiring nutritional support

Transfer considerations

›Transfer triggers

›Pediatric specialty care

08Treatment/t71

Rehydration and supportive care

›Oral rehydration therapy

›WHO oral rehydration solution (ORS)

›Preferred when tolerated; sodium 75 mmol/L, glucose 75 mmol/L formulation

›Children: 50 to 100 mL/kg over 3 to 4 hours for mild-moderate dehydration

›Dietary modification

›Bland, easily digestible foods during acute phase

›Avoid dairy products initially due to post-infectious lactase deficiency

›IV fluid resuscitation

›Indications

›Inability to tolerate oral rehydration

›Hemodynamic compromise

›Fluid choice

›Isotonic crystalloid (normal saline or lactated Ringer's)

›Bolus 10 to 20 mL/kg in children; 500 mL bolus in adults, reassess

First-line pharmacotherapy

›Tinidazole (preferred agent)

›Adult dosing

›2 g PO single dose

›Take with food to reduce GI side effects

›Pediatric dosing (age >= 3 years)

›50 mg/kg PO single dose (maximum 2 g)

›Single-dose promotes adherence

›Efficacy and safety

›Cure rate > 90%

›Disulfiram-like reaction with alcohol; avoid alcohol during and 72 hours after

›Contraindications

›First trimester pregnancy

09Special Populations/sp68

Pregnancy

›Antiparasitic safety in pregnancy

›First trimester considerations

›Tinidazole and metronidazole should be avoided in first trimester

›Teratogenicity concerns based on animal data; human data insufficient

›Paromomycin (preferred in pregnancy)

›25 to 35 mg/kg/day PO divided TID for 7 days

›Nonabsorbable aminoglycoside; minimal systemic exposure

›Lower cure rate (55 to 90%) than nitroimidazoles

›Defer treatment when possible

›Mild-moderate cases: defer to second or third trimester

›Treat if significant dehydration, weight loss, or malnutrition

›Pregnancy-specific complications

›Increased dehydration risk

›Superimposed on pregnancy-related nausea

›IV hydration threshold lower in pregnant patients

›Nutritional considerations

›Iron and folate malabsorption in chronic giardiasis

›Folate supplementation critical in first trimester

›Monitor fetal growth if chronic infection

Geriatric

›Physiologic considerations in older adults

›Dehydration risk

›Blunted thirst response

›Reduced renal concentrating ability

›Lower threshold for IV hydration

›Electrolyte management

›Higher risk of hyponatremia and hypokalemia

›Cardiac monitoring if significant hypokalemia (K < 3.0 mmol/L)

›Drug considerations in older adults

›Metronidazole and tinidazole in geriatric patients

›CNS effects including confusion and ataxia more common in elderly

›Reduce dose or prefer nitazoxanide if CNS side effects concern

›Drug interactions

›Warfarin: metronidazole and tinidazole inhibit CYP2C9, increasing INR

›Monitor INR closely if on warfarin during nitroimidazole treatment

›Atypical presentations in older adults

›Confusion or delirium from dehydration

›May present as altered mental status rather than diarrhea complaints

›Low threshold to test for Giardia in elderly with unexplained diarrhea

›Weight loss as predominant complaint

›Chronic giardiasis may present as unexplained weight loss

›Screen for parasitic infection before attributing to malignancy

Pediatrics

›Age-specific epidemiology

›Children < 5 years most commonly affected age group

›Daycare attendance is major risk factor

›Highest attack rates in institutional settings

›School-age children

›Recreational water exposure (swimming pools, lakes)

›Travel with family to endemic areas

›Pediatric dosing summary

›Tinidazole (age >= 3 years)

›50 mg/kg PO single dose (maximum 2 g)

›Preferred for adherence

›Nitazoxanide (age >= 1 year)

›Age 1 to 3 years: 100 mg BID for 3 days

›Age 4 to 11 years: 200 mg BID for 3 days

›Age >= 12 years: 500 mg BID for 3 days

›Metronidazole (alternative)

›5 to 10 mg/kg TID for 5 to 7 days (maximum 250 mg per dose)

›Pediatric complications

›Failure to thrive and growth stunting

›Growth monitoring essential during and after treatment

›Nutritional rehabilitation if significant malnutrition

›Cognitive and developmental impact

›Chronic infection associated with cognitive impairment in children

›School performance assessment in recurrent cases

›HIV-exposed or immunocompromised children

›Per HHS/CDC/IDSA pediatric opportunistic infection guidelines

›Nitazoxanide preferred for HIV-positive children

10Background/b74

Epidemiology

›Incidence and prevalence

›United States burden

›15,000 to 20,000 reported cases annually

›Most common intestinal parasitic infection in the United States

›Global prevalence

›200 to 300 million symptomatic infections per year worldwide

›Endemic in areas with poor sanitation and water quality

›Giardiasis is a nationally notifiable condition in the United States

›Mandatory public health reporting required

›Surveillance data used for outbreak detection

›Population distribution

›Age distribution

›Children < 5 years highest reported incidence

›Adults 25 to 39 years second most affected group

›Geographic and seasonal patterns

›Higher rates in summer and early fall (recreational water season)

›Developing world: hyperendemic with year-round transmission

›Outbreak settings

›Daycare centers

›Recreational water facilities

›Community water supply contamination events

Pathophysiology

›Organism biology

›Causative organism

›Giardia duodenalis (synonyms: G. lamblia, G. intestinalis)

›Flagellated protozoan with trophozoite and cyst life stages

›Infectious dose

›As few as 10 to 25 cysts can cause infection

›Cysts environmentally stable and chlorine-resistant

›Mechanism of disease

›Site of infection

›Proximal small intestine (duodenum and upper jejunum)

›Trophozoites adhere to intestinal epithelium via ventral adhesive disc

›Mucosal damage

›Villous blunting and crypt hyperplasia

›Disruption of brush border enzymes (lactase, sucrase)

›Tight junction disruption increasing intestinal permeability

›Malabsorption mechanism

›Fat malabsorption causing steatorrhea

›Carbohydrate malabsorption causing bloating and flatulence

›Micronutrient malabsorption (iron, vitamin D, B12, folate)

›Transmission

›Fecal-oral route

›Contaminated water (most common source in developed countries)

›Person-to-person direct transmission in daycare and institutional settings

›Zoonotic reservoir

›Beavers are classic reservoir (Beaver fever)

›Dogs, cats, and livestock as additional reservoirs

Therapeutic Considerations

›Treatment goals

›Eradication of infection

›Symptom resolution as primary endpoint

›Test of cure not required in immunocompetent patients if symptoms resolve

›Prevention of complications

›Malabsorption and nutritional deficiencies

›Post-infectious lactose intolerance (affects 20 to 40% of patients)

›Pharmacologic mechanisms

›Nitroimidazoles (tinidazole, metronidazole)

›Reduced intracellularly by Giardia ferredoxin to toxic nitro radical intermediates

›DNA strand breakage causes organism death

›Nitazoxanide

›Interferes with pyruvate:ferredoxin oxidoreductase enzyme-dependent electron transfer

›Broad activity against intestinal protozoa and helminths

›Paromomycin

›Nonabsorbable aminoglycoside; luminal action only

›Lower cure rate but safe in pregnancy

›Resistance considerations

›Nitroimidazole resistance

›Reported in 20 to 40% of treatment failures

›Switching to nitazoxanide or combination therapy for refractory cases

›Post-infectious sequelae management

›Lactose-free diet for 4 to 6 weeks post-treatment if lactose intolerant

›Nutritional supplementation for iron, vitamin D if deficient

›Long-term extraintestinal sequelae

›Approximately one-third of patients develop long-term symptoms

›Ocular, muscular, and metabolic complications reported (Emerging Infectious Diseases 2025)

11Patient Discharge Instructions/pdi48

copy discharge instructions

›Diagnosis and explanation

›You have been diagnosed with Giardiasis

›A parasitic intestinal infection caused by Giardia duodenalis

›Spread through contaminated water, food, or person-to-person contact

›Medication instructions

›Take your prescribed antiparasitic medication exactly as directed

›Tinidazole: single large dose taken with food

›Nitazoxanide: taken twice daily for 3 days with food

›Metronidazole: taken two to three times daily for 5 to 7 days with food

›Alcohol restriction

›Avoid alcohol while taking tinidazole or metronidazole

›Continue alcohol avoidance for 72 hours after finishing medication

›Diet and hydration

›Drink plenty of clear fluids (water, diluted juice, oral rehydration drinks)

›Aim for at least 2 liters of fluid per day

›Avoid sugary drinks which can worsen diarrhea

›Dietary recommendations

›Eat bland foods (bananas, rice, toast, boiled potatoes)

›Avoid dairy products for 4 to 6 weeks as lactose intolerance is common after infection

›Avoid fatty or spicy foods until diarrhea resolves

›Infection prevention and public health

›Hand hygiene

›Wash hands thoroughly with soap and water after bathroom use and before food preparation

›Alcohol-based hand sanitizers do not kill Giardia cysts; soap and water required

›Water safety

›Boil or filter water when camping, hiking, or traveling to endemic areas

›Do not drink untreated surface water or well water of unknown quality

12References/r34

Guidelines and key sources

›American Family Physician guidelines

›Pyzocha N, Cuda A. Common Intestinal Parasites. American Family Physician. 2023. PMID 37983700

›Covers epidemiology, diagnosis, and treatment of Giardia and other intestinal parasites

›Source of drug dosing tables and diagnostic test performance characteristics

›IDSA Infectious Diarrhea Guidelines

›Shane AL, Mody RK, Crump JA, et al. 2017 IDSA Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea. Clinical Infectious Diseases. 2017. PMC5850553

›Recommendations on stool testing indications (Choosing Wisely criteria)

›O&P testing guidance for < 7 day diarrhea in immunocompetent patients

›Cochrane systematic review on drug treatment

›Granados CE, Reveiz L, Uribe LG, Criollo CP. Drugs for Treating Giardiasis. Cochrane Database of Systematic Reviews. 2012

›Comparative efficacy of nitroimidazoles for giardiasis

›Disulfiram reaction data for metronidazole and tinidazole

›Network meta-analysis comparative efficacy

›Ordonez-Mena JM, McCarthy ND, Fanshawe TR. Comparative Efficacy of Drugs for Treating Giardiasis. Journal of Antimicrobial Chemotherapy. 2018. PMID 29186570

›Systematic update and network meta-analysis of randomized clinical trials

›Tinidazole superiority data; differential diagnosis evidence

Additional references

›Pathophysiology and biology

›Adam RD. Giardia Duodenalis: Biology and Pathogenesis. Clinical Microbiology Reviews. 2021. PMID 34378955

›Trophozoite adhesion mechanisms and mucosal damage pathways

›Villous blunting and brush border enzyme disruption

›Dixon BR. Giardia Duodenalis in Humans and Animals. Research in Veterinary Science. 2021. PMID 33066992

›Zoonotic transmission including beaver and domestic animal reservoirs

›Long-term sequelae

›Miko S, Kache PA, Imada E, Freeland AL, Haston JC. Postinfectious Syndromes and Long-Term Sequelae After Giardia Infections. Emerging Infectious Diseases. 2025. PMID 41570191

›Approximately one-third of patients with long-term extraintestinal symptoms

›Ocular, muscular, and metabolic complications data

Evidence & Review

Evidence & Review