Overview
Giardiasis is an infection of the small intestine caused by a microscopic parasite that is swallowed in contaminated water, food or from unwashed hands. CDC states that swallowing just a few of the parasites is enough to make someone ill.
It typically causes diarrhoea, bloating, foul-smelling stools and marked tiredness lasting weeks rather than days. The hardy form of the parasite survives for weeks to months in soil and water, which is why it spreads so readily where water is unsafe.
Disease Summary
- Category
- Infectious Disease
- Cause
- Giardia duodenalis
- Transmission
- Contaminated water and food, and the faecal-oral route
- Body System
- Digestive System
- Preventable
- Yes
- Curable
- Yes
Symptoms
Common Symptoms
Symptoms usually begin one to three weeks after swallowing the parasite and can last several weeks. Some people have no symptoms at all and still pass it on.
Diarrhoea
Often watery at first, then greasy and foul-smelling. It typically lasts longer than an ordinary episode of diarrhoea.
Bloating and Excess Wind
Marked bloating and gas are among the most characteristic features of this infection.
Stomach Cramps
Cramping pain in the abdomen is common and often comes in waves.
Greasy Stools That Float
The parasite interferes with fat absorption, which produces pale, greasy stools.
Nausea
Nausea with or without vomiting is frequently reported.
Weight Loss
Weight loss follows from poor absorption of nutrients when the illness continues for weeks.
Marked Tiredness
Fatigue is often the symptom people remember most, and it can outlast the diarrhoea.
No Symptoms at All
Many infected people never feel unwell but still pass the parasite in their stool.
Danger Signs
Seek urgent medical attention immediately if any of the following symptoms develop.
Signs of Dehydration
Sunken eyes, very little urine, drowsiness, or in a child no tears when crying. This is the main danger in young children.
Continued Weight Loss in a Child
Prolonged infection in a young child can affect growth and needs treating rather than waiting out.
Diarrhoea Lasting More Than Two Weeks
Prolonged diarrhoea deserves a stool test rather than repeated courses of general treatment.
Blood in the Stool
Giardia does not usually cause bloody diarrhoea, so blood points to a different or additional cause and needs assessment.
High Fever
High fever is not typical of giardiasis and suggests something else is going on.
Diagnosis
History of Exposure
Drinking untreated water, swimming in lakes or pools, travel, contact with a childcare setting, or another household member with the same symptoms all point towards it.
Examining Stool for the Parasite
The traditional method, looking for the parasite or its hardy cysts under the microscope.
Antigen or Molecular Stool Testing
More sensitive modern tests detect proteins or genetic material from the parasite and are now widely used.
Repeating the Sample
The parasite is shed intermittently, so a single negative stool test does not exclude the infection. Several samples across different days may be needed.
Considering Other Causes
Prolonged diarrhoea has many causes, so testing looks for other parasites and bacteria at the same time.
Laboratory Tests
Stool Examination for the Parasite
Finds the parasite or its cysts. Because shedding is intermittent, several samples over different days improve the chance of detection.
Stool Antigen Test
Detects proteins from the parasite. More sensitive than microscopy alone for this infection.
Molecular Stool Test
Detects the parasite’s genetic material and is often run as part of a panel testing for several causes of diarrhoea at once.
Tests for Other Causes of Diarrhoea
Bacterial and other parasitic causes are looked for at the same time, because the symptoms overlap and more than one can be present.
Blood Tests Where Illness Is Prolonged
Used to assess dehydration, salt balance and nutrition when the illness has continued for weeks.
Treatment
Prescription Antiparasitic Medicine
CDC states that a healthcare provider can prescribe medicine to treat this infection. Several effective options exist and the choice depends on age, pregnancy and local availability.
Rehydration
Replacing lost fluid and salts is the priority in children and anyone who is dehydrated, and it matters more than the antiparasitic medicine in the short term.
Nutrition Support
Because the infection interferes with absorbing food, continued feeding during and after the illness matters, particularly in young children.
Managing Symptoms That Persist After Cure
Some people have ongoing bloating and loose stools for a period after the parasite has been cleared. This is recognised and usually settles.
Treating Household Contacts Where Indicated
Where several household members are affected, treating together avoids passing the infection back and forth.
Prevention
Boil or properly filter untreated water
CDC advises boiling or using a certified filter for untreated water. Chlorine alone is not reliable against this parasite.
Wash hands with soap and water
Especially after using the toilet, changing nappies and before preparing food. This is the single most effective everyday measure.
Do not swallow water while swimming
CDC names recreational water among the transmission routes, including lakes, rivers and pools.
Wash, peel or cook produce
Particularly where the water used for irrigation or washing may be contaminated.
Careful hygiene in childcare settings
CDC identifies childcare settings as a common place for person-to-person spread.
Safer sexual practices
The parasite can pass through oral-anal contact, so barrier protection and hygiene reduce this route.
Risk Factors
Young Children and Their Carers
CDC identifies childcare settings as a common source of person-to-person spread.
- Children in nurseries and childcare centres
- Children still in nappies
- Parents and staff changing nappies
- Children are more likely than adults to become dehydrated
People Without Safe Drinking Water
Water is the classic route of infection.
- Households drinking untreated surface water
- Communities where sanitation is inadequate
- People using water that is chlorinated but not filtered
- Swimmers in lakes, rivers and untreated pools
Travellers and People Moving Between Countries
CDC notes that giardiasis ranks among the commonest diagnoses in travellers returning ill from several world regions.
- Travellers to areas with poor sanitation
- Longer stays carry higher risk than short ones
- Refugees, immigrants and internationally adopted children, who often test positive on screening
- Many of those who test positive have no symptoms at all
Complications
Dehydration
The most immediate risk, particularly in young children and older adults.
Weight Loss and Poor Absorption of Food
The parasite interferes with absorbing fat and nutrients, so prolonged infection causes real nutritional loss.
Faltering Growth in Children
Repeated or prolonged infection in early childhood can affect growth and development.
Temporary Lactose Intolerance
Difficulty digesting milk commonly follows the infection and usually settles over weeks.
Symptoms Continuing After the Parasite Has Gone
Some people have ongoing bloating and irregular bowels for a period after successful treatment. This is recognised and generally improves.
Prolonged Fatigue
Tiredness often outlasts the bowel symptoms by several weeks.
When to Seek Medical Care
Diarrhoea Lasting More Than a Few Days
Particularly if it is greasy, foul-smelling and accompanied by bloating.
Any Sign of Dehydration
Very little urine, sunken eyes, drowsiness, or no tears in a crying child.
Weight Loss or Poor Growth in a Child
This deserves a stool test rather than being attributed to fussy eating.
Blood in the Stool or High Fever
Neither is typical of giardiasis, so both point to another cause needing assessment.
If Several People in the Household Are Ill
This suggests a shared source such as water, and the source is worth identifying.
Prognosis
Most People Recover Fully
With or without treatment, the great majority of people recover completely.
Treatment Shortens the Illness
Prescribed medicine clears the parasite and shortens what can otherwise be several weeks of symptoms.
Some Symptoms Linger
Bloating, loose stools and difficulty with milk can persist for weeks after the parasite is gone. This does not mean treatment failed.
Reinfection Is Possible
Having had giardiasis does not prevent catching it again, so the same water and hygiene precautions still apply.
Differential Diagnosis
Amoebiasis
Another parasitic gut infection, but it typically causes bloody diarrhoea and can affect the liver, which giardiasis does not.
Bacterial Causes of Diarrhoea
Usually shorter and more often accompanied by fever and blood, and separated by stool testing.
Irritable Bowel Syndrome
Causes bloating and irregular bowels over months to years without weight loss, and is diagnosed after infection has been excluded.
Coeliac Disease and Other Absorption Problems
Cause greasy stools and weight loss like giardiasis, and are separated by blood tests and by the response to treatment.
Lactose Intolerance
Causes bloating and loose stools after milk. It can also follow giardiasis, so the two are sometimes both present.
References
CDC: About Giardia Infection
What giardiasis is, its symptoms, how common it is and who is at risk.
CDC: Giardia Infection, Causes and How It Spreads
Transmission routes, including water, food, person-to-person and animal contact.
CDC: Patient Care for Giardia Infection
Diagnosis, treatment options and management of persisting symptoms.
CDC: DPDx, Giardiasis
Life cycle of the parasite and the laboratory methods used to identify it.
Frequently Asked Questions
How do people catch giardiasis?
By swallowing the parasite. CDC lists contaminated drinking water, recreational water such as lakes and pools, contaminated food, surfaces and objects, person-to-person contact especially in childcare settings, sexual contact, and contact with infected animals. Swallowing just a few parasites is enough.
Does chlorine kill it?
Not reliably. CDC notes the parasite tolerates chlorine moderately well, which is why it spreads through swimming pools as well as untreated water. Boiling or a certified filter is what makes water safe.
How long does giardiasis last?
Symptoms typically begin one to three weeks after swallowing the parasite and can last several weeks without treatment. Prescribed medicine shortens this. Bloating and loose stools sometimes continue for a period after the parasite has gone.
Do I need treatment if I have no symptoms?
Not always. A symptom-free adult may not need medicine, though treatment is usually given to people who handle food, care for young children or live with someone whose immune system is weakened. Ask your clinician about your situation.
Why do I still feel unwell after treatment?
This is recognised. Temporary difficulty digesting milk and ongoing bloating are common after giardiasis and usually settle over weeks. If symptoms persist or return, ask about retesting, since reinfection is possible.