It is a stomach bug known to afflict some backpackers with bouts of uncomfortable diarrhoea.
Now scientists say they have discovered how the parasites that cause giardiasis - one of the world's most common gastric diseases - make people ill.
Giardia parasites mimic human cell functions to break apart cells in the gut and feed inside, researchers found.
This also allows bacteria already present in the body to join in and feed from the same nutrients, they said.
Some 500,000 cases of giardiasis are diagnosed each year, with people typically picking up the disease by drinking infected water or contaminated food.
Symptoms include severe diarrhoea, stomach pains, bloating, flatulence and fatigue and can last weeks or months without treatment.
Although it is found throughout the world, it is most prevalent in developing countries and is one of the most common gastric diseases caught by backpackers.
But although scientists have known of giardia's existence for hundreds of years, until now it remained unclear how it makes people sick.
Researchers found the giardia parasite produces two types of protein that enable it to cut through layers of protective mucus in the gut - breaking the links that knit cells together - in order to easily access the nutrients within them.
One of the proteins does this by mimicking a group of human proteins called tenascins, which regulate cell adhesion and break apart when necessary, such as during wound healing.
But the giardia tenascins are used instead to upset the body's balance by preventing healing of the junctions between cells that hold them together.
One of the study's authors, Dr Kevin Tyler, from UEA's Norwich medical school, said: "Because the giardia have broken down the cell barriers and made all these nutrients available, other, opportunistic bacteria can move in to take advantage of these 'ready meals' which can make giardiasis even more severe for some."
Those suffering from giardiasis are usually able to recover from the illness with or without antibiotics.
However, about half of those who get the parasite experience no symptoms of the illness.
Dr Tyler said the difference in the severity of disease might be explained by the proportion of "good" and "bad" bacteria in the gut.
Those who become more ill may have a higher proportion of "bad bacteria" feeding off nutrients released by the giardia parasite, Dr Tyler said.
Dr Tyler told the BBC: "Some people have a gut that is predominantly full of quite good bacteria that doesn't cause inflammation and illness and indeed may protect from it.
"What we think is that in people who have the bad bacteria, the pro-inflammatory bacteria, those start to use the nutrients that have been unleashed by the giardia.
"The giardia does the damage, allowing the nutrients to flow into the gut, and then if you have the wrong kind of bacteria you get this cycle of inflammation."
This is why probiotic drinks and supplements - which populate the gut with good bacteria - are helpful in treating giardiasis, he said.
The study was published in the journal GigaScience, external.
Source: Centers for Disease Control and Prevention
FAQs
Researchers found the giardia parasite produces two types of protein that enable it to cut through layers of protective mucus in the gut - breaking the links that knit cells together - in order to easily access the nutrients within them.
How does Giardia cause human illness? ›
Giardiasis is a common illness caused by a parasite that may result in diarrhea and stomach cramps. The Giardia parasite can spread through contaminated water, food and surfaces, and from contact with someone who has it. Antibiotics can treat giardiasis.
What kills Giardia in humans naturally? ›
Thankfully giardia is quite susceptible to many of our herbal antimicrobials including garlic (11), oregano, and guava leaf (12). Many other herbal antimicrobials are helpful, and in most cases natural treatment for Giardia can be effective in as little as two weeks of herbal treatment.
Does Giardia stay with you for life? ›
Giardia infections usually clear up within a few weeks. But you may have intestinal problems long after the parasites are gone. Several drugs are generally effective against giardia parasites, but not everyone responds to them. Prevention is your best defense.
Can your body fight Giardia on its own? ›
Children and adults who have giardia infection without symptoms usually don't need treatment unless they're likely to spread the parasites. Many people who do have problems often get better on their own in a few weeks.
Can I get Giardia from my dog licking me? ›
Can I get giardia from my dog licking me? Fortunately, the risk of humans contracting Giardia from dogs is relatively low, but it can happen.
How long does Giardia last in humans without treatment? ›
Symptoms usually begin 1 to 2 weeks after becoming infected with Giardia and last for 2 to 6 weeks.
What not to eat with Giardia? ›
Eat slowly and avoid foods that are hard to digest or may irritate your stomach, such as foods with acid (like tomatoes or oranges), spicy or fatty food, meats, and raw vegetables. You may be able to go back to your normal diet in a few days.
How to starve Giardia? ›
This can best be achieved by consuming a whole-food based, high-fiber, diet that is low in fat, lactose, and refined sugars. Additionally, ingestion of probiotics and wheat germ assists in parasite clearance. Numerous medicinal herbs show promise in the treatment of giardiasis.
Does apple cider vinegar treat Giardia? ›
Indeed, undiluted vinegar solution has also been reported to inactivate Giardia cysts [28] , although temperature, contact time, and concentration appear to be important considerations [27]. ...
Key points about giardiasis
It causes diarrhea. Symptoms include explosive, watery, greasy, foul-smelling stools, bloating, nausea, pain, gas, fatigue, and loss of appetite. Several medicines are available that cure the infection.
Can Giardia affect the brain? ›
Diarrhea caused by Giardia sp. or Cryptosporidium sp. has frequently been associated with stunting and lower cognitive function[40,68] (Table
What are the neurological symptoms of Giardia? ›
Vomiting. Low-grade fever (infrequent) Various neurologic symptoms (eg, irritability, sleep disorder, mental depression, neurasthenia)
What does Giardia feel like in humans? ›
Symptoms often begin by having diarrhea 2 to 5 times per day and feeling more and more tired. However, Giardia infections do not always cause symptoms. Common short-term symptoms include: Diarrhea.
Will probiotics get rid of Giardia? ›
Recent research has focused on the use of probiotics in the prevention and treatment of giardiasis, as several probiotic microbes have been found to have anti-Giardia effects in vitro and in vivo.
Why is Giardia so hard to treat? ›
Thus, drug resistance in Giardia is the ability of this parasite to survive in the presence of an antimicrobial drug dose that would normally kill it or limit its growth.
What is the relationship between Giardia and humans? ›
Infection with Giardia intestinalis most often results from fecal-oral transmission or ingestion of contaminated water. Contaminated food is a less common etiology. Person-to-person spread is common, with 30% of family members with infected children themselves becoming infected. Most infections are asymptomatic.
What is the pathogenesis of Giardia? ›
Abstract. Giardia intestinalis infection causes enterocytes damage and loss of brush border of the epithelial cells of the intestine that leads to shortening of microvilli and altered epithelial barrier function. This pathology results in aqueous diarrhoea, steatorrhea, nausea, abdominal pain, vomiting and weight loss.
How does Giardia cause malabsorption? ›
Previous studies using models in vivo and in vitro have established that Giardia causes malabsorption of glucose, sodium and water, and reduced disaccharidase activity, due to loss of epithelial absorptive surface area. Observations from humans infected with this parasite now confirm these findings.
What is the mode of transmission of giardiasis? ›
Mode of transmission of Giardia
Transmission occurs from person to person and animal to person via hand-to-mouth transfer of cysts from infected faeces or faecally contaminated surfaces. Waterborne outbreaks may occur as a result of faecal contamination of public water supplies or recreational swimming areas.
