Giardia
Giardia is a genus of protozoan flagellates recognized as one of the most prevalent pathogens causing intestinal diseases in humans globally. The infection it causes, known as giardiasis, is particularly common in developing countries, affecting about 33% of the population, while in developed regions, it impacts around 2% of adults and 6-8% of children. Giardia lamblia, the primary species responsible for this infection, was discovered in 1681 and can be transmitted through contaminated water, undercooked foods, and unsanitary conditions, such as those found in daycare centers. The organism's life cycle includes a protective cyst stage that can survive in cool, damp environments, allowing for transmission even in challenging conditions. An infection can be triggered by ingesting a small number of cysts, leading to symptoms that often affect those with compromised immune systems. Diagnosis typically requires multiple stool tests, and treatment usually involves medications like metronidazole or tinidazole, although resistance to these drugs is noted in some cases. Overall, awareness and preventive measures are key in managing the risk of giardiasis, especially in high-risk environments.
Giardia
- TRANSMISSION ROUTE: Ingestion
Definition
Giardia, a genus of protozoan flagellates, is one of the most common intestinal-disease-causing pathogen in humans worldwide. An estimated 33 percent of all people in developing countries have giardiasis, a protozoan infection caused by Giardia lamblia. Though the disease is less common in developed countries, it is still present, affecting an estimated 2 percent of adults and 6 to 8 percent of children. G. lamblia was first discovered in 1681 by Antoni van Leeuwenhoek, who examined his own diarrheal feces under a rudimentary microscope. This species is also sometimes known as G. intestinalis or G. duodenalis.
Natural Habitat and Features
Giardia's natural environment consists of rivers, lakes, and streams. People who hike or backpack may develop giardiasis by drinking water from these sources. Giardia resembles a pear cut in half lengthwise in its active disease-causing stage. Once consumed, the parasite rapidly activates.
Giardia can also be transmitted in undercooked or raw foods, although waterborne contamination is believed to be far more common. When transmitted in food, it is most often transmitted through contaminated water used to prepare food or by an infected food handler. Giardia has also been reported in daycare centers where infant diapers are changed, and proper sanitation has not been practiced. Animal contamination of food may cause infection in humans, and some cross-species reports of G. lamblia have been reported. Giardia also infects amphibians, rodents, birds, voles, and muskrats.
There are two key aspects to Giardia's life cycle, including encystation and active infection. In the cyst stage, Giardia protects itself by creating a hard cyst, in which it can survive for long periods in damp, cool environments. The cyst comprises 40 percent protein and 60 percent carbohydrates; some studies have shown that it is up to 0.5 micrometers (µm) in thickness. The cysts are oval-shaped and have outer and inner layers that form two protective membranes.
Ingestion of as few as ten cysts can trigger an active infection in a host; an average bowel movement of a person infected with giardiasis could contain three hundred million cysts. Once ingested, the parasite is activated by the gastric acid of the host, which triggers excystation (the end of the cyst stage) and the beginning of the second phase of the life cycle, in which the cyst is deactivated, and activation of Giardia occurs. Between two and four trophozoites (the active stage of the organism) may be excysted from each cyst. Each trophozoite reproduces itself at five- to ten-minute intervals. The reproduction stage is not completely understood, and experts disagree on whether the trophozoites reproduce via cell division or sexual reproduction (which has never been observed).
Excystation occurs in about fifteen minutes, starting with the flagella breaking through the cyst. The parasite uses its sucking disk like a suction cup to attach to the intestinal wall of the host. The trophozoite of G. lamblia is up to 9 µm wide and 15 µm long. G. lamblia consumes some of the nutrients eaten by the host before the host can use them. People most susceptible to infection are those whose immune systems are compromised. Young children and those who work with them, people exposed to contaminated water, those who work with animals, and individuals living in areas that lack proper sanitation are also at risk.
There are two primary genetic groups of G. lamblia that affect humans: assemblage A and assemblage B. Assemblage B appears to be more common in humans overall, though assemblage A predominates in some parts of the world.
G. lamblia can survive a moderate level of chlorine and, according to the Centers for Disease Control and Prevention (CDC), can remain alive for up to one hour in chlorinated pools. Children and adults can become infected in pools that house the germ by swallowing the pool water.
G. lamblia never enters the bloodstream of the infected host. Its actions are not entirely understood by experts, but it appears that the pathogen may release substances such as lectins or proteinases that can damage the epithelium of the host. Giardia may also trigger proteinase-activated receptors by the host, although Giardia itself never invades the skin. Some research also indicates that Giardia can cause a hypersecretion of chloride in infected humans and the malabsorption of sodium, glucose, and water, which together may be responsible for fluid accumulation of the host during infection.
The number of Giardia cases reports in the United States (US) varies each year. In some years, around 14,000 cases have been reported, while, in other years, over 27,000 have been reported. An average of 19,781 cases are reported in the US each year. However, there are regional differences in numbers. Further, cases have been steadily declining in the twenty-first century.
Pathogenicity and Clinical Significance
Up to three stool specimens are needed to identify Giardia, and only sensitive immunoassays should be used to identify this pathogen. The enzyme-linked immunosorbent assay (ELISA) is one means of testing for Giardia, as is direct fluorescence testing (DFA). Cysts can be readily seen with the fluorescence method.
Drug Susceptibility
The first-line treatment for G. lamblia is metronidazole, given in a dosage of 250 milligrams three times per day for up to ten days. Resistance to metronidazole has been reported in about 20 percent of cases. If such a resistance occurs, tinidazole may be given three times per day for seven days. In the twenty-fist century, tinidazole is often the preferred treatment. If the pathogen is resistant to both metronidazole and tinidazole, then other drugs may be prescribed, including albendazole, furazolidone, and quinacrine.
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