Gastrointestinal Amebiasis by Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that causes intestinal and extraintestinal infections in humans. It belongs to the phylum Sarcomastigophora, subphylum Sarcodina, class Lobosa, order Amoebida, and family Entamoebidae. It has two main forms: the cyst and the trophozoite.

The cyst is the infectious form that can survive in the external environment and is transmitted by fecal-oral route through contaminated food, water, or objects. The cyst is round or oval, measuring 10 to 16 micrometers in diameter, and has a thick wall that protects it from harsh conditions. The cyst contains one to four nuclei, depending on its stage of development. The mature cyst has four nuclei and can be distinguished from other similar parasites by its chromatoid bodies, which are rod-shaped structures with blunt ends.

The trophozoite is the vegetative form that lives and multiplies in the human colon. It is responsible for causing disease and tissue damage. The trophozoite is motile and moves by extending pseudopods (false feet). It measures 10 to 50 micrometers in diameter and has a single nucleus with a central karyosome (a mass of chromatin) and peripheral chromatin granules. The trophozoite feeds on bacteria, red blood cells, and host tissues by phagocytosis (engulfing). It can also secrete enzymes and toxins that lyse the cells and cause inflammation.

Entamoeba histolytica is a pathogenic species that can cause amoebic dysentery, amoebic colitis, amoebic liver abscess, and other extraintestinal complications. It can invade the intestinal wall and reach the bloodstream, where it can spread to other organs such as the liver, lungs, brain, or skin. Entamoeba histolytica can be distinguished from other nonpathogenic species of Entamoeba, such as Entamoeba dispar or Entamoeba moshkovskii, by molecular or antigenic tests.

Amebiasis is mainly transmitted by the fecal-oral route, which means that people can become infected by ingesting cysts (the dormant form of the parasite) that are present in the feces of an infected person or animal. The cysts can contaminate food, water, soil, or objects that come into contact with fecal matter. Some of the common sources of contamination are:

• Drinking water that is not properly treated or filtered. Cysts can survive in water for weeks and are resistant to chlorine disinfection. Therefore, drinking water from wells, streams, lakes, or other sources that may be contaminated with human or animal feces can pose a risk of infection. Travelers to areas where sanitation is poor should avoid drinking tap water or using ice cubes made from tap water.
• Eating food that is washed, cooked, or prepared with contaminated water. Fruits, vegetables, salads, and other raw foods that are grown or irrigated with fecal-contaminated water can harbor cysts on their surfaces. Similarly, cooked foods that are not heated enough to kill the cysts or that are recontaminated after cooking can also be a source of infection. Food handlers who are infected or who do not practice good hygiene can also spread the parasite to the food they prepare.
• Touching objects or surfaces that are contaminated with cysts and then putting them into the mouth. This can include utensils, plates, cups, toys, diapers, toilet seats, or anything else that may have been in contact with fecal matter. Children who play in dirt or sandboxes that are contaminated with animal feces can also become infected by putting their hands or toys into their mouths.
• Having oral-anal sexual contact with an infected person. This can occur among men who have sex with men or among heterosexual couples who practice oral-anal sex. The parasite can be transmitted through saliva or mucus that comes into contact with the anus of an infected person.

Amebiasis is an infection caused by a protozoan parasite called Entamoeba histolytica. The parasite has two forms: a cyst and a trophozoite. The cyst is the infective stage that can survive outside the human body and can be transmitted through fecal-oral route by contaminated food, water, or objects. The trophozoite is the active stage that lives in the colon and can cause tissue damage and inflammation.

When a person ingests E. histolytica cysts, they reach the small intestine and excyst into trophozoites. The trophozoites then migrate to the large intestine and colonize the lumen and the mucosa. Most of the infections are asymptomatic, but some trophozoites can invade the intestinal wall and cause ulceration, bleeding, and diarrhea. This is called intestinal amebiasis or amebic dysentery.

Some of the invasive trophozoites can enter the bloodstream through the portal vein and reach other organs, such as the liver, lungs, brain, or skin. This is called extraintestinal amebiasis and can cause abscesses, necrosis, and systemic complications. The most common site of extraintestinal amebiasis is the liver, where E. histolytica can cause amoebic liver abscesses.

The pathogenesis of amebiasis involves various molecules secreted by E. histolytica that help it to adhere to, kill, and digest host cells and tissues. Some of these molecules are:

• Galactose-inhibitable lectin: a surface protein that binds to galactose residues on host cells and facilitates attachment and phagocytosis.
• Cysteine proteases: enzymes that degrade host proteins, such as collagen, immunoglobulins, complement components, and cytokines.
• Amebapores: pore-forming peptides that disrupt host cell membranes and cause cell lysis.
• Lipophosphoglycan (LPPG): a surface glycolipid that modulates host immune responses and protects the parasite from complement-mediated lysis.

The pathogenesis of amebiasis also depends on host factors, such as nutritional status, immune system, intestinal flora, and genetic susceptibility. Some people may have resistance or tolerance to E. histolytica infection, while others may develop severe or chronic disease.

• Amebiasis is a global disease, especially prevalent in tropical and subtropical regions with poor sanitation and hygiene.
• Amebiasis affects about 50 million people worldwide and causes 100,000 deaths annually.
• Most infections are asymptomatic, but 10% to 20% of infected people develop intestinal or extraintestinal disease.
• In developed countries, amebiasis is mainly seen in travelers, immigrants, refugees, institutionalized people, and men who have sex with men.
• Travelers who spend more than six months in endemic areas are more likely to acquire E. histolytica infection than those who spend less than one month.
• E. histolytica is not a common cause of travelers` diarrhea and accounts for only 0.3% of cases in one study of German travelers to the tropics.
• E. dispar is more common than E. histolytica and is generally considered nonpathogenic. E. moshkovskii and E. bangladeshi are emerging species with unclear pathogenic potential.

Most people who are infected with E. histolytica do not develop any symptoms and remain asymptomatic carriers.

However, some people may experience mild to severe symptoms of intestinal or extraintestinal amebiasis, depending on the stage and location of the parasite.

Intestinal amebiasis usually occurs within 2 to 4 weeks after ingestion of the cysts and causes symptoms such as:

• Loose stools
• Abdominal cramping and pain
• Flatulence and bloating
• Nausea and vomiting
• Weight loss and malnutrition
• Fever and chills

Amebic dysentery is a severe form of intestinal amebiasis that involves the invasion of the parasite into the intestinal wall and causes symptoms such as:

• Frequent watery and bloody stools
• Severe abdominal pain and tenderness
• Dehydration and electrolyte imbalance
• Perforation or rupture of the colon
• Peritonitis or inflammation of the abdominal cavity

Extraintestinal amebiasis occurs when the parasite spreads to other organs through the bloodstream and causes symptoms such as:

• Amebic liver abscess: The most common complication of amebiasis that involves the formation of pus-filled lesions in the liver. Symptoms include fever, weakness, abdominal swelling, nausea, jaundice, cough, and upper-right quadrant pain.
• Amebic brain abscess: A rare but life-threatening complication of amebiasis that involves the infection of the brain tissue. Symptoms include headache, confusion, seizures, coma, and neurological deficits.
• Amebic pleuropulmonary abscess: A complication of amebiasis that involves the infection of the lungs or chest cavity. Symptoms include chest pain, shortness of breath, cough, hemoptysis, and pleural effusion.
• Amebic cutaneous or genital lesions: A complication of amebiasis that involves the infection of the skin or genital area. Symptoms include ulcers, nodules, or fistulas on the perianal or genital regions.

Amebiasis is diagnosed by detecting the presence of E. histolytica in stool samples, body fluids, or tissue specimens. However, microscopic examination alone cannot distinguish E. histolytica from other nonpathogenic species such as E. dispar, E. moshkovskii, and E. bangladeshi. Therefore, additional tests based on immunological or molecular techniques are required to confirm the diagnosis and differentiate the species.

Some of the commonly used laboratory methods for amebiasis are:

• Microscopy: This involves examining fresh or preserved stool samples for cysts and trophozoites of E. histolytica. Wet mounts, iodine stains, and permanent stains (such as trichrome) can be used to enhance the visibility of the parasites. Aspirates or biopsies from the intestine, liver, or other organs can also be examined for trophozoites. However, microscopy has low sensitivity and specificity and cannot differentiate between pathogenic and nonpathogenic species.
• Antigen detection: This involves using enzyme immunoassays (EIA) or rapid diagnostic tests (RDT) to detect specific antigens of E. histolytica in stool samples or other specimens. Antigen detection has higher sensitivity and specificity than microscopy and can distinguish between pathogenic and nonpathogenic infections. However, antigen detection may not be available in all settings and may cross-react with other parasites.
• Antibody detection: This involves using EIA or indirect hemagglutination (IHA) tests to detect specific antibodies against E. histolytica in serum samples. Antibody detection is most useful for patients with extraintestinal disease (such as liver abscess) when organisms are not found in stool samples. However, antibody detection may not be useful for acute intestinal infection, asymptomatic carriers, or recent travelers, as antibodies may take several days to weeks to develop or may persist for years after infection.
• Molecular methods: This involves using polymerase chain reaction (PCR), loop-mediated isothermal amplification (LAMP), or other nucleic acid amplification tests (NAAT) to detect and differentiate E. histolytica DNA or RNA in stool samples or other specimens. Molecular methods have high sensitivity and specificity and can identify mixed infections with other species. However, molecular methods may not be widely available, standardized, or affordable in resource-limited settings.

The choice of laboratory method depends on the availability of resources, the clinical presentation of the patient, and the epidemiological context of the infection. A combination of methods may be needed to achieve an accurate diagnosis of amebiasis.

Amebiasis is a treatable disease that can be cured with antibiotics. The type and duration of treatment depend on the severity and location of the infection. The treatment of amebiasis consists of two stages: first, an antimicrobial drug is given to kill the invasive trophozoites, and then a luminal agent is given to eliminate the cysts from the intestine and prevent relapse.

The most commonly used antimicrobial drugs for amebiasis are metronidazole and tinidazole. These drugs are effective against both intestinal and extraintestinal infections. They are usually taken orally for 7 to 10 days, but they can also be given intravenously in severe cases. They may cause side effects such as nausea, vomiting, headache, metallic taste, and disulfiram-like reaction with alcohol.

The most commonly used luminal agents for amebiasis are paromomycin and diloxanide furoate. These drugs are not absorbed by the body and act only in the intestine to eradicate the cysts. They are usually taken orally for 10 days after completing the antimicrobial therapy. They may cause side effects such as abdominal cramps, diarrhea, and rash.

Some patients with amebiasis may require surgical intervention in addition to medical treatment. This may be necessary in cases of fulminant colitis, intestinal perforation, bowel obstruction, or liver abscess that does not respond to antibiotics or threatens to rupture. Surgery may involve resection of the affected bowel segment or drainage of the abscess.

The treatment of amebiasis should be guided by laboratory tests that confirm the diagnosis and identify the species of Entamoeba. This is important because some species, such as E. dispar and E. moshkovskii, are non-pathogenic and do not require treatment. Moreover, some strains of E. histolytica may be resistant to certain drugs and require alternative therapy.

The prevention and control of amebiasis depend on improving sanitation and hygiene practices in endemic areas. This includes ensuring safe drinking water, proper disposal of human feces, washing hands before eating and after using the toilet, cooking food thoroughly, peeling or washing fruits and vegetables before consumption, and avoiding sexual practices that involve fecal-oral contact. Travelers to endemic areas should also follow these precautions and seek medical attention if they develop symptoms of amebiasis.

Amebiasis is a preventable disease that can be avoided by following some simple measures of hygiene, sanitation, and food and water safety. Some of the recommended practices are:

• Wash your hands thoroughly with soap and water before eating, after using the toilet, after changing diapers, and after contact with fecal matter. Hand washing can help prevent up to 50% of diarrheal diseases caused by parasites like E. histolytica.
• Drink only safe water that has been boiled for at least one minute, filtered, or treated with chlorine or iodine. Avoid ice cubes unless they are made from safe water. Bottled water with an unbroken seal or carbonated drinks in sealed cans or bottles are also safe.
• Eat only cooked food that is served hot and avoid raw fruits and vegetables unless you peel them yourself. Do not eat food from street vendors or food that has been left out for a long time.
• Avoid contact with human feces or sewage, especially in areas where sanitation is poor. Do not use human feces as fertilizer for crops or irrigation for food plants.
• Practice safe sex and avoid anal-oral contact or sharing of sex toys with people who may be infected with E. histolytica.
• If you travel to an endemic area, take precautions to avoid infection and seek medical attention if you develop symptoms of amebiasis. You may need to take a stool test to confirm the diagnosis and get appropriate treatment.
• If you are diagnosed with amebiasis, follow your doctor`s instructions and take the prescribed medication as directed. Do not stop taking the medication even if you feel better. You may need to take another stool test after treatment to make sure you are cured.
• If you are a food handler, health care worker, or child care provider, do not work while you have symptoms of amebiasis and until you are cleared by your doctor. You may spread the infection to others through contaminated food, water, or objects.

By following these prevention and control measures, you can reduce your risk of getting amebiasis and protect yourself and others from this potentially serious disease.

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