Amebiasis

Synonyms

4

Overview

Amebiasis is an intestinal illness that's typically transmitted when someone eats or drinks something that's contaminated with the parasite Entamoeba histolytica (E. histolytica)-the parasite is an amoeba, a single celled organism. 

Most infected people, about 90%, are asymptomatic, but this disease has the potential to make the sufferer dangerously ill if not treated. It is estimated that about 40,000 to 100,000 people worldwide die annually due to amoebiasis. Infections can sometimes last for years. Until symptoms appear, can take a few days to weeks to develop and manifest.

Symptoms

Symptoms can range from mild diarrhea to severe dysentery with blood and mucus.

Mild symptoms:

  • Abdominal cramps
  • Diarrhea
  • Passage of 3 - 8 semiformed stools per day
  • Passage of soft stools with mucus and occasional blood
  • Fatigue
  • Intestinal gas (excessive flatusexcessive flatus)
  • Rectal pain while having a bowel movement (tenesmustenesmus)
  • Unintentional weight lossUnintentional weight loss 

Severe symptoms:

  • Abdominal tenderness
  • Bloody stools 
  • Passage of liquid stools with streaks of blood
  • Passage of 10 - 20 stools per day
  • Fever
  • Vomiting 

In 90% of people with amebiasis there are no symptoms.

Causes

In places with poor sanitation, amebiasis is acquired by ingesting food or water that is contaminated with feces. Fruits and vegetables may be contaminated when grown in soil fertilized by human feces, washed in polluted water, or prepared by someone who is infected. Amebiasis may occur and spread in places with adequate sanitation if infected people are incontinent or hygiene is poor (for example, in day care centers or mental institutions). Amebiasis can also be spread through certain sexual practices such as oral-anal sex.

Prevention

Travelers to countries where sanitary standards are low can reduce their chances of acquiring amebiasis by:

  • Drinking only water that has been bottled in sanitary conditions or boiled (water-purifying tablets are ineffective against amebic cysts)
  • Eating only cooked or peeled vegetables or fruits
  • Protecting food from fly contamination
  • Washing hands after defecation and before preparing or eating food

To help prevent the spread of amoebiasis at home:

  • Clean bathrooms and toilets often; pay particular attention to toilet seats and taps.
  • Avoid sharing towels or face washers.

To help prevent infection:

  • Avoid raw vegetables when in endemic areas
  • Boil water or treat with iodine tablets.
  • Avoid eating street foods especially in public places where others are sharing sauces in one container

Good sanitary practice, as well as responsible sewage disposal or treatment, are necessary for the prevention of E.histolytica infection on an endemic level. E.histolytica cysts are usually resistant to chlorination, therefore sedimentation and filtration of water supplies are necessary to reduce the incidence of infection.

E. histolytica cysts may be recovered from contaminated food by methods similar to those used for recoveringGiardia lamblia cysts from feces. Filtration is probably the most practical method for recovery from drinking water and liquid foods. E. histolytica cysts must be distinguished from cysts of other parasitic (but nonpathogenic) protozoa and from cysts of free-living protozoa as discussed above. Recovery procedures are not very accurate; cysts are easily lost or damaged beyond recognition, which leads to many falsely negative results in recovery tests.

Diagnosis

Diagnostic tools may include:

  • Medical history, physical examination, lab tests, and stool studies
  • Blood tests for revealing antibodies to the organism
  • Sigmoidoscopy to evaluate the intestinal wall
  • Radiological studies including ultrasound and CT (computed tomography) scans to detect liver abscesses

With colonoscopy it is possible to detect small ulcers of between 3–5mm, but diagnosis may be difficult as the mucous membrane between these areas can look either healthy or inflamed.

Asymptomatic human infections are usually diagnosed by finding cysts shed in the stool. Various flotation or sedimentation procedures have been developed to recover the cysts from fecal matter and stains help to visualize the isolated cysts for microscopic examination. Since cysts are not shed constantly, a minimum of three stools should be examined. In symptomatic infections, the motile form (the trophozoite) can often be seen in fresh feces. Serological tests exist and most individuals (whether with symptoms or not) will test positive for the presence of antibodies. The levels of antibody are much higher in individuals with liver abscesses. Serology only becomes positive about two weeks after infection. More recent developments include a kit that detects the presence of amoeba proteins in the feces and another that detects ameba DNA in feces. These tests are not in widespread use due to their expense.

Microscopy is still by far the most widespread method of diagnosis around the world. However it is not as sensitive or accurate in diagnosis as the other tests available. It is important to distinguish the E. histolytica cyst from the cysts of nonpathogenic intestinal protozoa such as Entamoeba coli by its appearance. E. histolytica cysts have a maximum of four nuclei, while the commensal Entamoeba coli cyst has up to 8 nuclei. Additionally, in E. histolytica, the endosome is centrally located in the nucleus, while it is usually off-center in Entamoeba coli. Finally, chromatoidal bodies in E. histolytica cysts are rounded, while they are jagged in Entamoeba coli. However, other species, Entamoeba dispar and E. moshkovskii, are also commensals and cannot be distinguished from E. histolytica under the microscope. As E. dispar is much more common than E. histolytica in most parts of the world this means that there is a lot of incorrect diagnosis of E. histolytica infection taking place. 

Typically, the organism can no longer be found in the feces once the disease goes extra-intestinal.[citation needed] Serological tests are useful in detecting infection by E. histolytica if the organism goes extra-intestinal and in excluding the organism from the diagnosis of other disorders. An Ova & Parasite (O&P) test or an E. histolytica fecal antigen assay is the proper assay for intestinal infections. Since antibodies may persist for years after clinical cure, a positive serological result may not necessarily indicate an active infection. A negative serological result however can be equally important in excluding suspected tissue invasion by E. histolytica.

Prognosis

In the majority of cases, amoebas remain in the gastrointestinal tract of the hosts. Severe ulceration of the gastrointestinal mucosal surfaces occurs in less than 16% of cases. In fewer cases, the parasite invades the soft tissues, most commonly the liver. Only rarely are masses formed (amoebomas) that lead to intestinal obstruction.(Mistaken for Ca caecum and appendicular mass) Other local complications include bloody diarrhea, pericolic and pericaecal abscess.

Complications of hepatic amoebiasis includes subdiaphragmatic abscess, perforation of diaphragm to pericardium and pleural cavity, perforation to abdominal cavital (amoebic peritonitis) and perforation of skin (amoebic cutis).

Pulmonary amoebiasis can occur from hepatic lesion by haemotagenous spread and also by perforation of pleural cavity and lung. It can cause lung abscess, pulmono pleural fistula, empyema lung and broncho pleural fistula. It can also reach brain through blood vessel and cause amoebic brain abscess and amoebic meningoencephalitis. Cutaneous amoebiasis can also occur in skin around sites of colostomy wound, perianal region, region overlying visceral lesion and at the site of drainage of liver abscess.

Urogenital tract amoebiasis derived from intestinal lesion can cause amoebic vulvovaginitis (May's disease), rectovesicle fistula and rectovaginal fistula.

Entamoeba histolytica infection is associated with malnutrition and stunting of growth.

Treatment

Tinidazole (Tindamax) - FDA-approved indication: Treatment of intestinal amebiasis and amebic liver abcess caused by E. histolytica in both adults and pediatric patients older than three years of age. It is not indicated for the treatment of asymptomatic cyst passage.

Resources

  • Refer to Research Publications
  • NIH