1.Introduction:
Entamoeba histolytica is an anaerobic parasitic amoebozoa, part of the genus Entamoeba. Predominantly infecting humans and other primates causing amoebiasis, E. histolytica is estimated to infect about 35-50 million people worldwide. E. histolytica infection is estimated to kill more than 55,000 people each year. Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, E. dispar. Mammals such as dogs and cats can become infected transiently, but are not thought to contribute significantly to transmission.
Binomial nomenclature of Entamoeba histolytica:
Kingdom: Protista
Subkingdom: Protozoa
Phylum: Sarcomastigophora
Subphylum:Sarcodina
Super class: Rhizopoda
Class: Lobosea
Orders: Euamoebida
Genus: Entamoeba
Species: E. histolytica
2.Structure of Entamoeba histolytica:
Life cycle of histolytica is relatively simple and consists of infective cyst and invasive trophozoites stage. Life cycle completes in a single host, i.e. humans get infected with E. histolytica cyst from contaminated food and water. Infection can also be acquired directly by sexual contact.
2.1) Cyst
Infection occurs by ingestion of mature cysts through fecally contaminated water or food. Due to protection from walls, cysts survive several days and sometimes weeks. They are responsible for transmission.
• It is the infective form of parasite.
• Shape: It is round or round or oval in shape
• Size: 12-15 µm in diameter
• It is surrounded by a highly refractile membrane called cyst wall. The cyst wall is resistant to digestion l by gastric juice in human stomach
• Nucleus: A mature cyst is quadrinucleate.
• Cytoplasm: Cytoplasm shows chromatid bars and glycogen masses but no RBCs or food particles.
• Mature cyst passed out in stool from infected patients and remained without further development in soil for a few days.
2.2) Excystation
Excystation is the process by which cysts transform into trophozoites. When the cysts enter the ileum of the small intestine of the host, the process of excystation begins. Trophozoites are released in the small intestine and from here they migrate to the large intestine.
It is the intermediate stage between trophozoite and cyst
It is smaller in size; 10-20µ
It is round or slightly ovoid with blunt pseudopodia projecting from periphery
No RBC or food materials are found on its endoplasm.
2.3) Trophozoite
Trophozoites are unicellular parasites that measure from 14 to 18 mm in diameter. They multiply in the small intestine by binary fission to produce cysts that exit via human stool. Several trophozoites remain inside the lumen of the small intestine. The rest attach themselves to the intestinal mucosa, enter the bloodstream and further grow in the extraintestinal regions of the host like lungs, liver, brain.
• It is the growing and feeding stage of parasite
• Shape; not fixed because of constantly changing position
• Size: ranging from 18-40 µm; average being 20-30 µm
• Cytoplasm: cytoplasm is divided into two portions; a clear transparent ectoplasm and a granular endoplasm. Ingested RBCs, tissue granules and food materials are also found in endoplasm
• Nucleus: It is single, spherical in shape and size ranging from 4-6µ Nucleus contains central karyosome and fine peripheral chromatin.
• Trophozoites are actively motile with the help of pseudopodia.
• Trophozoites are anaerobic parasite, ( present in large intestine)
3.Life Cycle of Entamoeba histolytica:
•The mature Cyst is resistant to low pH of the stomach, so remains unaffected by the gastric juices.
•The cyst wall is then lysed by intestinal trypsin and when the cyst reaches the caecum or lower part of ilium excystation occurs. The neutral or alkaline environment as well as bile components favor excystation.
•Excystation of a cyst gives 8 trophozoites. Trophozoites are actively carried to the large intestine by peristalsis of the small intestine. Trophozoites then gain maturity and divide by binary fission.
•The trophozoites adhere to mucus lining of the intestine by lectin and secretes proteolytic enzymes which causes tissue destruction and necrosis. Parasite, when gaining access to blood, migrates and causes extra-intestinal diseases.
•When the load of trophozoites increases, some of the trophozoites stop multiplying and revert to cyst form by the process of encystation.
•These cysts are released in feces completing the life cycle.
4.Pathogenesis:
4.1. Mode of infection:
• Faeco-oral route
• Ingestion of cyst contaminated foods and water
4.2. Virulence factors:
• i. Cyst wall: cyst wall is resistant to low pH and gastric juice of the stomach.
• ii. Lectin: Surface of trophozoite contains lectin that is specific to (N-acetyl-galactosamine and galactose sugar) present in the surface of intestinal epithelium.
• iii. Ionophore-like protein: It causes leakage of ions such as Na+, K+, Ca++ from target cells.
• iv. Hydrolytic enzymes: Phosphatase, proteinase, glycosidase and RNase cause tissue destruction and necrosis.
• v. Toxin and haemolysin
4.3. Pathogenesis;
• The parasites express a large number of virulence factors including lectin, lytic peptide, cysteine, proteinases and phospholipase.
• Excystation of cyst in intestine releases 4 trophozoites which then colonizes the large intestine. The binding of trophozoites with the colonic epithelium is a dynamic process in pathogenesis. After adherence trophozoite lyse the target cell by its ionophore-like protein that causes leakage of ions from cytoplasm. The proteolytic enzymes secreted by the amoeba causes tissue destruction giving flask shaped amoebic ulcer, is a
typical feature of intestinal amoebiasis.
Trophozoites penetrates the columnar epithelium of
mucosa causes lysis and moves deep inside till they reach the submucosa layer and multiply rapidly. Ultimately amoeba destroys considerable area of the submucosa leading to an abscess formation which breaks down to form ulcer. The ulcer is flask shaped with a narrow neck and broad base. The ulcer may be localized in the ileo-caecal region or generalized throughout the large intestine.
• From the intestine, the parasites may be carried to other vital organs such as the liver, heart, brain etc through blood circulation. Pulmonary and hepatic amoebic abscesses are frequent and rarely cerebral, cutaneous and splenic amoebic abscesses.
5.Clinical manifestation:
•Infection ranges from asymptomatic to invasive intestinal amoebiasis and extra-intestinal amoebiasis
5.1. Intestinal Amoebiasis
i. Asymptomatic infection: 90% of E. histolytica infection is mild or asymptomatic
ii. Symptomatic infection:
•Non dysenteric amoebic colitis (mild diarrhea)
•Acute amoebic dysentery: it is more common and characterized by abdominal pain, fever and tenderness. Stool contains RBCs, charcot-leyden crystals and trophozoites. Complications: toxic megacolon, fulminant amoebic colitis, ameboma, amoebic peritonitis, perianal ulceration
5.2. Extra intestinal amoebiasis:
• i. Hepatic infection: non suppurative hepatitis, liver abscesses, other complications
• ii. Pulmonary infection: chest pain, dyspnoea, non-productive cough
• iii. Cerebral infection: it is rare and occurs as a complication of liver of pulmonary amoebiasis
• iv. Genitourinary infection: involves kidney and genital organs
• v. Splenic infarction
• vi. Cutaneous amoebiasis
6.Lab Diagnosis:
Specimen: stool, pus or liver abscesses, sputum and biopsy samples
• i. Stool macroscopy: in amoebic dysentery stool is offensive, semi-solid, dark brown color and acidic in nature, mixed with blood, mucus and fecal materials.
• ii. Microscopy: Normal saline preparation of fresh fecal material reveals trophozoites with RBCs in its cytoplasm and its amoebic motility.
• iii. Stool Ag detection: ELISA to detect 170 KD lectin of E. histolytica
• iv. Stool culture: Robinson’s medium and NH polyxenic culture medium are used to culture E. histolytica
• v. Serology: IHA, IFA etc are used to detect antibody in serum against E. histolytica
• vi. PCR: It is sensitive test , used to differentiate E. histolytica with other Entamoeba species
• vii. Radiological finding: X-rays, MRI, CT scan, ultrasonography etc for extra intestinal amoebiasis..
• viii. Blood test: blood count, Liver function test, Kidney function test
• ix. Intradermal test
7.Treatment:
Amoebicidal drugs used to destroy the parasite inside human body may be grouped under following categories
7.1. Tissue Amebicides:
These are the drugs which directly act on the trophozoite stage of the parasite residing inside the tissues-
(a) Emetine and dehydro- emetine (DHE) are the drugs of choice to kill trophozoites residing inside intestinal wall, liver and other metastatic lesions.
(b) Chloroquine (4 aminoquinoline) is used specifically for the parasite present in liver and lung.
7.2. Luminal Amebicides:
• These are the drugs which act when they come in contact with the trophozoites as well as cystic forms of E. histolytica present only in the intestinal lumen. That is why, they are also known as contact amoebicides.
• The important luminal amebicides are Di-iodohydroxyquinoline (diodoquin), iodochlor hydroxy quinoline (clioquinol) chlorphenoxamine (mebinol), chlorbetamide (mantomide), acetarsone (stovarsol), carbarsone (milibis), emetine bismuth iodide (EBI), paromomycin (humatin) etc.
7.3. Both luminal and tissue amebicides:
• The new group of drugs administered orally act on parasite residing in tissue as well as the lumen of intestine are
• Niridazole group (Ambilhar) and Metronidazole group (flagyl, Metrogyl etc.,)
8.Conclusion:
Entamoeba histolytica invasion in human hosts vary from showing zero symptoms to being fatal. This parasite, discovered in 1859 by Lambl. S. Chaudin, showed the difference between pathogenic and non-pathogenic forms of amoeba.
