Intestinal Protozoa Entamoeba Histolytica: a. Strains For this protozoa , there are two strains. One pathogenic , the other non-pathogenic. The Pathogenic form is called Entamoeba Histolytica, meaning cell lysis due to ameobiasis. The non- pathogenic strain is called Entamoeba Dispar, both strains are morphologically indistinguishable. b. Identification Cyst form : Round in shape ~10 to 20 m in diameter They have smooth reflective coating Immature cysts contain dark sausage shaped bodies known as chromatin bodies which disappear as the cyst matures Immature cysts have less than four nuclei, and four up on maturity Trophozoite form: Amoeboid shape ~10 to 60 m Reflectile hyaline ectoplasm, sharply separated from the endoplasm The granulated cytoplasm contain no bacteria , but sometimes RBCs in different stages of breakdown Single eccentric nucleus Chromatin evenly lines the inner wall of the nucleus A central dark karyosome is present from which a linin network of fine fibrils radiate to the periphery c. Life Cycle Infection takes place when the cyst form of the parasite is ingested. The cyst is the resistant infective form of the parasite. They are formed in the intestinal lumen of the definitive host (man), and are passed out on the stool, there are reservoir hosts but are negligible. Upon ingestion of the cyst , it is immediately infective. Only mature cysts are able to withstand the acidity of the gastric juices of the stomach, which will start to act upon the cyst coating. Upon passage to the upper part of the small intestine, and due to the change in acidity of the environment, the cyst wall disintegrates liberating four metacystic amoeba, which eventually divide into eight trophozoites. The parasites move from the small intestine to the large bowels, and they rely on intestinal stasis in the cecal region to establish a site of infection. Sometimes this is done in the sigmoidorectal region. In cases of hyper intestinal motility , the trophozoite might be passed out of the body completely. Cyst formation takes place in the lumen, and is passed out with the stool. d. Epidemiology World Wide Distribution (WWD) – since this is an intestinal disease- it is highest in developing countries, due to lack of hygiene , and contaminated water and food supply Infection spread is due to low hygiene, and takes place by contamination of water and food supplies. Mechanical transmission of the disease is also done by insects. The ratio of Dispar to Histolytica is about 10:1 respectively 10% of cases develop extraintestinal complications There is no gender bias regarding intestinal ameobiasis, but complications are higher in men , children, and pregnant women. e. Pathogenesis The pathogenesis of E. Histolytica is primarily intestinal, and secondarily extraintestinal , and is associated with ulceration of the colonic mucosa. The parasite is invasive . With superficial ulcers damage is contained with in the mucosa muscularis. As the parasite digs deeper into the colonic wall, and into the submucosa, it extend laterally from its point of penetration, thus widening the ulcer , and giving rise to the classic flask ulcer shape(pathogonomic). The severity of the ulcerations depend on a number of factors such as : Number of parasites present, immunity status of mucosa (ie. IgA gamma globulin), condition of the IG tract at point of penetration, and later on the cellular mediated immunity if and when the parasite gains access to the blood. The reason of the flask shaped ulcer seen in E. Histolytica infection is due to the remaining of the parasite on the lateral wall of the ulcer. As it penetrates laterally, the center of the ulcer fills up with necrotic tissue, while the parasite keeps going laterally, this is a hallmark of invasive ameobiasis. Examination of the necrotic tissue will result in absence of the trophozoite due to it staying on the perimeter of the ulcer. The conditions favoring the invasive nature of the parasite is thought to be provided by the intestinal flora. Mechanical damage caused by the parasite suckers , as it tries to hold to the colon wall cause destruction of the epithelium and blunting of the mircovilli, and removal of the Lactase and Galactase enzymes present on the boundaries of the micro villi. Since both Galactose and Lactose are osmotically active molecules, water is drawn into the lumen of the intestine, and hence we get the clinical manifestation of diarrhea. Malabsorption , and toxin translocation is also possible due to mucosal damage. Finally , E. Histolytica is a phagocytic parasite, ingesting RBCs and to a lesser extent bacteria, is indicative of the invasive phase. f. Clinical Manifestations Acute Case: Classic Dysentery ( inflammation of the colon + abdominal pain) : Abdominal pain – Straining and pain upon defecation (Tenesmus) – Bloody diarrhea – Constipation In sharply acute cases fulminant colitis my develop with sever bloody diarrhea Fibroblast buildup may occur and project into the lumen. Usually mistaken for carcinoma Asymptomatic Case: No clinical signs , and the DH is unaware of the parasite The DH is a silent cyst carrier May persist or turn to acute ameobiasis Chronic Case: Recurrent attacks of dysentery Intervening periods of moderate constipation Localized abdominal tenderness Usually secondary complication symptoms are present g. Complications Complications arise when Extraintestinal ameobiasis occurs. Amebic Liver Abscess (ALA) : by gaining access to the portal circulation , the parasite maybe transported to the liver, usually it infects the posterior aspect of the right lobe. Enlargement and tenderness of the liver arises. Pain maybe referred to the right shoulder due to common origin of the phrenic nerve and the brachial nerves. As in the intestine , the parasite would tend to form an abscess in the liver tissue, with necrotic cell in the center and the parasite on the parameter. This is important to distinguish ALA from Pyogenic liver infection, where bacteria would form an abscess, but will be through out the necrotic tissue. Patients suffering from ALA show the following manifestations: Hepatomegally and tenderness Remittent fever (40˚ C ) with chills Recurrent diarrhea Pain in the right hypochondrium Most patients show leucocytosis, (increased numbers of polymorphonuclear neutrophils distinguish Amebic hepatitis from viral hepatitis. Liver functions will be abnormal with high Alkaline Phosphatase reading, but unlike pyogenic liver abscess, no hyperbilirubineamia Jaundice can occur in cases of super bacterial infection with ALA ALA may develop more complications such as rupture into the pleural cavity and lung, in such a case a bronchopleural fistula can develop . The patient will cough up sputum which will be dark brown due to necrotic liver tissue. Rupture may occur into the peritoneum , and in very few cases the pericardium in involved. ALA has less then 1% mortality if uncomplicated and treated in time. ALA present in the left lobe maybe very difficult to detect since, right lobe functions normally and no signs are detected Peritonitis and diverticulitis are both a common complication of invasive ameobiasis, due the parasite eroding the bowel wall and getting access to the peritoneal cavity. This is usually accompanied by bacterial leakage into the peritoneum from the intestinal flora, causing local or general peritonitis Cutaneous Ameobiasis is rare Abscesses in the brain , kidney, spleen as well as ulcers and lesions in the rectum, uterus, vagina, cervix, and testes are extremely uncommon. h. Diagnosis Key 1. Clinical examination No symptoms Go to 14 Symptoms of intestinal Ameobiasis Go to 2 2. Immediate stool Examination E. Histolytica cyst present Confirm intestinal histolytica . Go to 6 E. Histolytica trophozoite present Confirm intestinal histolytica. Go to 6 Nothing can be seen Go to 3 ; If done more than 3 times Go to 4 for confirmation 3. Take stool examination three times due to acyclic or uneven Go to 2 production of cysts 4. Endoscopy Flask shaped ulcer seen 95% confirm intestinal histolytica. Go to 5 Other shaped ulcer or no ulcer Unlikely Histolytica 5. Biopsy from ulcer edges and exudate Trophozoite identified by mircoscopy and clean exudate Confirm intestinal histolytica. Go to 6 Exudate contain viable organism Histolytica unlikely 6. Symptoms of extraintestinal ameobiasis Symptoms present Go to 7 Symptoms not present Extraintestinal ameobiasis unlikely . Go to 15 ,but confirm with serology . Go to 7 7. Serum examination +ve for histolytica antibodies Go to 8 -‘ve for histolytica antibodies Go to 9 8. Check past infections and dates If past infection present , and recent, with correlation of endemic areas Serum test unreliable, due to antibody masking from previous infection. Go to 10 If no past infections are noted Extra intestinal histolytica confirmed. Go to 10 9. Retake serum test after 5-7 days to confirm –‘ve result Confirmation +’ve Parasite in early extraintestinal phase. Go to 10 Confirmation –‘ve No extraintestinal ameobiasis confirmed Go to 15 10. CT or Sonography imaging of upper right abdominal quadrant Abscess found Go to 11 No Abscess present No ALA , but may be too small to see, because still in initial stages 11. Aspirate collected from abscess using CT guided needle (not to be done if hydatid cyst is suspected due to risk of spillage & anaphylaxis) Clean aspirate Confirm histolytica liver ameobiasis confirmed Go to 12 & 13 Other organism found in aspirate Abscess unlikely due to histolytica, most likely caused due to other super infection Go to 12 12. Liver functions test Functions abnormal Go to 13 Functions normal Histolytica unlikely to be cause of liver abscess, but maybe secondary infection 13. Blood chemistry panel High bilirubin Histolytica unlikely to be cause of liver abscess Normal bilirubin Go to 16 14. Diagnosis: Healthy or E. Dispar infected person, confirm with stool exam. 15. Diagnosis: Patient is suffering from intestinal ameobiasis only , no extraintestinal complications have been seen. 16. Diagnosis: Patient is suffering from ALA and intestinal ameobiasis. i. Treatment Antibiotic therapy is usually effective against invasive intestinal, and extraintestinal ameobiasis. However consideration of secondary complications may require treatment modification, to allow best result of recovery. Treatment of invasive ameobiasis is done in two stages: Invasive disease treatment, and removal and eradication of the intestinal carriage of the organism Stage 1: The drug of choice is Metronidazole , given over a 5-10 day period ,effective in 90% of cases. This is also the drug of choice for treatment of ALA Critically ill patient , with fulminating colitis, should be given Metronidazole and Dehydroemetine ( this is a rapid amoebicidal agent) for the 1st three days, then only Metronidazole for the next week or so. In some cases Metronidazole will not have an appreciable effect. In such a case Tinidazole should be administered, and it has been reported to be effective in a single dose. In cases of peritonitis , the above treatment should be administered with antibacterial therapy and antidiarrheal drugs for relief of dysentery Stage 2: After the above treatment is complete, the patient should be treated with luminal drugs to eradicate the parasite. The drugs of choice are Paromomycine or Iodoquinol In cases of peritonitis the same treatment is advised In cases of pregnant patients , only the nonabsorbable Paromomycin should be given Usually medical treatment is enough to recover full health for such a parasitic infection. However severe complications may warrant surgical intervention. Cases in which toxic megacolon develops or severe ulcerative destruction of a large section of the colon , which may lead to massive hemorrhage, may need resectioning operations to be performed.
Pages to are hidden for
"entamoeba histolytica"Please download to view full document