The most important human disease due to amoebae is amoebiasis, which is caused by Entamoeba histolytica. This intestinal pathogen can be differentiated from other enteric amoebae such as ntamoeba hartmani, Entamoeba coli and Endolimax nana, since E. histolytica is the only amoeba found in the intestine that phagocytoses RBCs. It occurs worldwide, although much higher incidence rates are found n the tropics and subtropics. It can be found in active male homosexuals who carry the pathogen (usually strains of low virulence) and between whom it is spread by sexual contact.
LIFE-CYCLE AND PATHOGENESIS
The organism exists both as a motile trophozoite and as a cyst that can survive outside the body. Cysts are transmitted chiefly by ingestion of contaminated food or water or spread directly by person-to-person contact. Trophozoites emerge from the cyst in the small intestine and then pass on to the colon, where they multiply. Many individuals can carry the pathogen without obvious evidence of clinical disease (asymptomatic cyst passers). However, under certain conditions, E. histolytica trophozoites invade the colonic epithelium, probably with the aid of their own cytotoxins and proteolytic enzymes. The parasites continue to multiply and finally frank ulceration of the mucosa occurs. If penetration continues trophozoites may enter the portal vein, via which they reach the liver and cause hepatitis and intrahepatic abscesses. This invasive form of the disease is particularly serious and unless treated promptly is often fatal.
The incubation period is highly variable and may be as short as a few days or as long as several months or even a year. The presentation of amoebic colitis may be:
RADUAL ONSET with mild intermittent diarrhoea and abdominal discomfort, usually progressing to bloody diarrhoea with mucus. Systemic manifestations such as headache, nausea and anorexia are often present.
EVERE ACUTE (AMOEBIC) DYSENTERY, closely resembling that due to Shigella (bacillary dysentery).
FULMINATING COLITIS. Typically, patients with amoebic colitis appear less unwell than those with bacillary dysentery, fever is low-grade or absent, and dehydration is unusual.
Complications are unusual, but include:
PROGRESSION OF FULMINANT COLITIS to toxic dilatation of the colon with perforation and peritonitis.
CHRONIC INFECTION leading to stricture formation.
AMOEBOMA, i.e. a mass of fibrotic granulation tissue, develops most commonly in the caecum or rectosigmoid region. Amoebomas occur in 10% of patients and may bleed, cause obstruction, intussuscept and are sometimes mistaken for a carcinoma.
AMOEBIC LIVER ABSCESS, which often develops in the absence of a recent episode of colitis. Tender hepatomegaly, a high swinging fever and profound malaise are characteristic, although early in the course of the disease both symptoms and signs may be minimal.
The amoebic fluorescent antibody titre (FAT) is positive in at least 90% of patients with liver abscess and 75% with active colitis. Seropositivity is low in asymptomatic cyst passers.
Direct examination of colonic exudate obtained at sigmoidoscopy or of freshly passed stool as a saline-wet mount is the most rapid and least expensive way of confirming amoebic infection. E. histolytica trophozoites must be distinguished from non-pathogenic amoebae and from polymorphonuclear leucocytes, with which they are sometimes confused. Cysts may also be present in the stool. Sigmoidoscopy and barium enema examination may show colonic ulceration but are rarely diagnostic.
Liver abscess should be suspected if the serum alkaline phosphatase is elevated, even when clinical signs are absent. Hepatic ultrasound scan should confirm the presence of an abscess, which may be either single or multiple. Pus from an amoebic abscess has a classic ‘anchovy sauce’ appearance and may contain trophozoites.
Metronidazole 800 mg three times daily for 5 days is given in amoebic colitis and a more prolonged course for 10- 14 days in liver abscess or other extra-intestinal spread. An alternative drug is the other nitroimidazole derivative, tinidazole. Dehydroemetine is used when nitroimidazoles fail (rarely). Diloxanide furoate is a luminal amoebicide and may be a helpful adjunct in clearing cysts. Large, tense abscesses in the liver may require percutaneous drainage, using an ultrasound scan to localize accurately the abscess and to position the drainage neeclle.
CONTROL AND PREVENTION
This disease will be difficult to eradicate because of the substantial human reservoir of asymptomatic cases. There is no immediate hope of vaccine development, particularly as the same individual may experience several episodes of amoebic infection, indicating that only partial protective immunity develops after exposure to the pathogen. Improved standards of personal hygiene and water quality are important. Cysts are destroyed by boiling water for at least 10 min, but the effects of chlorination are variable.
Balantidium coli is the only ciliate that produces clinically significant infection in humans. It is found throughout the tropics, particularly in Central and South America, Iran, Papua New Guinea and the Philippines. It is usually carried by pigs and infection is most common in those communities that live in close association with swine. Its life-cycle is identical to that of E. histolytica. B. coli produces a dysenteric illness owing to invasion of the distal ileal and colonic mucosa. The colitis may be acute and fulminant and if untreated may be fatal. Trophozoites rather than cysts are found in the stool. Treatment is with tetracycline, ampicillin or metronidazole.
Giardia lamblia is a flagellate that is found worldwide. It causes small-intestinal disease, with diarrhoea and malabsorption. Prevalence is high throughout the tropics. It is an important cause of traveller’s diarrhoea worldwide usually occurring on return from travel. In certain parts of Europe, the former USSR, and in some rural and mountainous areas of North America, large water-borne epidemics have been reported. Person-toperson spread is common in day nurseries and residential institutions and between male homosexuals. Like E. histolytica, the organism exists both as a trophozoite and a cyst, the latter being the form in which the protozoon is transmitted. The organism colonizes and multiplies within the small intestine and may remain there without causing detriment to the host. Severe malabsorption may occur and is thought to be related to morphological damage to the small intestine; changes in villous architecture vary from mild partial villous atrophy to rarely subtotal villous atrophy. The mechanism by which Giardia causes alteration in mucosal architecture and produces diarrhoea and intestinal malabsorption is unknown. There is evidence that the morphological damage may be immune mediated. Bacterial overgrowth has also been found in association with giardiasis and may contribute to fat malabsorption.
Many individuals excreting Giardia cysts have no symptoms and are therefore carriers. Others develop symptoms within 1 or 2 weeks of ingesting cysts. These include diarrhoea, often watery in the early stage of the illness, nausea, anorexia, abdominal discomfort and distension. Stools may then become paler, with the characteristic features of steatorrhoea. If the illness is prolonged, weight loss ensues, which, even in previously healthy adults, can be marked. Chronic giardiasis can result in growth retardation in children.
Both cysts and trophozoites can be found in the stool, but negative stool examination does not exclude the diagnosis since the parasite may be excreted at irregular intervals. The parasite can also be seen in duodenal aspirates and in histological sections of jejunal mucosa. Raised specific anti-Giardia IgG and, in acute infections, IgM antibodies are found.
Metronidazole 2 g as a single dose on three successive days will cure the majority of infections, although sometimes a second or third course is necessary. Preventive measures are similar to those outlined for E. histolytica.Alternative drugs include mepacrine, furazolidone and albendazole.
This organism is found worldwide, cattle being a major natural reservoir. It has also been demonstrated in drinking water supplies. It produces a devastating diarrhoeal illness in patients with immunodeficiency, particularly those with AIDS. It has recently become a recognized cause of gastroenteritis, particularly in children. The parasite is able to reproduce both sexually and asexually and has a life-cycle in the intestine very similar to that of Plasmodium. The disease is spread by oocysts excreted in the faeces.
In healthy individuals cryptosporidiosis is a self-limiting illness lasting for 7-10 days. Acute watery diarrhoea is associated with fever and general malaise, but otherwise the disease follows a benign course. In the immunocornpromised patient diarrhoea is followed by severe weight loss and general debility, contributing significantly to the downhill course of AIDS. Occasionally toxic dilatation of the colon can occur. A syndrome of right upper quadrant abdominal pain, raised alkaline phosphatase and the typical bile duct abnormalities of sclerosing cholangitis is seen in AIDS patients.
The parasite can be detected in intestinal biopsies but is now most commonly found in faeces (as oocysts) usingoncentration techniques and a modified Ziehl-Nielsen stain.
As yet there is no effective antimicrobial treatment for this infection. AZT can reduce diarrhoea temporarily in _ IDS as can paromomycin, although the agent does not affect the cryptospiridiosis itself. Good hygiene, especially hand washing, prevents spread of organism.
B. hominis is a strictly anaerobic protozoan pathogen that inhabits the colon. For decades, its pathogenicity for umans was questioned but there is increasing evidence at it may cause diarrhoea. It is sensitive to metronidazole.
Recently cyanobacterium-like bodies were detected in the ools of travellers returning from Nepal with diarrhoea. This coccidian parasite, which has not been detected “ithin enterocytes, is thought to cause diarrhoea and has entatively been named Cyclospora cayatenensis.
This is now a common cause of diarrhoea in patients with HIV infection. Spores can be detected with high accuracy in the stools. Albendazole is effective in eradication. Trichomon iasis Trichomonas vaginalis is a flagellate that causes vaginitis and urethritis