(1995; 152 pages) [French] [Spanish]
Amoebiasis and giardiasis
Entamoeba histolytica is a protozoan parasite which is usually transmitted from person to person through faecal contamination of food or hands, but may also be transmitted by sexual contact in homosexual men. Ingested cysts release trophozoites that lodge in the caecum and ascending colon where they multiply and form more cysts which are excreted in the faeces. Only certain varieties are pathogenic, and asymptomatic carriers are common in endemic areas. Diagnosis presents difficulties, particularly in epidemiological surveys, because the microscopical techniques used require highly skilled personnel seldom available where infection is most prevalent. Globally, as many as 500 million people may harbour these parasites and several tens of thousands die each year as a consequence of fulminating colitis or liver abscess.
Amoebic dysentery occurs when the parasites invade the intestinal wall and abscesses may develop in the liver or, less frequently, in the lung or brain as a result of haematogenous spread. Skin lesions may also occur. Pregnant women and individuals who are malnourished or immunocompromised are most vulnerable to systemic infection.
Sporadic cases of invasive amoebiasis occur worldwide, but the disease is most prevalent throughout south-east Asia including the Indian subcontinent, south-east and west Africa, and Central and South America.
Where there is a high risk of reinfection neither chemoprophylaxis nor mass chemotherapy offers an effective means of control. Prevention is dependent upon eliminating faecal contamination of food, hands and water supplies by:
• instructing primary health care workers on how the disease is transmitted and recognized;
• training communities in personal and family hygiene; and
• efficient sewage disposal and provision of an adequate and safe supply of water.
The available drugs are classified broadly as luminal amoebicides, active primarily against organisms in the colonic contents, and systemic amoebicides, active against organisms responsible for invasive disease.
In non-endemic areas, carriers should be treated with a luminal amoebicide which reduces the risk of transmission and protects the patient from invasive amoebiasis. Diloxanide furoate is most widely used, but other compounds, including clefamide, etofamide and teclozan, are also effective.
When the risk of reinfection is high, treatment is not warranted except for mothers responsible for preparing food within a family or for individuals who, as a result of their occupation or lifestyles, are particularly likely to infect others.
All patients with invasive disease require treatment, firstly with a systemically active compound and, subsequently, with a luminal amoebicide in order to eliminate any surviving organisms in the colon. Combined preparations have also been used with success. The pathology and clinical expression of amoebiasis vary from region to region and drug regimens are best devised on the basis of local experience.
The availability of metronidazole - and several other 5-nitroimidazoles, including ornidazole, tinidazole and secnidazole - has made the management of most cases simpler and safer (see table on page 8). Parenteral formulations of metronidazole, ornidazole and tinidazole are available for patients too ill to take drugs by mouth. Preliminary studies suggest that the more recently introduced compounds may sometimes act more rapidly, and comparative clinical studies are being conducted. Until their results become widely known the cheapest available preparation should be used. In severe cases of amoebic dysentery, tetracycline lessens the risk of superinfection, intestinal perforation and peritonitis when it is given in addition to a systemic amoebicide.
Dehydroemetine, which is too irritant to be taken orally, is claimed by some authorities to remain the most effective tissue amoebicide (but it is closely matched by parenterally administered 5-nitroimidazoles). It is reserved for dangerously ill patients, but these are likely to be most vulnerable to its cardiotoxic effects.
Patients treated with dehydroemetine for hepatic abscess should also receive chloroquine, which has amoebicidal activity and is selectively concentrated in the liver. Needle aspiration is advisable, both when the size of the abscess is likely to compromise effective penetration of the drugs, and when severe hepatic pain and tenderness indicate that rupture is imminent.
Giardia intestinalis is a flagellated protozoan parasite which frequently coexists with E. histolytica and is transmitted in the same way. It occurs worldwide, particularly where sanitation is poor and it is a common cause of both acute and persistent diarrhoea among children in developing countries. Reported prevalence rates range from less than 1% to over 50% and it has been estimated that about 200 million infections occur annually in Africa, Asia and Latin America. Localized epidemics frequently occur in children’s institutions. In addition, several large waterborne epidemics have occurred in northern regions of the former USSR, and also in Canada and the USA, where beavers may provide a reservoir of infection.
Ingested cysts release trophozoites that attach firmly to the mucosa of the jejunum. These multiply and eventually form another generation of cysts which are excreted intermittently in the faeces. Many carriers are symptomless, but others lose weight and complain of diarrhoea or gastrointestinal discomfort. Diagnosis requires skilled microscopy, and false-negative tests are common because cysts are excreted in the stools irregularly. Confirmatory examination of jejunal aspirates may be necessary. Extensive infections result in intestinal malabsorption and impairment of growth. Severe symptoms are more likely to occur in patients who are malnourished, hypochlorhydric or immunocompromised.
Treatment with tinidazole in a single dose or with another 5-nitroimidazole is highly effective and should be offered, when practicable, to all infected patients. Family and institutional contacts should also be treated. Larger epidemics are difficult to eradicate because of the high proportion of symptomless carriers and because excreted cysts can survive for long periods outside the human host.