The Topic of This Month Vol.24 No.4(No.278)

Amebic dysentery April 1999 - December 2002

(IASR 2003; 24:79-80)

The etiological agent of amebic dysentery is a parasitic protozoon, Entamoeba histolytica , which is transmitted through ingestion of foods or water contaminated with cysts. The trophozoite emerges from the ingested cyst after excystation in the small intestines, forming ulcerous lesions on the mucosal surface of the colon. The trophozoite encysts in the colon and the infective cyst is passed in stool. The cyst excretion may sometimes continue for several years. Symptomatic disease develops in about 5-10% of infections, exhibiting such dysentery symptoms as excreting stools with mucus and blood, liquid stools, tenesmus, flatulence, and/or lower abdominal pain at passage. Typical cases may excrete strawberry jelly-like mucosanguineous stools, showing alternate worsening and improving at intervals of a few days-weeks. In fulminant colitis cases, intestinal perforation may occur. About 5% of colitis cases may show lesions of extraintestinal infection, particularly abscess formation in the liver, lung, brain and skin; liver abscess being most frequent. Severe symptoms follow when abscess rupture occurs.

In the former Communicable Diseases Prevention Law, dysentery included shigellosis and amebic dysentery. In addition to those who have dysentery symptoms, those who have extraintestinal infection and also asymptomatic carriers excreting cysts were notified as amebic dysentery in a broad sense of amebiasis. In the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) enacted in April 1999, amebic dysentery was classified as one of the category IV notifiable infectious diseases. In the case definition for notification in the National Epidemiological Surveillance of Infectious Diseases (NESID), the clinical syndrome is classified largely into intestinal amebiasis and extraintestinal amebiasis and the necessity for notification of extraintestinal amebiasis is more clearly shown. On the other hand, asymptomatic cyst carriers have been excluded from the subject of notification.

Incidence: In other countries, there are 48,000,000 yearly cases with about 70,000 deaths, mainly in developing tropical countries (WHO, 1998). In Japan, many domestic cases are seen among male homosexuals (see p. 81 of this issue) and in facilities for mentally-handicapped (see p. 81-85 of this issue), with rather few cases imported from endemic areas. In industrialized countries in Europe and USA, most infections among male homosexuals and mentally handicapped are due to nonpathogenic E. dispar , whereas in Japan, the characteristic status is that most infections are due to E. histolytica for an unknown reason.

1) The National Epidemiological Surveillance of Infectious Diseases (NESID): The cases of amebic dysentery notified in compliance with the Infectious Diseases Control Law numbered at 1,544 during the 45 months from April 1999-December 2002 (Fig. 1). The annual cases are slightly increasing: reported cases during 2000-2002 numbered at 377, 434, and 457. Seasonal or monthly ups and downs are not notable. Males accounted for 90% of all infections (Table 1). The ages of male cases are ranging widely from 20s-70s and showed the largest number at 50-54 years old, while those of female cases peaked at 25-29 years (Fig. 2). Cases reported by prefecture (Fig. 3) show a marked accumulation in such prefectures that have large populations as Tokyo, Kanagawa, Chiba, Aichi, Osaka, and Hyogo. Sixty-four percent of cases were reported as domestic infection and 19% as overseas infection (Table 1).

2) Deaths: Of a total of 1,544 cases of amebic dysentery notified, there were 11 deaths at the time of notification. They were all males aged 44-73 years; there was no death among female cases. Liver abscess was recorded in four cases and intestinal perforation in one. Intestinal lesion was suggested for other three cases, in which no cause of death was identified. No site of lesion was identified in the other three cases. Confirmatory diagnosis was based on a combination of pathogen detection and serologic test in two cases, pathogen detection only in five cases and post-mortem examination in four cases.

3) Route of infection: Many reports have indicated that many of amebic dysentery cases in Japan are complicated with other sexually-transmitted infections, such as syphilis, HIV infection, hepatitis B, genital herpes, etc. (see p. 81 of this issue). The sex difference seen among amebic dysentery cases notified, the incidents concentrated to large cities, and the predominance of domestic infections agree well with the conventional knowledge that a majority of amebiasis cases are male homosexuals. Of notified cases, however, reports of infection through homosexual and heterosexual contacts accounted for only 27% of all. For prevention of infection, it is important to identify the route of infection. It is desirable that the physician try to obtain accurate information from patients. Since domestic infection is also seen among females (Table 1), attention must be paid to the increase in domestic cases among both homosexuals and heterosexuals due to diversification of sex behaviors.

Diagnosis and treatment: Since treatment of nonpathogenic E. dispar is unnecessary, differential diagnosis from E. histolytica infection is important, but morphological differentiation of these two amebas is impossible.

At present, diagnosis depends on
1) morphological detection of ameba (trophozoite or cyst) in stools, colonic epitherium, or needle biopsy of abscess,
2) detection of E. histolytica -specific antigen,
3) detection of E. histolytica DNA by PCR,
4) detection of serum antibody, and
5) supplementary diagnosis including colonoscopy, ultrasound, and CT scan.

The most reliable methods to differentiate between E. histolytica and E. dispar are methods 2) and 3). Method 4) is often used; E. dispar does not invade tissues and gives negative antibody reactions, whereas E. histolytica infection results in positive antibody reaction. Attention must be paid to the fact that the positive result of method 4) only indicates past history of infection with E. histolytica , not necessarily showing the present infection.

According to the current case definition for notification, when the presence of protozoa is demonstrated by at least one of methods 1)-4), amebic dysentery is confirmed. As method 1) requires a lot of skill, combination of method 1) with detection of the etiological agent by method 2) or 3) or with detection of serum antibody, 4), is recommended. In the notified cases, use of method 2) or 3) was seldom seen, and combination of methods 1) and 4) is only seen in 5-10%. In 50% of the cases, detection of the etiological agent was not attempted; diagnosis was made only by observation of clinical symptoms and detection of serum antibody by method 4).

DNA typing of E. histolytica has recently been developed. This has been shown to be useful in identification of the transmission route (see p. 85 of this issue), and will be useful in preventing intrainstitutional and interinstitutional occurrence and spreading of amebiasis.

The therapy of amebiasis patients depends usually on oral administration of metronidazole, giving high therapeutic effects. For therapy of asymptomatic cyst carriers, diloxanide furoate (unlicensed in Japan), that is not absorbed from the digestive tract, is being used (see p. 86 of this issue). The efficacy rate for eradication of cysts is not sufficient in some cases (see p. 81 of this issue).

Outbreak control: In the notifications after enactment of the Infectious Diseases Control Law, only three cases were reported among institutionalized population of mentally-handicapped. This may have been due to the lack of the input column in the case-notification form. It may also suggest that many cases diagnosed as amebic dysentery and suspected to be involved in an outbreak were not properly notified. Many infected cases may not be found because of asymptomatic status, and even if found, asymptomatic cases are not required to report by the current requirement for notification. For prevention of outbreaks, as a part of control of nosocomial infection, it is indispensable to find at an early stage E. histolytica cyst carriers, the source of infection, and to prevent infection among institutionalized population by adequate diagnosis and treatment. Rapid communication of accurate information on amebiasis infection at each institution is also important to limit interinstitutional infection.

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