Amoebiasis : diagnosis and prevalence in León Nicaragua

Abstract

About ten yearly cases of liver abscess are presented in University Hospital, León, Nicaragua. Based on serology most of them have been shown to be amoebiasis cases. This raised the question of Entameoba histolytica prevalence in the population. Based on microscopy of stool specimens and serology, about 20% prevalence was found. With the new realization that pathogenic E. histolytica is morphologically indistinguishable from some common apathogenic species such as E. dispar (cysts detected by microscopy are reported as E. histolytica/ E. dispar), the aim was to determine the true prevalence of E. histolytica using various tests designed to differentiate between pathogenic and apathogenic species.

In 480 apparently healthy individuals, the prevalence of E. histolytica/ E. dispar was 12% (58/480) as determined by microscopy. Out of these 58 stool samples an E. histolytica specific PCR was positive in 5%; thus the prevalence E. histolytica was 0.6%. In a group of 134 diarrhea patients, the most common finding was E. histolytica/ E. dispar (24%) at the health center laboratory level. In the Microbiology Department E. histolytica/E. dispar was found only in 4.5%. With the Triage Parasite test, only one case of E. histolytica/ E. dispar was found. By PCR, E. dispar was recognized in 10 (7.5%) and E. histolytica in two cases (1.5%). Over diagnosis was also confirmed in a quality control study where León health centers were examining 10 different stool samples. We found that the health center technicians continue to mix up E. histolytica/ E. dispar with other amoebas. The consequences of the apparent widespread over diagnosis of E. histolytica were studied retrospectively in 100 records of patients with intestinal symptoms. We found that all patients received treatment with metronidazole or related drugs. In 41% these treatments were not based on any laboratory findings at all. In 32 % E. histolytica/E. dispar were found. Other parasites (Entamoeba coli, Giardia intestinalis, Endolimax nana, Enterobius. vermicularis, Iodamoeba bütschlii) were seen in 27%. To explain the high seroprevalence of anti-E.histolytica antibodies in sero-epidemiological surveys we considered two possibilities: cross reactivity due to the common intestinal apathogenic E. dispar and antibodies to ubiquitous freeliving environmental amoebas. A study was undertaken to identify environmental amoebas and to determine cross-reactivity using antibodies from amoebiasis patients. Amoebas isolated from environmental water samples were characterized by morphological and immunohistochemical methods. In fresh water Acanthamoeba spp. were found in 21 %. Fifty three percent of tested wells in the geothermal area contained thermotolerant amoeboflagellates. Naegleria spp. was identified in 24 out of 39 (62 %) of isolated amoeboflagellates. Absorption studies did not support the idea that environmental free- living amoebas induce antibodies cross-reacting with E. histolytica. Antigenic cross reactivity between E. dispar and E. histolytica remains a possible explanation for the high seroprevalence in the population. This is supported by the observation that IF antibody titers in sera from healthy individuals are similar with both antigens. Also the ratio of antibody reactivity was similar when measured by a densitometric method. In contrast, sera from patients with invasive amoebiasis reacted preferentially with E. histolytica.

We conclude that amoebiasis is not a major problem in the community. Overdiagnosis and overtreatment of diarrhea patients thought to suffer from amoebiasis are serious problems. Thus there is an urgent need for education and quality assessment.