Sero-epidemiology of amoebiasis in Peninsular Malaysia

Protective immunity to entamoeba histolytica infection in subjects with antiamoebic antibodies residing in a hyperendemic zone

To investigate the relationship between amoebic gut infection and the presence of serum antiamoebic antibodies in a community where amoebiasis is highly endemic, a cross-sectional population survey was conducted covering 982 subjects in 2 adjacent villages in rural India. E. histolytica was detected in fecal smears in 18.1% (88/487) and antiamoebic antibodies by micro-ELISA in 28.7% (251/874) subjects. In 450 subjects in whom there was no clinical evidence of invasive amoebic disease or history of ingestion of antiamoebic drugs in the previous year and from whom both stool and matched serum samples were obtained, correlation of gut infection with seropositivity was attempted. Seropositivity in 80 cyst passers (23.8%) was significantly lower than in 370 non-cyst passers (35.1%; p less than 0.05). Conversely the rate of gut infection with E. histolytica was lower in seropositive subjects (19/149; 12.8%) as compared to seronegative subjects (61/301; 23%, p less than 0.05). In the population where E. histolytica infection was highly endemic, the rate of gut infection with the parasite was inversely related to seropositivity. Our data suggests that invasive E. histolytica infection (mostly asymptomatic) evokes good gut immunity in the host with clearing of the parasite from the colon and/or resistance to reinfection. High prevalence rate of amoebic antibodies indicates good "herd" immunity.

Seroepidemiological study of antibody responses to the zymodemes of Entamoeba histolytica

In 303 apparently healthy subjects and 54 patients with confirmed invasive amoebiasis amoebae were isolated where possible, and Entamoeba histolytica was classified into its various zymodems. 20% of the healthy subjects were antibody-positive; 17 healthy subjects proved to be symptomless carriers of pathogenic zymodemes and, like patients with invasive disease, all these were seropositive. Furthermore, of subjects infected with pathogenic zymodemes 94-100% were strongly seropositive, compared with 2-4% of subjects with non-pathogenic zymodemes. Pathogenic zymodemes are therefore believed to be in constant contact with the host's tissues, even in symptom-free individuals; the presence of these potentially invasive organisms is thought to be directly related to the seropositivity in a given population, and seroepidemiological surveys would therefore provide information only on the distribution of this form of the parasite. Quantitative serological data constitute valuable information on the endemicity of an area, and the results will gain reliability from use of two complementary tests instead of one.

Amoebiasis and giardiasis in Bangladesh: parasitological and serological studies

To determine the prevalence of infections with Entamoeba histolytica and Giardia lamblia in Bangladesh, we screened stool specimens from patients with diarrhoea attending urban (N = 2,246) and a rural (N = 2,791) hospital and a group of healthy urban residents (N = 440). Sera from 200 healthy villagers were also examined for antibody to E. histolytica using the immunofluorescent antibody test. The prevalence of E. histolytica cysts or trophozoites in all groups assessed by examination of a single stool specimen ranged from 0% among infants (less than 1 year) to 34% among healthy adults 30 to 44 years old. G. lamblia was uncommon in infants and found most frequently in urban hospital patients aged five to nine (21%). 12% of one- to two-year-old children had serum antibody to E. histolytica at a titre of 1:40. By age 14 years, 80% of those tested were seropositive, and this prevalence declined thereafter with increasing age. Amoebiasis and giardiasis occur frequently in Bangladeshi populations and the morbidity and mortality associated with these infections need to be assessed.