Asymptomatic Toxocara Infection

We performed a serological survey of Toxocara canis infection in junior high school students from three districts in northern Sulawesi. Almost all of the 117 subjects from two rural districts near Manado allowed dogs in their houses, and there was an 84.6% prevalence of T. canis infection in this group. Fifty-three subjects (45.3%) had serum samples with a high titer of specific anti-Toxocara antibody. By contrast, 41 students tested in one urban district showed a 12.2% prevalence. To confirm the clinical symptoms of visceral larva migrans (VML) and ocular larva migrans (OLM) caused by Toxocara, we administered a questionnaire survey, serological liver function tests, and an ophthalmoscopic examination in 34 subjects having high anti-Toxocara antibodies. One rural district showed a high prevalence; 58 out of 71 subjects (81.7%) had a high titer of anti-Toxocara antibodies according to a plate-ELISA test, although none showed clinical signs. Five of these subjects exhibited hypereosinophilia. These results indicated that T. canis infection in northern Sulawesi is latent in many more cases than previously estimated, and suggest that people living in environments polluted by Toxocara eggs become easily infected with T. canis and show a high prevalence of infection. while a presumptive diagnosis may be based on clinical and laboratory findings and on the presence of significant anti-Toxocara antibody titer (Glickman et al, 1978). In developed countries, there have been numerous serological surveys on toxocariasis in young children, but few such studies have been done in developing countries. We assumed that this was partly due to a lack of suitable tools for easy on-site testing. While the introduction of the enzyme-linked immunosorbent assay (ELISA), which is based on the use of excretory-secretory (ES) antigen, has resulted in greatly increased specificity, it is still difficult to perform this test in developing countries. However, since toxocariasis remains a pressing public health problem in developing countries, serological epidemiological surveys are essential to provide detailed information regarding the prevalence of T. canis infection and its clinical features if the transmission of T. canis infection to humans is to be prevented. Although these diseases can cause a severe vision defect or liver function disorder, there are few reports on the precise relationship beSOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 1400 Vol 36 No. 6 November 2005 tween the seropositive rate and the clinically symptomatic rate. The purpose of the present study was to evaluate the factors contributing to high serological prevalence of toxocariasis and to investigate the relationship between these pathogens and clinical symptoms. We performed a serological survey of junior high school students in two rural districts and in one urban district, and found a very high prevalence rate in the rural district, with most cases being of the covert type. MATERIALS AND METHODS Specific antibody detection Serum specimens were obtained from 158 junior high school students, aged between 10 and 15 years, from three distinct geographical areas in northern Sulawesi, Indonesia. These areas consisted of an urban area of Manado city (district A); an agricultural village (district B), which is located 32 km southwest from Manado city; and a fishing village (district C), which is located 120 km west from Manado city. Serum samples were collected from 41 individuals from district A, 71 individuals from district B, and 46 individuals from district C. Informed consent was obtained before the blood drawing in all cases. The samples were tested in the field by Toxocara-CHECK, a rapid-test kit for Toxocara infection, to screen for the specific anti-Toxocara antibody (Akao et al, 1997; Dubinsky et al, 2000). Each sample was diluted 1:10 and 1:50 in a dilution buffer (0.5% BSA in PBS, pH 7.2) and tested simultaneously. According to the test kit instructions, samples having a positive result at a serum dilution of 1:10 and a negative result at 1:50 were considered to be weakly positive, those having a positive result at 1:50 were considered to be strongly positive, and those having a negative result even at 1:10 were considered to be non-significant for toxocariasis. In addition to the on-site examination, we carried out a plate-ELSA with 158 serum samples from the three districts using the larval excretorysecretory (ES) antigen of T. canis. We objectively assessed the measurement of the optical density (OD) of the results of the plate-ELISA and used this as an index value for comparison with the three control samples: strongly positive, weakly positive, and negative results. We assigned an index value of 10 to the strongly positive control serum, 3 to the weakly positive control serum, and 1 to the negative control serum. Samples registering above 4 on the index value were considered to be positive. Questionnaire survey All subjects were given a questionnaire about their lifestyle, water supply, eating habits, method of defecation, feeding of animals, and so forth. To evaluate the clinical symptoms caused by VML or OLM, questions about the condition of the skin rash and visual trouble were also included. Ophthalmoscopic examination The ocular fundi of 34 subjects that had a high antibody titer were carefully observed using a portable ophthalmoscope (Hand-Held Fundus Camera GENESIS, Kowa Corporation, Japan) after the pupils were dilated with tropicamide (one drop for each eye; Mydrin-P, Santen Pharmaceutical, Japan). The images of the fundi were recorded photographically. No complications were reported after the examinations. Eosinophil counts and assessment of liver function Eosinophil counts were performed using a thin smear stained with May-Grunwald-Giemsa staining for 34 students having a high antibody titer. To evaluate the liver functional disorder caused by VLM, we measured serum asparatate aminotransferase (AST), alanine aminotransferase (ALT), and gamma-glutamyltranspeptidase (gamma-GTP) in 125 students from the three districts. An eosinophil rate of over 6% was considered to indicate hypereosinophilia; while an AST and ALT over 40 IU/l, and a gamma-GTP over 50 IU/l were considered to indicate a significant abnormality. Dot-ELISA for detecting the antibodies for pathogenic parasitic diseases For 71 students in district B, we also conducted a screening test by dot-ELISA to detect pathogenic parasitic diseases such as those caused by Entamoeba histolytica, Fasciola hepatica, Paragonimus miyazaki, Ascaris suum, ASYMPTOMATIC TOXOCARA INFECTION IN MANADO Vol 36 No. 6 November 2005 1401 Dirof i lar ia immit is, Anisakis s implex, and Spirometra erinacei. All but A. simplex and S. erinacei were derived from adult-stage antigens.