Evaluation of the dot enzyme-linked immunosorbent assay in comparison with standard ELISA for the immunodiagnosis of human toxocariasis
William Roldán1 ; William Cornejo; Yrma Espinoza
Instituto de Medicina Tropical " Daniel A. Carrión ", Facultad de Medicina, Universidad Nacional Mayor de San Marcos, Calle José Santos Chocano 199, Callao, Perú
Mem. Inst. Oswaldo Cruz vol.101 no.1 Rio de Janeiro Feb. 2006.
A dot enzyme-linked immunosorbent assay (dot-ELISA) was standardized using excretory-secretory antigens of Toxocara canis for the rapid immunodiagnosis of human toxocariasis. Thirty patients with clinical signs of toxocariasis, 20 cases with other parasitic diseases, and 40 healthy subjects were tested. A total of 0.2 ng of antigen per dot, serum dilution of 1:160 and dilution conjugate of 1:1000 were found optimal. The sensitivity and specificity of the assay were 100 and 95%, respectively. Comparable sensitivity of dot-ELISA and the standard ELISA was obtained, but only 3 cross-reactions occurred in the dot-ELISA, compared with 6 in the standard ELISA. Dot-ELISA is simple to perform, rapid, and low cost. Large-scale screening studies should be done to evaluate its usefulness under field conditions.
Key words: Toxocara canis – toxocariasis – dot enzyme-linked immunosorbent assay – ELISA
Human toxocariasis is a world-wide helminthic zoonosis due to the human infection by larvae of Toxocara canis, the common ascarid of dogs, and also by the cat ascarid T. cati (Schantz & Glickman 1983, Despommier 2003). The incidence of human toxocariasis is unknown because toxocariasis is not a communicable disease in the majority of the countries. However, many cases of this disease have been reported throughout the world (Glickman & Schantz 1981, Despommier 2003).
Humans are infected by ingestion of embryonated T. canis eggs. Children playing in areas contaminated with dog faeces are in higher risk, because of their likelihood of ingesting soil. Prevalence of Toxocara infestation of dogs and the resulting contamination of the ground is relatively high in many countries all over the world. Reported data range from 0 to 93% for dog infestation (Glickman & Schantz 1981) and 15 to 78% for soil contamination (Gillespie 1988, Magnaval et al. 2001). It has been determined that in Peru from 24 to 80% of soil samples in public playgrounds and parks are contaminated with Toxocara eggs (Buitrón 1976, Guerrero 1995, Lescano et al. 1998, Chavez et al. 2000, Dávalos et al. 2000).
Although the clinical features vary, two syndromes are recognized: visceral larva migrans (VLM) and ocular larva migrans (OLM). VLM is usually detected in young children (1 to 5 years of age) with a history of geophagia and/or exposure to puppies. It is a self-limited, rarely lethal disease characterized by fever, cough, wheeze, pallor, malaise, asthma, weight loss, hepatomegaly, and marked eosinophilia (Schantz 1989). OLM occurs unilaterally in children and young adults and cause visual loss, strabismus and, more rarely, eye pain (Shields 1984). Another clinical manifestations of Toxocara infection are " common toxocariasis " in adults and " covert toxocariasis " in children (Schantz 1989, Magnaval et al. 2001).
Diagnosis of toxocariasis is based on clinical and serological data because of the difficulty in detecting larvae from tissues. The test currently employed for the serodiagnosis of toxocariasis is ELISA using excretory-secretory antigens from T. canis second-stage larvae (TES) (De Savigny et al. 1979, Jacquier et al. 1991). However, this technique has some drawbacks, including the need for trained personnel, requirement of special equipment, the lack of reproducible reading due to plate-to-plate variation, and may be troublesome to perform under field conditions. Dot-ELISA test, a modification of the standard ELISA test, offers a simple and less expensive tool for toxocariasis detection. The dot-ELISA has been successfully adapted for the detection of parasitic diseases in humans as leishmaniasis, schistosomiasis, toxoplasmosis, and hydatidosis (Pappas et al. 1984, Boctor et al. 1987, Rogan et al. 1991, Elsaid et al. 1995). A dot-ELISA for diagnosis of human toxocariasis was described (Camargo et al. 1992). The present study was conducted to standardize and evaluate a dot-ELISA to establish the optimal conditions for the detection of IgG antibodies to toxocariasis in comparison to the standard ELISA test.