Toxocariasis: Serological Diagnosis by Indirect Antibody Competition Elisa

Abstract

Toxocariasis is caused by infection of man by Toxocara canis and Toxocara cati larvae, the common roundworm of dogs and cats. Because larvae are difficult to detect in tissues, diagnosis is mostly based on serology. Non specific reactions are observed mainly due to cross-reactivity with Ascaris sp antigens. This investigation aimed at developing and evaluating an indirect antibody competition ELISA (IACE) employing a specific rabbit IgG anti-Toxocara canis excretory-secretory antigens as the competition antibody, in order to improve indirect ELISA specificity performed for toxocariasis diagnosis. For that, the rabbit IgG was previously absorbed by Ascaris suum adult antigens. Sensitivity and specificity of IACE were first evaluated in 28 serum samples of mice experimentally infected with T. canis embryonated eggs. Adopting cut-off value established in this population before infection, sensitivity and specificity were 100% after 20 days post-inoculation. For human population IACE was evaluated using sera from 440 patients with clinical signs of toxocariasis and the cut-off value was established with 60 serum samples from apparently healthy individuals. Using as reference test the indirect ELISA performed by Adolfo Lutz Institute, sensitivity was 60.2%, specificity was 98% and concordance was 77.3%. Repeatability of IACE was evaluated by the inter-reactions variation coefficient (2.4%).

Evaluation of an enzyme-linked immunoelectrotransfer blot test for the confirmatory serodiagnosis of human toxocariasis

“…However, some components of TES antigens are not specific, demonstrating moderate cross-reactivity with other helminth species (Speiser & Gottstein 1984, Lynch et al 1988, Jacquier et al 1991, Nunes et al 1997. Sera from patients with ascariasis, filariasis, schistosomiasis and strongyloidiasis also react with TES antigens (Maizels et al 1984, Nunes et al 1999, Yamasaki et al 2000. In northern countries, the TES antigens are more specific for serodiagnosis of toxocariasis, but the level of specificity is unsatisfactory in countries where co-infections with other helminths are frequent.…”

Freqüência de soropositividade para antígenos de Toxocara canis em crianças de classes sociais diferentes

“…A possibilidade de reação cruzada com antígenos de outros parasitas -causa de erros diagnósticos conhecida -, foi descartada, no presente estudo, pela absorção preliminar das amostras sorológicas com antígenos do A. suum, nematóide relacionado antigenicamente com o Ascaris lumbricoides, que é um parasita intestinal comum em crianças, e cuja infestação poderia falsear o teste de ELISA para identificação de anticorpos contra o T. canis 15 . Embora a amostra populacional estudada não permita definir a prevalência da toxocaríase para a população de Brasília, os resultados apontam para provável relevância desta zoonose no Distrito Federal.…”

Field evaluation of a rapid diagnostic test to detect antibodies in human toxocariasis

“…The enzyme-linked immunosorbent assay (ELISA) using T. canis excretory-secretory (TES) antigens harvested from T. canis larvae is currently the serological test for confirming toxocariasis (Magnaval et al, 2001) but some cross reactions have been reported with other nematodes (Kennedy et al, 1987(Kennedy et al, , 1989Yamasaki et al, 2000). Other toxocariasis diagnostic methods that were developed based on TES to improve sensitivity and specificity included the indirect antibody competition ELISA (Nunes et al, 1999), dot-ELISA (Carmago et al, 1992), and antigen capture assays using monoclonal antibodies (Gillespie et al, 1993;Ngah et al, 1999). An alternative to producing large amounts of Toxocara TES and improved diagnosis for toxocariasis is through the development of highly specific recombinant antigens which share important antigenic and immunogenic structures with native TES.…”