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The Detection of Antibodies to Infections with the Nematode, Toxocara Canis, a Causative Agent of Visceral Larva Migrans

The results of these experiments, in general, reveal that specific antibodies are produced following active infection or artificial immunization with Toxocara canis. These antibodies are detectable in a reproducible manner by the flocculation, and in a less consistent manner by the complement fixation and the precipitin tests, with the use of an acid-soluble protein fraction of adult Toxocara worms as the source of antigen. Similar results were obtained with the precipitin test in which crude extracts of the adult worms were used as antigen. Indirect additional evidence in favor of a serological response to this infection was obtained by observing a marked hypergammaglobulinemia in animals infected with Toxocara.

Experiments in rabbits indicated that the flocculation test with a purified antigen can detect antibodies at a relatively early date after exposure. A sufficiently high level of reactivity could be detected for several months after infection. Similar results with the flocculation test could be obtained when the same sera were repeatedly tested at different times with the same antigen. However, when different lots of antigen were employed, quantitative differences in the results of the test were observed. The results of flocculation tests with sera from human beings and animals infected or immunized with different viruses, rickettsiae, bacteria, protozoa and helminths failed to indicate any extensive degree of cross reaction with any of these organisms with the exception of Ascaris lumbricoides. This observed cross reaction may not always be present, since a relatively low percentage of positives was obtained with sera from Korean prisoners of war, most of whom were infected with A. lumbricoides, and from wild animals trapped in south Georgia, many of which were infected with large intestinal roundworms.

In spite of the encouraging and suggestive results obtained, sufficient information is not yet available regarding the sensitivity and specificity of these tests to justify their use for the laboratory diagnosis of visceral larva migrans. In fact, in view of occasional inconsistent results obtained when different lots of antigens were employed and of the frequency of non-specific reactions at serum concentrations of 1:5 or greater, serological tests for visceral larva migrans must still be regarded to be in the experimental stage.