To determine epidemiological and clinical associations with Toxocara canis seropositivity, we studied 333 (87%) children of a cohort of 383 five- to seven-year-olds. The prevalence of seropositivity (antibody titer to T canis, greater than or equal to 1:32) was 23.1%. Black children were more frequently seropositive than were white children, as were children of parents who did not graduate from high school. In a sample of seropositive and seronegative children, seropositivity was associated with both a history of pica and puppy ownership, but not with a greater frequency of symptoms and signs that occur in visceral larva migrans or with poor growth. No child had evidence of ocular toxocariasis on retinal examination. For the whole sample, poor reading achievement, marked distractibility, and lower intelligence were associated with seropositivity, but by using multiple regression analysis, we found that these associations may be attributable to confounding variables.
In an attempt to standardize the procedure for the enzyme-linked immunospecific assay, several parameters were examined. It was determined that horseradish peroxidase was the enzyme of choice and that several brands of polystyrene tubes and plates could be used. The test was considerably shortened by omitting long incubation steps previously used for eliminating background fixation of conjugate. The reproducibility of the procedure proved to be excellent, but reactivity curves indicated that use of a single dilution is not adequate for quantitative tests. A "standard" procedure was proposed.
The effects of age and sex of the cat on oocyst shedding, multiplication of Toxoplasma gondii in tissues of cats, and acquisition of immunity were investigated after oral inoculation of cats with Toxoplasma cysts. Twenty-five cats varying in age from 1 week to 39 months were killed 7-97 days after inoculation with T. gondii. Homogenates of brain, heart, mesenteric lymph nodes, retina, and blood from these cats were inoculated into mice to test for Toxoplasma infectivity. Toxoplasma was isolated more frequently and in higher titers in mice receiving inocula from cats of the youngest age group (1 week old). Toxoplasma gondii was isolated from tissues of only 2 of 21 cats older than 2 months (at the time of inoculation), although all of the animals shed oocysts within 1 week after ingesting the parasites. The number of oocysts shed varied among littermates of the same sex and between sexes. Generally, cats younger than 12 months shed more oocysts than older cats. The number of oocysts shed by older cats varied considerably; males generally shed more oocysts than the females. However, the numbers of cats examined were too small for statistical comparison. Nevertheless, the observations suggest that cats older than 12 months should not be used in experiments where numbers of oocysts shed is critical.
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