Laboratory Diagnosis In Humans

Due to the fact that Toxocara larvae remain arrested in the tissues and do not develop to adulthood in the intestine, neither parasites nor their products are detected in the faeces. Diagnosis usually depends upon indirect measures, including the detection of Toxocara-specific antibodies in serum. A variety of parasite antigens are used, which may vary in their specificity. Additional information may include the presence of certain clinical symptoms and signs and knowledge of risk factors. Larvae can be detected in biopsy materials and have been found at post-mortem examination, but in general, biopsy is regarded as unrewarding (due to the small numbers of larvae present and the difficulty in finding them). Also an appropriate specimen is rarely available (Glickman et al., 1986). Symptoms and signs can also be nonspecific, which is why an accompanying serological test is recommended. Such an immunodiagnostic test is required to be highly sensitive and specific, capable of distinguishing T. canis infection from other parasites (Glickman et al., 1986).

Transient Myositis

Transient myositis has been reported in children, with isolated swelling of the calf or lower part of the leg (Taylor et al., 1988; Walsh et al., 1988). In two cases the symptoms resolved within 72 hours, while in the third resolution was much slower.

Serological Tests

There are many serological tests available for diagnosing toxocariasis. These have been summarised in the review by Glickman et al. (1986) and include skin tests, complement fixation, bentonite flocculation, larval precipitation tests, gel diffusion, capillary tube precipitation tests, indirect haemagglutination, direct or indirect immunofluorescence, ELISA and radioimmuno-assay. Antigens used in the tests have included both somatic extracts of adult and larval parasites. Tests using antigens derived from adult worms have been shown to lack specificity (Glickman et al., 1978), whereas antigens derived from eggs require sera to be pre-absorbed with Ascaris antigens to retain specificity (Glickman et al., 1985). At present, the test that is now widely recommended is the ELISA, utilising Toxocara larval ES antigens (TES) (de Savigny et al., 1979). These so-called ES antigens are excretory-secretory antigens derived from in vitro culture of infective Toxocara larvae (de Savigny, 1975).

New Approaches

More recently, a variety of new approaches to diagnosis of toxocariasis in humans have been reported, including the use of IgE-specific methods and antigen capture ELISA. Magnaval et al. (1992a) developed an immunoenzymatic assay with ES antigen of Toxocara to detect specific immunoglobulin E (sIgE ELISA). The value of this assay for post treatment follow-up and its specificity and sensitivity characteristics were evaluated. The authors concluded that, due to only moderate specificity and sensitivity, the test could not be used alone but could act as a complementary method for the detection of specific IgG. Furthermore, it was the only assay to detect positivity in sera from hypereosinophilic patients and it revealed reductions in sIgE post-treatment, and so had some value as a follow-up assessment after treatment.

An antigen capture ELISA that can detect a carbohydrate epitope on the excretory-secretory (ES) antigens of Toxocara was evaluated by Gillespie et al. (1993b). The sera from patients with acute visceral larva migrans, ocular disease and inactive toxocariasis were assessed, along with healthy controls and patients with other helminth infections. Over half the patients with acute toxocariasis tested positive, in contrast to low numbers from the inactive disease or ocular complications. False positives were, however, detected in 25% of the patients with schisto-somiasis and filariasis. For this reason, the authors concluded that this assay was useful for case confirmation only.

PCR-based methods were used to detect ascarid larvae from animal tissues including cats, dogs and foxes, and species differentiation between T. canis, T. cati and Toxascaris leonina was possible (Jacobs et al., 1997). These methods may prove to be good candidates for further development for the detection and/or identification of ascarid larvae in human tissues.

Ocular Disease

A definitive diagnosis of ocular larva migrans can be obtained by histological detection of a larva, but suitable specimens are rarely available. Whilst the concentration of Toxocara antibody in serum is usually raised in ocular disease, the concentrations are generally lower than in visceral disease (Glickman and Schantz, 1981). There are therefore difficulties in diagnosis associated with low levels of IgG antibodies. Some other workers have advocated the use of IgE for the serological diagnosis of ocular disease, since it is thought that ocular disease is caused by a lower number of infective larvae compared to visceral larva migrans, and that the smaller amounts of circulating antigen may stimulate the production of IgE rather than IgG (Genchi et al., 1986).

Immunological reactions in aqueous and vitreous humour may be a more reliable indicator of toxocaral eye disease but such measurements are made only infrequently. Petithory et al. (1993) reported a comparison of sera and vitreous humour antibody studies in 10 subjects. In eight patients sera were negative for T. canis antibody, while vitreous humour antibody was found in nine subjects. Furthermore, for six (out of a total of nine) patients, antibody to T. cati was detected in the vitreous humour. Petithory et al. (1987) have suggested the following criteria for the diagnosis of ocular larva migrans: (a) positive immunologic tests for nematode antigens in aqueous or vitreous humour; (b) eosinophilia of aqueous or vitreous humour (c) ocular lesions. However, few ophthalmologists have aqueous or vitreous humour material available to them, and in most cases the diagnosis is based on the appearance of the ocular lesions, a supportive history and exclusion of other likely causes.

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