Toxocariasis is a helminth zoonosis caused by
infection with the larvae of Toxocara spp. ascarid worms. Only two species, Toxocara canis and Toxocara cati, are recognised as causative agents of human disease. The best choice for serodiagnosis of the generalised forms of
toxocariasis,
visceral larva migrans (VLM) or covert
toxocariasis, relies upon the initial use of
TES-ELISA, after which any positive result should subsequently be tested by Western blotting (WB). Covert
toxocariasis is mostly a benign
infection, so a large majority of infected subjects are asymptomatic or have very few symptoms and therefore go undiagnosed. In this form, this helminthosis is often self-limiting, leaving residual specific
antibodies. A positive serodiagnosis caused by residual
antibodies that do not have any diagnostic significance can be associated with any infectious or
non-infectious disease. If separated from the ongoing clinical and laboratory context, such a positive result has no diagnostic value and should be only taken into account after the possible etiologies of any observed syndromes have been ruled out. Unlike the methods used for the immunodiagnosis of bacterial, viral or protozoal (
toxoplasmosis)
infections, it is not possible with
toxocariasis to assess the age of the presence of specific
IgG using the levels of specific
IgM because
IgM antibodies can be found throughout the course of
helminthiasis. The detection of other classes of
immunoglobulins, namely
IgE and
IgA, the subclasses, namely
IgG4 or circulating Ag was proven to be unable to discriminate between active and self-cured generalised toxocaral
infections. Currently, the diagnosis of an active covert
toxocariasis relies upon indirect arguments, e.g., the presence of otherwise unexplained symptoms along with blood
eosinophilia and/or elevated levels of
eosinophil cationic protein (ECP). This situation is far from ideal and more research should be carried out to solve this difficult problem.