Immunodiagnosis

Abstract

Immunodiagnosis is an indirect way of diagnosing the disease by detecting not the parasitic stages but footprints of the infection in the form of presence of antibodies or antigens either in the host or in its excretions. Since this is a trial to find an indirect evidence of the infection, it is never considered superior to parasitological diagnostic methods, which are still considered “golden diagnostic tests,” but are preferred due to higher sensitivity. Antibodies are considered specific to their specific antigens; in this case for schistosomes or their eggs, hence theoretically detection of antibodies should confirm presence of schistosome infection. Yet this is not the case in practice as serum, collected for detecting schistosome antibodies, contains other antibodies as well which at times cross-react with schistosome antigens during the test, jeopardizing specificity of immunodiagnostic tests. Linked with this is the question of aim of application of these immunological tests in animal schistosomiasis—whether positive cases should be treated against schistosomiasis and whether positive cases indicate excretion of schistosome eggs in host’s excreta. Ironically, we cannot claim either of these as antibody-dependent tests do not differentiate between present and past infections—either schistosomiasis may still be persisting or positiveness is witnessing only past history of the infection. Even titers of the immunodiagnostic tests do not indicate parasite status in the host. In contrast, immunodiagnosis has played a pivotal role in viral and bacterial diseases, owing to its ability to reflect immune status of the host and for undertaking vaccination program with available vaccines. There is no vaccine which may be used against schistosomiasis or any other helminthic infection (lung worm vaccine production by Indian Veterinary Research Institute (IVRI) has been discontinued). Then, how can the detection of these serologically positive cases help in controlling these parasitic infections? Are these immunodiagnostic tests a flop show for parasitic infections, under the present scenario? In our opinion, the present information on serologically positive cases is merely indicative of prevalence rate of schistosomiasis in particular area and may be used in epidemiological work. To use it in treatment or control in the future, there is a need of accumulating more knowledge on the subject, and the data will be of great value only after using a vaccine against schistosomiasis in the field.