The reactivity of sera from 151 confirmed human cases of
Capillaria philippinensis infection was examined by double diffusion and indirect hemagglutination (IHA) tests chiefly against Capillaria obsignata
antigen because of the present unavailability of C. philippinensis
antigen.
Antigens from additional parasites and other human and animal sera representing a variety of helminthic
infections were used for comparison. Of 71 pre-treatment human sera, 56.3% were reactive by double diffusion test and 85.9% by IHA test (titer greater than 1:16) with C. obsignata
antigen. C. philippinensis sera were also reactive with Trichinella spiralis and Trichuris vulpis
antigens but not with Schistosoma japonicum
antigens. Sera from other
infections such as with T. spiralis, T. vulpis, and S. japonicum were also reactive with C. obsignata
antigen but sera from
Trichuris trichiura infection were not. With C. obsignata
antigen, IHA titers in human C. philippinensis sera are apparently not related to clinical severity of the disease; the titers remain at fairly stable levels during the course of the illness but may tend to decrease after
chemotherapy. The cross-reactivities observed dictate caution in the use and interpretation of any serologic procedure for human
intestinal capillariasis; nevertheless, the IHA test using C. obsignata
antigen may be a useful addition tool in the study of C. philippinensis
infection both for clinical and epidemiologic purposes especially when the efficiency of stool examination is decreased by changes in the reproductive activity of the helminth.