SKIN REACTIONS TO TOXOPLASMIC ANTIGEN IN PEOPLE OF DIFFERENT AGES WITHOUT KNOWN HISTORY OF INFECTION

Abstract

One hundred forty-two residents of the Cincinnati area ranging in age from 3 months to 81 years were surveyed for their reaction to the intracutaneous injection of toxoplasmic antigen prepared from infected chorioallantoic membranes of chicken eggs. The skin test antigen is also present in abundance in mouse peritoneal exudate freed of Toxoplasma by centrifugation and its concentration appears to be proportional to that of the complement-fixing antigen. The positive reactions, consisting of red, indurated, swollen, tender and itching lesions ranging in size, from 60 x 50 mm. to 10 x 10 mm. (the latter being relatively rare), reach their maximum in 30 to 48 hours. The incidence of positive reactions among at least 20 individuals in each age group listed was as follows: 0 to 4 years—0; 5 to 9 years—5%; 10 to 19 years—14%; 20 to 29 years—20%; 30 to 39 years—50%; 40 to 49 years—65%; 50 to 81 years—65%. A positive skin test was invariably associated with demonstrable toxoplasmic antibody in the serum. Toxoplasmic antibody in varying titer was also found in the serum of 7 of 18 persons (39%) with negative skin tests. Further evidence in favor of the high incidence of toxoplasmic antibody among the "normal" adult population was obtained when a 15 to 20 fold concentrated, commercial preparation of gamma globulin prepared from the plasma of more than 20,000 American adults was found to contain toxoplasmic antibody in a titer of 1:256 as determined by the dye test. Since no agent having an antigen in common with Toxoplasma and capable of giving rise to antibodies for this protozoon parasite has been found thus far, one must consider the possibility that inapparent infection with Toxoplasma may be widespread, although it is rather curious that the incidence of antibodies (and presumably infection) should rise so sharply after the 20th year of life. The toxoplasmic skin test is not useful for diagnosis because a strongly positive test bears no relationship to titer of antibody and a negative test is encountered too often in the presence of antibody.