Antibodies to cysts of Giardia lamblia in primary giardiasis and in the absence of giardiasis

The significance of serum antibodies binding to cysts of Giardia lamblia was evaluated by indirect immunofluorescence. Titers of 10 or higher were found in 85.6% of adults, who had probably never had giardiasis. Titers in 118 adults (geometric mean, 29.9) were higher than in 35 children (16.4), and titers in women (42.3) were higher than in men (20.0). Titers in 150 patients with primary giardiasis (80.4) were higher than in control adults, but overlapping precluded serological diagnosis. Titers increased with the duration of infection. Female patients had higher titers (100.5) than did males (66.1), but men who had been infected longer than 45 days had high titers (132). Titers in second serum samples taken from 26 patients 2 weeks to 3 months after successful nitroimidazole treatment (58.1) were lower than before treatment (151.7) but higher than in 118 controls. We conclude that most people have antibodies which cross-react with G. lamblia but which are induced by other immunogens and that primary giardiasis induces protracted systemic antibody responses.