Observation of an Intraoval Precipitin (IOP) Reaction in Schistosoma mansoni Eggs

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In performing the circumoval precipitin (COP) test using Schistosoma mansoni eggs freshly isolated from mouse tissues for the serodiagnosis of S. mansoni, we observed intraoval precipitin (IOP) reactions in addition to the classical COP formations as described by Oliver-Gonzalez (1954, J. Inf. Dis. 95: 86-91). Typically, the IOP reaction consisted of bleb formation within the fresh eggs (Fig. 1). These intraoval reactions, which were similar to those recently described (Kamiya and Kamiya, 1980, Jap. J. Vet. Res. 28: 155-160; Kamiya, 1981, Jap. J. Parasitol. 12: 161-165) with S. japonicum eggs in tissue sections overlaid with species-specific antisera, may form following the rapid entrance of specific antibody into the egg shell producing antigen-antibody complexes manifest as minute blebs. To evaluate this observation, serum from eight parasitologically defined patients with S. mansoni were pooled and compared with pooled sera from 10 parasitologically negative controls. Classical COP reactions were recorded in 37.0 ? 12.7% of eggs in 10 tests conducted on the parasitologically positive sera and in none of the eggs in 10 tests run in parallel on the parasitologically negative control sera. However, IOP reactions were also seen in 8.7 ? 4.2% of eggs with the 10 parasitologically positive sera. No IOP reactions were noted with the 10 parasitologically negative control sera. When S. mansoni miracidia from freshlyhatched eggs were incubated with pooled human antisera at 37 C for 48 hr, precipitates developed on the surface of the miracidia (Fig. 2). When miracidia were permitted to "bathe" themselves by swimming about in water at 20 C for 2 to 4 hr before incubation with antisera at 37 C for 48 hr, precipitate formation was much diminished or absent suggesting that the circum-miracidial reaction may have involved a diffusible antigen. Slight, bleblike precipitates also could be seen on the surface of glass slides when drops of hatch