CEF6

A serological assay was developed to assess the outcome of the treatment of intestinal schistosomiasis with praziquantel, in patients with Schistosoma mansoni infection. Each of 33 patients (seven found to be excreting S. mansoni eggs and 26 egg-negatives found seropositive for antibodies against antigens from S. mansoni eggs) had two to five serum specimens assayed, the sera being collected at the time of diagnosis and at least once after praziquantel treatment. The sera were tested in ELISA against three antigen preparations: the unfractionated soluble egg antigens of S. margrebowiei and S. mansoni (SmSEA) and a cationic antigen fraction (CEF6) purified from the SmSEA. The dynamics of the post-treatment antibody levels were variable. In a minority of the patients, antibody levels declined relatively rapidly (within 5-12 months), to ELISA negativity, with the levels of the anti-CEF6 antibodies declining more rapidly than those of the anti-SmSEA antibodies. In the remaining patients, however, the levels of these specific antibodies declined only slowly or not at all over a 2- to 3-year period. The post-treatment monitoring of the levels of anti-schistosome-egg antibodies, particularly those of anti-CEF6 antibodies, may help to distinguish the treatments that result in parasitological cure from those that are only partially successful.

Aims:To find out whether serology can reliably speciate human schistosomiasis using a simple enzyme linked immunosorbent assay (ELISA) technique.Methods:Stored sera from 66 patients with microscopically confirmed schistosomiasis were subjected to ELISA using a panel of three antigens, namely: unfractionated Schistosoma mansoni soluble egg antigen (SEA); CEF6, a cationic fraction of SEA; and crude S margrebowiei egg antigen, prepared from an animal schistosome closely related to S haematobium.Results:The optical densities (ODs) obtained using CEF6 as antigen were significantly higher in sera from S mansoni infected patients than in sera from S haematobium infected patients (median OD, 0.810 v 0.595). Using S margrebowiei egg antigen, the optical densities were significantly higher in S haematobium sera than in S mansoni sera (median OD, 0.794 v 0.544). There was no significant difference in optical densities between S mansoni and S haematobium sera using SEA (median OD, 0.725 v 0.737). The ratio of ODs (CEF6 to S margrebowiei egg antigen) was calculated: a ratio of >1 indicated S mansoni infection (sensitivity, 88%) and a ratio of <1 indicated S haematobium infection (sensitivity, 84%). The odds ratio for S haematobium having an OD ratio of <1 was 36.8 (95% confidence interval, 7.0 to 194).Conclusions:The identity of the infecting species of schistosome can be determined using the panel of antigens described. SEA should be used to screen serum samples, and the CEF6 : S margrebowiei egg antigen ELISA optical density ratio can be used where serological speciation is required.

Circulating IgG antibody reactivity and excreted egg counts were investigated in 489 Kenyans given chemotherapy for schistosomiasis mansoni. Antibody reactivity was measured in ELISA, using either unfractionated aqueous soluble constituents of Schistosoma mansoni eggs (SEA) or CEF6 (a soluble fraction of S. mansoni eggs containing two cationic antigens) as the antigen source. Antibody reactivity for each antigen source was strongly associated with egg counts, both pre- and post-treatment. Approximately 6 months after chemotherapy, egg counts were zero in 84% of the subjects. The mean optical densities (OD) measured in the post-treatment ELISA were 60% (CEF6) or 45% (SEA) lower than the pre-treatment values, the reduction in the OD with CEF6 as antigen source being significantly greater than that observed with SEA (P <0.001). The usefulness of an assay for antibody reactivity in monitoring the effects of the treatment of schistosomiasis is discussed.