Serological diagnosis of Echinococcus granulosus infection in sheep using cyst fluid antigen processed by antibody affinity chromatography

SUMMARY Serum antibody responses in sheep naturally or experimentally infected with Echinococcus granulosus and/or other larval cestodes were examined using an enzyme‐linked immunosorbent assay (ELISA) with various antigens prepared from sheep hydatid cyst fluid (SHCF). Serum donors included: sheep experimentally infected with E. granulosus and their age‐matched non‐infected controls; sheep experimentally infected with other helminth parasites; sheep naturally infected with E. granulosus both from Tasmania and the Australian mainland; sheep from Tasmania naturally infected with larval cestodes other than E. granulosus; and naturally reared sheep completely free from infection with larval cestodes. Attempts were made to eliminate serological reactions which were not specific for E. granulosus by using a series of antibody affinity chromatography steps to deplete crude SHCF antigen; these included adsorption with a monoclonal antibody, 3EgH 29–2, removal of host IgG using rabbit anti‐sheep IgG antibody, and removal of antigens which bound non‐specifically to normal sheep immunoglobulin. The final affinity‐depleted antigen product was designated AD SHCF. Specific serological reactivity in infected sheep was very low. Affinity depletion of SHCF using 3EgH 29–2 did not appear to increase the specificity of serological diagnosis of E. granulosus infection when experimentally infected sheep were compared with their non‐infected controls provided the latter were age‐matched with experimental animals. The other affinity adsorption steps significantly reduced non‐specific background binding to antigen by normal sheep serum. Despite this reduction in background in the ELISA, only low levels of antibody could be detected in naturally‐infected sheep. With sheep from Tasmania, diagnosis of infection was possible on a flock basis, but overlap of serological reactions between infected and non‐infected sheep made individual diagnosis unreliable. Heavily infected sheep from mainland Australia were serologically indistinguishable from non‐infected animals. This suggested that different “strains” of E. granulosus may stimulate different types or titres of certain antibodies in the host even though overall titres of antibody are low. The low levels of specific antibody in serums from infected sheep compared with sera from humans with hydatid disease suggests a fundamental difference in the immunological relationship between E. granulosus and these 2 host species.