Serodiagnosis of alveolar echinococcosis: detection of antibody against EM18 in patients and rodents.

An international collaborative study on echinococcosis has been carried out for the establishment of a simple means for differential serodiagnosis of alveolar echinococcosis (AE) from other parasitic diseases including cystic echinococcosis (CE). The main candidate epitope is Em18 (previously undescribed epitope of a low molecular weight protein of 18.5 kDa). Evaluation of the usefulness of Em18 is introduced in this review paper. Serum samples showing antibody response against Em18 are exclusively from AE. The predominant IgG subclass recognizing Em18 is IgG4 or IgG1 or IgG4 + IgG1 but never IgG2. There are good correlations between (1) the antibody response against Em18 and the presence of active lesions and (2) the antibody response against Em18 and the Em2-ELISA values. Em18 is, therefore, expected to be reasonably reliable and useful for differentiation of active AE from inactive AE. A new ELISA system using a partially purified Em18 enriched fraction (PP-Em 18/16-ELISA) has been evaluated for serodiagnosis of AE compared with Em2plus-ELISA. A total of 194 serum samples were examined: 127 sera from AE (79) and CE (48) in China where both AE and CE are endemic, 21 sera from CE in Australia where CE only exists, 28 sera from cysticercosis (21), paragonimiasis (5) or sparganasis (2) in Korea where no indigenous AE nor CE exists and 11 normal sera. Antibody levels by PP-Em18/16-ELISA were much higher in AE than in CE and it was also true for commercially available Em2plus-ELISA. Some of CE from China showed exceptionally higher levels of antibody in comparison with those of CE from Australia. It is suggested that these strongly positive cases of CE from China may have been exposed to both species of Echinococcus. Although most of sera from paragonimiasis showed high antibody levels by Emplus-ELISA, they were negative by PP-Em18/16-ELISA. Therefore, PP-Em18/16-ELISA is expected to be more reliable for differentiation of AE from CE and others especially in Asian countries where paragonimiasis is still not rare. Antibody responses in rodents naturally infected with E. multilocularis: Serum samples from the wild vole, Clethrionomys rufocanus bedfordiae, infected with E. multilocularis showed similar antibody responses as in AE patients, whereas those from Norway rats, Rattus norvegicus, showed almost none. The latter rodents were simultaneously infected with Taenia taeniaeformis but showed no antibody response against T. taeniaeformis either. Therefore, we speculate that Norway rats may only be infected with E. multilocularis under some immunosuppressed conditions or genetic unresponsiveness. It is stressed that Em18 is highly specific to E. multilocularis, and antibody response against Em18 is reasonably reliable for differentiation of AE from other helminthic infections by Western blot and ELISA in humans and may be useful for detection of domestic animals contaminated with E. multilocularis in the endemic area.