West Greenland harbour porpoises assayed for antibodies against Toxoplasma gondii: false positives with the direct agglutination method

Abstract

We assayed blood/tissue fluid samples from 20 harbour porpoises Phocoena phocoena from western Greenland coastal waters for antibodies against the protozoan parasite Toxoplasma gondii by the direct agglutination test (DAT). Nine individuals (45%) were interpreted to be seropositive at 1:40 dilution and 4 (20%) were seropositive up to 1:160. Samples from these individuals were assayed by an enzyme-linked immunosorbent assay (ELISA), and tissue samples of the DAT-positive animals were tested by a nested polymerase chain reaction (nPCR). Results from both methods were negative, suggesting the absence of infection in the tested animals. After chloroform clean-up, all were negative when re-assayed by DAT. We concluded that infection with T. gondii was absent in all 20 animals, despite the initially positive DAT results, and that the false positives resulted from non-specific adherence to tachyzoites in the DAT assay which could be removed by the chloroform clean-up method. Our results suggest that detecting antibodies against T. gondii using the DAT or the modified agglutination technique, particularly on samples from Arctic marine animals which often are rich in lipids, may lead to false positive results. For such samples, the use of ELISA or PCR on available tissue samples may be advocated as confirmatory tests in order to avoid false positives and overestimating seroprevalence.