Establishing and evaluation of a polymerase chain reaction for the detection of Echinococcus multilocularis in human tissue

Abstract

Alveolar echinococcosis (AE) is caused by metacestode larva of the tapeworm Echinococcus multilocularis. AE diagnostics currently rely on imaging techniques supported by serology, but unequivocal detection of AE is difficult. Although polymerase chain reaction (PCR)-based methods to detect tapeworm DNA in biopsies have been suggested for several species, no validated protocol adhering to accepted guidelines has so far been presented for AE diagnostics. We herein established a PCR protocol for metacestode biopsies and technically evaluated the method using isolated parasite DNA and cells, biopsies of clinically relevant material, and formalin fixed paraffin-embedded (FFPE) human tissue blocks. We compared the results with an immunochemical (IHC) approach using the monoclonal antibody Em2G11 specific for the antigen Em2 of E. mulitlocularis. Based on tapeworm 12S rDNA sequences we established and validated a PCR protocol for robust detection of as little as 50 parasite cells per specimen and report 127 cases of positive identification of Echinococcus species in samples from humans and animals. For further validation, we analyzed 45 liver, heart, brain, and soft tissue samples as well as cytological probes of aspirates of FFPE-material from 18 patients with clinically confirmed AE. Of each patient we analyzed (i) fully viable lesions with laminated layer; (ii) tissue with mAbEm2G11-positive small particles of E. multilocularis (spems); (iii) mAbEm2G11-negative tissue adjacent to the main lesion; and (iv) lymph node tissue with mAbEm2G11-positive spems. To identify the areas for the PCR-based approach, we performed IHC-staining with the monoclonal antibody Em2G11. Micro-dissected tissue of these areas was then used for PCR-analysis. 9 of 15 analyzed samples with viable E. multilocularis lesions with laminated layer were positive by PCR. Of this group, all samples preserved for less than 6 years (6/6) were tested positive. 11 of 15 samples of spems and 7 of 9 samples of the control group mAbEm2G11-negative tissue were negative by PCR. We further show that all probes from lymph nodes with spems are PCR negative. We present a sensitive PCR method for the detection of E. multilocularis in human tissue, particularly in fresh biopsy material and tissue blocks stored for less than 5 years. While the diagnostic sensitivity of material containing only spems was higher using IHC, PCR detection was possible in IHC negative liver tissue and in patients with negative serology. Our results support the view that spems do not contain parasitic DNA or viable cells of the parasite. spems thus most probably do not directly contribute to metastasis formation during AE. Alveolar echinococcosis (AE) is a potentially lethal zoonosis during which metacestode larval tissue of the tapeworm Echinococcus multilocularis grows like a malignant tumor, infiltrating the human liver. Current AE diagnosis, which mainly relies on imaging techniques (ultrasound, computed tomography, magnetic resonance tomography) supported by serology, is not always suitable to unequivocally identify an infection. In this work the authors present a PCR-based approach to detect the parasite in biopsy material taken from patients. The method was first validated for routine laboratory use employing isolated parasite DNA as well as defined numbers of parasite cells in clinically relevant settings. The method was then verified under clinically relevant settings using pathology samples from patients with defined AE. Particularly in cases when these samples had been stored for less than 6 years, the PCR methodology was highly suitable to identify the infection, even when serology was negative. The authors show that small particles of E. multilocularis (spems), containing surface material of the parasite and often located in the draining adjacent lymph nodes, are PCR negative and, thus, most probably do not contain viable parasite cells. The authors conclude that in cases of doubtful serology and imaging, a combination of immunochemical staining methods with PCR are reliable means to unequivocally detect the infection.