Food contamination by Echinococcus and other taeniid species: Typically low egg numbers, as estimated by digital PCR

Echinococcus multilocularis and Echinococcus granulosus sensu lato are the causative agents of two major zoonotic diseases, alveolar echinococcosis and cystic echinococcosis, respectively, and are ranked among the four most important foodborne parasites worldwide. Although interest in food contamination by taeniid eggs is increasing, the estimation of the number of eggs present in food items is still lacking. Using E. multilocularis eggs isolated from fecal samples of experimentally infected red foxes, an average of 5261 (±1253) mitochondrial DNA copies per egg was estimated by digital PCR (dPCR). Based on this estimation, 47 taeniid DNA samples previously obtained from different food items and identified as positive for E. multilocularis, E. granulosus sensu stricto or Taenia species, were submitted to dPCR to estimate the number of eggs present. In 95.7% of the samples, the contamination was estimated to be caused by one to five eggs, with only one egg in 83.0% of the samples. As detection of such low-level contamination requires sensitive detection methods, a performance comparison between microscopy and molecular methods was conducted using 15 pellets obtained from lettuce wash residues spiked with 0 to 63 E. multilocularis eggs. Two operators performed blinded microscopy, followed by DNA extraction from the examined pellets for real-time PCR detection and then an estimation of the number of eggs by dPCR. A higher sensitivity was obtained with real-time PCR (88.5%) compared to microscopy (40%) in samples containing one to five eggs. The correlation between the number of eggs spiked into the samples and the estimated egg number was strong using dPCR (r = 0.99, p < 0.01), while only moderate with microscopy (r = 0.67, p < 0.05). Estimation of the number of taeniid eggs in food samples by dPCR provides the first quantitative basis for exposure assessment and is important for evaluating the risk of human foodborne infections, with future work needed to address egg viability.