Effects of tonicity, digestive enzymes and bile salts, and nutrient media on the survival of excysted metacercariae of Echinostoma caproni

The effects of tonicity, digestive enzymes and bile salts, and various nutrients added to Locke's solution were studied on the chemically excysted metacercariae of Echinostoma caproni. Metacercariae were maintained at 37.5degreesC in multiwell chambers, ten per 0.5 ml of test solution; each experiment was replicated five times. Most metacercariae maintained in deionized water or Locke's 2x solution were dead within 2 h. About 85% and 55% of the metacercariae were alive at 8 h in Locke's 1x and Locke's 0.5x, respectively. Metacercariae of this species are osmoconformers, as is the case for adult digeneans. All metacercariae were dead in an acid saline or acid pepsin medium by 2 h. About 50% of the metacercariae were alive in an alkaline trypsin-bile salts medium at 4 h. These results suggest that the acidic pepsin environment in the stomach of a definitive host would be detrimental to the survival of excysted metacercariae, but prolonged survival in alkaline trypsin-bile salts would facilitate establishment of this larval stage in the mucosa of the host small intestine. Studies on excysted metacercariae in Locke's 1x supplemented with various nutrients showed that optimal survival occurred in Locke's plus 0.1% glucose and in Locke's 1x plus 1% hen's egg yolk. Significant survival of excysted metacercariae in Locke's 1x supplemented with either 0.1% proline, 0.1% threonine, or 0.1% serine did not occur.