This study determined the in vitro effects on the viability of internal parasites of grazing undrenched weaner deer on either chicory (Cichorium intybus) or perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pasture. One experiment investigated the hatching and development of gastrointestinal nematode eggs and larvae, and the development and motility of Li lungworm (Dictyocaulus eckerti) larvae, and a second experiment used larval migration inhibition assays to test the viability of LI lungworm larvae extracted from the faeces of weaner deer grazed on either chicory or pasture when they were incubated with rumen and abomasal fluids from fistulated deer also grazing on chicory or pasture. The incubations were undertaken with and without added condensed tannins purified from chicory and with or without polyethylene glycol (PEG) to bind the tannins. Chicory had no effect on the hatching and development of gastrointestinal nematode eggs and larvae. Grazing chicory reduced the number of lungworm larvae developing to the L3 stage, and Li lungworm larvae from the faeces of chicory-grazed deer were less viable in rumen and abomasal fluid than larvae from pasture-grazed animals. Abomasal fluid was significantly (P<0.001) less inhibitory to the migration of Li lungworms than rumen fluid. When the larvae were incubated in rumen and abomasal fluids from chicory-grazed deer, their passage through sieves was significantly (P<0.001) reduced in comparison with when they were incubated in the fluids from pasture-razed deer. Adding condensed tannins to rumen fluid increased the inhibition of the migration of Li lungworm larvae but PEG removed this inhibition; this effect was not observed with abomasal fluid.
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