Effects of four tanniferous plant extracts on the in vitro exsheathment of third-stage larvae of parasitic nematodes

Summary

The anthelmintic properties of tanniferous plants and of their secondary metabolites represent one possible alternative to
chemotherapy that is currently being explored as a means of achieving sustainable control of gastrointestinal nematodes in
ruminants. Previous in vivo and in vitro results suggest that tanniferous plants can have direct anti-parasitic effect against
different stages of nematodes. However, the mode of action of the bioactive plant compounds remains obscure. The
objectives of the current study were (1) to examine the hypothesis that extracts of tanniferous plants might interfere with
the exsheathment of third-stage infective larvae (L3); (2) to assess the role of tannins in the process by examining the
consequence of adding an inhibitor of tannins (polyethylene glycol: PEG) to extracts. The effects of 4 tanniferous plant
extracts on exsheathment have been examined on L3 of Haemonchus contortus and Trichostrongylus colubriformis. Artificial
exsheathment was induced in vitro by adding hypochlorite solution to larval suspension. The evolution of exsheathment
with time was measured by repeated observations at 10-min interval for 60 min. The selected plants were: genista
(Sarothamnus scoparius), heather (Erica erigena), pine tree (Pinus sylvestris), and chestnut tree (Castanea sativa), with tannin
contents ranging from 1.5 to 24.7% of DM. Extracts of a non-tanniferous plant (rye grass, tannin content: 0.3% of DM)
were included in the assay as negative controls. The extracts were tested at the concentration of 600 mg/ml and the effects
were compared to the rate of exsheathment of control larvae in PBS. No statistical differences in the pattern of
exsheathment was observed after addition of rye grass or genista extracts for both nematode species and with heather
extracts for T. colubriformis. In contrast, pine tree extracts on larvae of both species and heather extracts with H. contortus
induced a significant delay in exsheathment. Last, contact with chest nut extracts led to a total inhibition of the process for
both nematodes. These results suggest that extracts of tanniferous plants might affect a key process in the very early stages
of larval invasion of the host. In most cases, the addition of PEG led to a total or partial restoration towards control values.
This suggests that tannins are largely involved in the inhibitory process. However, other secondary metabolites may also
interfere with the process that would help to explain some of the differences in response observed between the two
nematode species.