Controlling resistant nematodes in dairy sheep: a balance between drug efficacy and milk residues

LIFSCHITZ, A.; CRISTEL, S.; SALLOVITZ, J.; SUAREZ, V.; IMPERIALE, F.; FARIAS, C.; LANUSSE, C.

Bs As

Congreso; XXIII International Conference of the World Association for the Advancement of Veterinary Parasitology; 2011

The extra-label use of the macrocyclic lactone (ML) endectocide compounds to control endo- and ecto-parasites in dairy sheep and goats is rather common in an attempt to increase milk production. The intensive use of these compounds led to the worldwide development of drug resistant in nematode populations. Different pharmacological strategies have been assessed to improve the efficacy of ML against resistant nematodes. However, the pattern of milk residues must be considered when a parasite control program is established in lactating dairy animals. The aim of the work reported here was to simultaneously evaluate the comparative anthelmintic efficacy, the plasma-milk exchange and milk residual profiles for eprinomectin (EPM) and moxidectin (MXD) obtained after their oral administration to dairy sheep naturally infected with resistant nematodes. Twenty four (24) milking ewes naturally infected with ivermectin-resistant gastrointestinal nematodes (mean faecal nematodes eggs count: 1324) were used. Animals were allocated into three (3) experimental groups (n=8): untreated control, EPM and MXD orally administered at 0.2 mg/kg. A topical EPM formulation was given orally (as an experimental extra-label use). Individual fecal samples were collected at 0 and 15 days post-administration to run the faecal nematodes eggs count reduction test (FECRT). Blood and milk samples were collected from dairy ewes in each treated group over 15 days post-treatment. Drug concentrations in plasma and milk residues were measured by HPLC. A higher efficacy, indirectly estimated through the FECRT, was obtained after MXD administration 98.8% compared to the EPM treatment (71.4%). The third-stage larvae differentiation study demonstrated that Haemonchus spp was the only nematode genus surviving after both treatments. As it was expected, higher plasma concentration profiles and greater milk residues were measured after MXD compared to the EPM treatment. MXD persisted for prolonged time both in the bloodstream and milk. Considering the established MRL, the milk collected from MXD-treated dairy ewes should be discharged during 3 days post-treatment. However, the low plasma-milk exchange of EPM permitted to derive the milk from treated sheep without a withdrawal period, in spite of the extra-label use by the oral route. The parasite control scheme to be used in dairy small ruminant production should be carefully selected according to parasite susceptibility and the risk of milk residues over the permitted maximum levels.