The Control of Trichostrongyle Infections in Grazing Cattle of Argentina in a Context of Multiple Anthelmintic Resistances

A field program to control multiple anthelmintic resistant worm infections in cattle of Argentina is described. In 2003 around 140 calves died as a consequence of high worm burdens despite the administration of monthly alternate treatments with ivermectin (IVM) or fenbendazole (FBZ). A fecal egg count reduction test (FECRT) showed a clinical efficacy of 73%, 49.1% and 90.4% for IVM, FBZ and levamisole (LVM) respectively. An efficacy controlled test (ECT) showed that efficacy of IVM was 76.1% and 22.6% against Haemonchus spp. and Cooperia spp. respectively; for FBZ efficacy was: 0 % Ostertagia spp.; 28.3% Haemonchus spp. and 24.2% Cooperia spp. A rational control program based on epidemiological surveillance was established since 2004 up to 2012 based on nematode egg excretion and speciation of worms in coprocultures. The LVM was used when necessary. After ten years the results of a FECRT indicated a clinical efficacy of 79% for IVM (Cooperia spp. resistant), 89.4% for FBZ (Ostertagia spp. resistant) and 95.8% for LVM. The ECT showed that efficacy of IVM was ≥99% against Haemonchus spp. and Ostertagia spp. and 56% against Cooperia spp. The FBZ showed an efficacy of 100% against Haemonchus spp. and Cooperia spp, and 74% against Ostertagia spp. These findings demonstrate that resistance to IVM and FBZ persisted for Cooperia spp. and Ostertagia spp. respectively despite both compounds had not been used throughout a ten year period. The control programme successfully stopped mortality, minimized subclinical losses and reduced the number of treatments.