Evaluation of ivermectin effectiveness under contrasting anthelmintic resistance scenarios in beef cattle: a comparative study in Italy and Argentina

Beef cattle farming plays a crucial role in the livestock-related economies of both Italy and Argentina. Although both countries have grazing systems of meat production, the heterogeneity of those systems leads to different parasitological scenarios regarding epidemiology and the presence of resistance to commonly used anthelmintic drugs. Considering ivermectin (IVM) is the most widely used anthelmintic for treating gastrointestinal nematode (GIN) infections in cattle, the current studies evaluated the efficacy of ivermectin (IVM) given subcutaneously (SC) to both young calves and adult cows in ten (10) commercial cattle farms from Italy (A to E) and Argentina (F to J). The work was complemented with the assessment of the IVM plasma exposure and disposition kinetics both in young and adult animals. While adult cows and young calves were included in the study performed in Italy (Campania region), only calves aged 8-14 months old were selected to perform the efficacy trials in Argentina (central area of the Buenos Aires province). Fifteen (15) calves/cows naturally infected with GINs were treated with IVM (0.2 mg/kg) in each of the Farms. The therapeutic response (efficacy) was determined at 14 days after treatment by the faecal egg count reduction test. Six (6) adult cows and eight (8) young calves treated with IVM were randomly selected to perform the pharmacokinetic (PK) study with blood samples being taken 2 h and 21 days post-treatment. Drug concentrations were measured by HPLC. Similar IVM PK trends were obtained for both adult and young cattle. The IVM systemic exposure (expressed as AUC) obtained for adult cattle (380{+/-}158 ng.d/mL) was similar to that observed in calves group (313{+/-}85.5 ng.d/mL). No statistical differences between animals from both ages were observed for the most representative PK parameters (P>0.05). Surprisingly, while in Argentina IVM resistance was present in all the farms evaluated (90% CI = 0%-95%), in Italy a fully susceptible nematode population was presented in three farms (90% CI = 96%-100%) and only a very low level of IVM resistance was detected in the rest of the farms (90% CI = 92%-98%). Whereas Cooperia spp. and Haemonchus spp. were identified as the principal genera exhibiting resistance to IVM in Argentina, Cooperia spp., Ostertagia spp., and Oesophagostomum spp. were reported to display low levels of resistance under the Italian field scenario. In conclusion, IVM can be safely and effectively used in both young and adult cattle with similar disposition kinetic patterns. While its therapeutic response is failing under the conditions investigated in the Argentinian farms, its efficacy remains notably high in the Italian region under study. However, the use of IVM, as well any other active antiparasitic principle, should always be preceded by a diagnostic assessment of the nematode populations resistance status.