The threat of reduced efficacy of anthelmintics against gastrointestinal nematodes in sheep from an area considered anthelmintic resistance-free

Abstract

Background: The worldwide increased difficulty to combat gastrointestinal nematode (GIN) infection in sheep, due to progressing anthelmintic resistance (AR), calls for an enhanced and standardized implementation of early detection of AR. This study provides a snapshot of the current AR status against benzimidazoles and macrocyclic lactones in southern Italy, generated with standardized techniques.

Methods: On 10 sheep farms, the efficacy of albendazole (ALB) and either eprinomectin (EPR) or ivermectin (IVM) was evaluated based on the faecal egg count reduction test (FECRT) performed with the Mini-FLOTAC. For each tested drug, 40 sheep were rectally sampled at D0 and sampled again 14 days after the treatment (D14). The FECRT was calculated from individual samples and pooled samples which consist of 5 individual samples. Efficacy was classified as 'reduced, 'suspected' and 'normal'. Coprocultures were set for D0 and D14 faecal samples of each group. From farms with FECR < 95%, an in vitro egg hatch test (EHT) and a follow-up FECRT using fenbendazole (FBZ) were conducted.

Results: Based on the FECR, high efficacy (from 95.7% to 100%) was observed for ALB and IVM in eight farms (Farms 3-10). On Farm 1 and Farm 2, the efficacy for the macrocyclic lactones was classified as 'normal', but 'reduced' efficacy was observed for ALB on Farm 1 (FECR = 75%) and 'suspected' efficacy on Farm 2 (FECR = 93.3%) with the predominant GIN genus Trichostrongylus followed by Haemonchus at D14. The FEC results of pooled samples strongly correlated with those of individual samples, for FEC at D0 (r_s = 0.984; P < 0.0001) and at D14 (rs = 0.913; P < 0.0001). The classifications of efficacy in Farm 1 (FECR = 86.0%) and Farm 2 (FECR = 93.0%) in the follow-up FECRT with FBZ coincide with the main FECRT trial. The in vitro EHT confirmed AR in both farms (Farm 1: 89%; Farm 2: 74%).

Conclusions: In regions like southern Italy, where the negative impacts from AR have played a minor role, efficient monitoring of AR is important in order to evaluate potential risks and being able to promptly respond with countermeasures.

Keywords: Anthelmintic resistance; Egg hatch test; Faecal egg count reduction test; Gastrointestinal nematodes; Pooling faecal samples; Sheep.

Fig. 1

The number of sheep farms, anthelmintics used, sampling time and number of samples per farm used for the field trial setup, follow-up trial and in vitro egg hatch tests

Fig. 2

Frequency distribution of gastrointestinal nematode (GIN) egg counts among different individual animals of all examined farms sampled at D0

Fig. 3

Variability of gastrointestinal nematode (GIN) egg counts in the different sheep farms sampled at D0: mean eggs per gram (EPG) of faeces and standard errors (SE)

Fig. 4

The correlation in faecal egg counts at D0 (a) and at D14 (b) based on the examination of individual and pooled faecal samples of all examined farms