Target selected treatment with levamisole to control the development of anthelmintic resistance in a sheep flock

Levamisole phosphate, chosen based on its 100 % efficacy demonstrated by a previous fecal egg count reduction test (FECRT), was used as the exclusive anthelmintic treatment in the Embrapa Southeast Livestock sheep flock from 2009 to 2014 in a target selected treatment scheme. In the present study, the effectiveness of this nematode control scheme was evaluated after 5 years by FECRT, larval development test (LDT), and a molecular test to assess the development of levamisole resistance in Haemonchus contortus. Animals were submitted to treatments with albendazole, levamisole, closantel, ivermectin, moxidectin, and monepantel. Eggs per gram of feces (EPG) counts and fecal cultures were performed, and anthelmintic efficacy was calculated by the RESO 4.0 program. The helminths of the flock (GIN Embrapa2014) were compared to susceptible (McMaster) and resistant (Embrapa2010) H. contortus isolates in the LDT to estimate the LC₅₀ and LC₉₀ of levamisole and in a molecular test to evaluate the 63-bp indel in the acr8 gene associated with levamisole resistance. In the FECRT, parasites were susceptible to monepantel (99.6 %) and closantel (98.3 %), but resistant to moxidectin (93.8 %), levamisole (70.4 %), ivermectin (48.1 %), and albendazole (0 %). In the coproculture on D14, and the control group presented 80 % H. contortus and 20 % Trichostrongylus sp., while in the monepantel group L₁ were observed as well as Oesophagostomum sp. L₃. LDT and resistance factors provided good separation between susceptible and resistant parasites. The genotypic frequencies of the 63-bp insertion in the acr8 gene in H. contortus were 11.9, 6.7, and 0 % in GIN Embrapa2014, Embrapa2010, and McMaster isolates, respectively. After 5 years of exclusive use, the nematodes developed resistance to levamisole, detected by FECRT and by increase in LC₅₀ and LC₉₀ for levamisole in the LDT. The 63-bp indel was not confirmed as a molecular marker of levamisole resistance in our isolates. The target selected treatment scheme was effective to control helminths in the sheep flock for 5 years, when levamisole’s inefficacy was perceived because of no change in the clinical situation of treated animals. Through this scheme, it was possible to promote reversion towards susceptibility or increase of efficacy for other chemical classes. Thus, this is a valid recommendation to control worms and to delay the development of resistance, preserving other anthelmintic classes for future use.