Limited efficacy of pour-on anthelmintic treatment of cattle under Swedish field conditions

Abstract

A study on the effect of topical macrocyclic lactones (ML) against gastrointestinal nematodes (GIN) in Swedish first season grazing cattle (FSG) was performed during the grazing seasons of 2009 and 2010. Herds were recruited through farming press and both dairy and beef cattle farms were invited. A questionnaire revealed that 64% of participating farmers dewormed their animals in previous years, and of these 76% used topical formulations with ML. Four to six weeks after turnout, 107 (2009) and 64 (2010) farmers sent in individual faecal samples from 6-10 FSG. Faecal egg counts (FEC) were determined by the FECPAK®-method in 2009 and the McMaster-method in 2010, when also larvae were cultured. Average FEC of ⩾100 eggs per gram faeces (EPG) was seen in 39% of the herds in 2009 and 42% in 2010 and with arithmetic means of 258 ± 110 and 252 ± 350 EPG, respectively. Interestingly, FSG in dairy and beef herds had similar mean FEC. In herds with mean FEC of ⩾100 EPG, farmers dewormed all FSG in the tested grazing group with ivermectin (IVM) or doramectin (DOR) pour-on. In 2009, 33 (31%), and in 2010, 26 (40%) of the herds were retested 7-16 days post treatment. Mean reduction was 89% and 88%, respectively, and in only 12 (36%) and 10 (38%) herds it was ⩾95%. Beef herds had mean reductions similar to those of the dairy herds. No significant difference (P = 0.66) in reduction was seen between the groups treated with three different pour-on formulations, nor was there any correlation between the previous year's usage of anthelmintics and the efficacy. Larvae from post-treatment cultures analysed in 2010 with a species-specific ITS2 qPCR showed that Cooperia oncophora was the predominant species after deworming. Four (15%) groups also harboured surviving Ostertagia ostertagi post treatment.

Keywords: Anthelmintic resistance; Doramectin; Gastrointestinal parasites; Ivermectin.

Graphical abstract

Fig. 1

Geographical distribution of farms participating with faecal samples both pre and post treatment during 2009 and 2010.

Fig. 2

Faecal egg count reductions in percentage, compared to days between deworming and second sampling during grazing seasons of 2009 and 2010.

Fig. 3

Box plots of species-specific ITS2 qPCR on larval culture material from 26 Swedish farms in 2010. In pre-treatment samples, 35% of total ITS2 copies were specific for O. ostertagi and 65% for C. oncophora. In the 26 cultures from post-treatment samples, C. oncophora was the predominant species (99% of total copies). Four groups showed strong positive (>2000 copies) results for O. ostertagi, (4166-583050 ITS2 copies) while the other 22 groups were close to negative.