Moredon Scientific Ltd

Parasitology

Millennial Anthelmintic Resistance Survey

Dr Frank Jackson

Figure 1: Average prevalence of white drench resistance on lowland, upland and hill farms in Scotland.

The survey, which was intended to provide a clear indication of the current situation in Scotland, was conducted using Moredun Foundation's Scottish sheep farmer members. A total of 227 anaerobic faecal sample collection kits and accompanying questionnaires was sent to the participants in the survey all of whom had at least 30 ewes in their flocks. Eggs and or larvae extracted from samples were used in egg hatch assays and larval development assays to detect the presence of benzimidazole (BZ, Group I), imidazothiazole/tetrahydropyrimidine (IT, Group II), or macrocyclic lactone (ML, Group III) resistant nematode populations.

In Scotland as a whole 60.8% of the surveyed farms showed evidence of benzimidazole resistant worm populations with Teladorsagia (Ostertagia) being the predominant genera implicated in the resistance. There was no evidence of any age effect upon the distribution of resistant worm populations, which appeared to be equally prevalent in both ewes (61.1%) and lambs (58.0%).There was evidence that the type of farm influenced the prevalence of resistant nematodes (Figure 1).

Region

Participating Farms

Prevalence of BZ resistance (%)

Highland and Islands
Grampian
Tayside
Fife
Lothian
Borders
Central
Dumfries and Galloway
Argyll and Bute
Strathclyde

15
8
9
3
6
16
2
11
5
14

40.0
62.5
55.6
100
66.6
93.8
50.0
45.5
60.0
64.3

Figure 2: Regional participation and prevalence of benzimidazole resistance.

Figure 2 shows the regional participation and prevalence data. The survey provided no evidence of resistance against drugs within the group II and III drug families.

Participating farmers administered an average of 2.7 anthelmintic doses annually to their ewes and 3.2 treatments to their lambs. Farms where BZ resistance was apparent tended to treat their lambs more frequently than farms where there was no resistance (3.5 vs 3.0 treatments per annum). The results from the survey provide a graphic illustration of the adaptability of indigenous parasite populations and focus attention on the need to develop sustainable control strategies which minimise these adaptive responses.