Patterns of faecal nematode egg shedding after treatment of sheep with a long-acting formulation of moxidectin

•We survey moxidectin use by sheep farmers in south-east Scotland.•It is widely used to control sheep scab and the periparturient worm egg count rise.•We perform repeated faecal worm egg counts on sheep treated with 2% moxidectin.•Nematode egg shedding recommenced earlier than expected after moxidectin treatment.•Nematode egg shedding commenced earlier and peaked higher in spring than autumn.

Much of the current information on the effects of long-acting anthelmintics on nematode populations derives either from research farms or mathematical models. A survey was performed with the aim of establishing how moxidectin is currently being used on sheep farms in the south-east of Scotland. A study was undertaken on a subsection of the surveyed farms to examine the effects of long-acting moxidectin treatments in both spring and autumn on faecal nematode egg output. The survey showed that whole flock treatments of injectable 2% moxidectin were used to control sheep scab on 21% of farms. Injectable 2% moxidectin and oral moxidectin were used to control the periparturient rise in faecal nematode egg shedding by ewes on 13% and 55% of farms respectively. The effects of injectable 2% moxidectin treatment on faecal nematode egg shedding post-treatment in both the autumn and spring were investigated by faecal nematode egg counts at the time of treatment and at 2-weekly interval thereafter on eight and six farms in the autumn and spring, respectively. Faecal egg shedding recommenced at 8 weeks (autumn) and 4 weeks (spring) post-treatment. Counts increased to a peak and then declined. The mean (95% confidence interval) peak counts post-treatment were 2.8 (0.6, 5.1), 3.6 (1.7, 5.5) and 53.5 (25.1, 82.0) eggs per gram (EPG) for autumn-treated ewes, autumn-treated lambs and spring-treated ewes respectively. The spring treated sheep showed a statistically significantly earlier return to faecal egg shedding (p = 0.0125, p = 0.0342) compared to both other groups, statistically significantly higher peak in egg counts than the autumn treated sheep (p < 0.001) and a statistically significantly longer period of positive egg counts (p = 0.0148). There was no statistically significant difference in the timing of the peak FECs between autumn and spring (p = 0.211). The FECs of all groups of sheep treated with an injectable long-acting formulation of moxidectin became positive earlier than would be expected from the period of persistence given on the datasheet, but post-treatment FECs were very low compared to pre-treatment counts.