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

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.