In 2007 we tested an improved medicated grit with a more persistent coating. Weathering trials of this new grit have shown that 70% of the drug is still present after nine months of exposure on the hill. This raises the concern that the drug could pass into the human food chain if access to it is not withdrawn before the shooting season.
To address this, the new grit is designed to be used only in grit boxes with two compartments: one containing medicated grit, the other non-medicated. A flip or slide lid regulates access to each compartment and hence determines the duration of exposure of the grouse to the drug. The initial questions we wanted to answer from this study were
In March 2007, we selected two plots of 25 hectares on each of three Pennine moors, allocating medicated grit to boxes in one plot and non-medicated to the other. We spaced all boxes 100 metres apart so that there were 25 boxes per plot. Each box was a shallow tray (with each of the two compartments measuring 30 x 30 cm and 7.5 cm deep), and contained one kilogramme of grit. We collected fresh caecal pats from grouse from each plot each month from February to May to assess worm infection. At the same time, we recorded then removed evidence (feathers/faeces) of grouse activity.
The index of box use by grouse differed between sites and was highest on the moor where grouse densities were also highest (see Table 1).
|Monthly percentage of grit boxes being used by red grouse containing either medicated or non-medicated grit|
In February, before treatment, there were similar levels of worm eggs on all plots. By the end of March (after one month exposure) monthly egg counts were 66% to 92% lower on the medicated plots and 90% to 99% lower by the end of April (two months exposure). By the end of May, we found an average of only 100 worm eggs per sample on the medicated plots compared with over 12,000 eggs per sample on the non-medicated plots (see Figure 1).
Figure 1. Grouse caecal pats containing fewer than 100 eggs (effectively worm free) on medicated and non-medicated plots
The percentage of grouse caecal pats that contained fewer than 100 eggs (effectively worm free) increased with box use by grouse on medicated plots but not on plots with non-medicated quartz grit. Each data point is a mean.
On all three moors, breeding success was higher on the medicated plots, with an average of 8.5 young per hen (36 hens) compared with 6.1 on non-medicated sites (40 hens), a difference of 19%.
Following the grouse population crash in 2005, red grouse populations are increasing. With increasing numbers of grouse, and the very mild autumn in 2007, there has been a considerable increase in the worm numbers, in particular in the young birds (see Figure 2). Parasite burdens in these young birds have increased more than 10 times from 2006 to 2007. However, worm burdens in adult birds have remained fairly static. Correct application of medicated grit should help to keep parasite burdens down in 2008.
|Figure 2. Mean worm burdens in adult and young red grouse on eight moors in the North of England|
Information about the best practice use of medicated grit for the treatment of strongylosis in red grouse can be found in our best practice guidelines.