Strongylosis - testing the new medicated grit

A new form of medicated grit has been designed that weathers better on the moor and is consequently more effective than previous formulations. This new grit has to be withdrawn from grouse a month prior to, and during, the shooting season, so we have designed grit boxes equipped with a sliding lid that deny grouse access. Following a preliminary trial in 2007, we tested the system on a larger scale in 2008.

We used eight moorland plots, each of about 400 hectares, in the Upper Teesdale Estate, County Durham. Within each plot, we placed grit boxes 150 metres apart on a grid between January and March 2008. We randomly allocated medicated or non-medicated grit between the plots so that four contained medicated grit and four not.

We collected fresh grouse caecal pats monthly between January and May on each plot and counted their strongyle worm eggs. Worm egg counts showed a sharp increase in April and May on plots without medicated grit, but showed no change on the plots with medicated grit (see Figure 1).

Figure 1. Number of worm eggs (geometric mean) per gram of caecal material from four untreated and four medicated plots in Upper Teesdale in spring 2008

	Medicated
	Non-medicated

We counted grouse in all eight plots in March and again in July, and using the method of distance sampling, we estimated densities. Neither grouse densities nor their breeding success differed between plots containing medicated grit and those that did not.

Given the relative lack of an intensive lattice system of grit heaps on the Upper Teesdale plots before we started this experiment, and the lateness of distributing the grit boxes in some plots in early spring, grit uptake by grouse may have been lower and later than expected. So, the increase in worm egg burdens on the non-medicated plots in April and May probably occurred too late to influence hen breeding condition and, subsequently, breeding success did not differ between the treatments in 2008.

We attended all shooting days on the eight plots recorded the number of grouse shot on each day, as well as their age. We estimated worm burdens from freshly shot birds (at least 10 old and 10 young) monthly between August and November on each plot. Average worm burdens in adult grouse on medicated plots were less than on non-medicated plots (see Figure 2). There was no significant difference in worm burden in young grouse, because all birds got only non-medicated grit from the first week of July.

Figure 2. Worm burdens (geometric mean) in both adult and young grouse shot between August and November 2008 on four untreated and four treated plots in Upper Teesdale

	Medicated
	Non-medicated

Grouse were shot on all plots, but the harvest rate was low and winter grouse densities remain high. This, coupled with currently high worm burdens, suggests that in the non-medicated plots, birds will be in poor condition this spring and will either breed less well, or worse still, we will see the start of a worm-induced population crash. Within the medicated plots, medicated grit was restored to the grouse on 10 December (the last day of the shooting season) and is being taken up well. We anticipate that worm burdens will fall later this winter and into early spring so that grouse on these plots will endure neither suppressed breeding success nor a population crash. We will report whether these predictions come true in 2010.

Extensive comparisons of medicated and non-medicated moors

We count grouse annually and their strongyle worm burdens on 25 moors in northern England. Some of these moors regularly use medicated grit, and now use the new form of grit, whereas others do not. A comparison of worm burdens from grouse shot in August and September 2008 at 11 sites that use medicated grit and a further 11 sites that do not, clearly shows that medicated grit reduces worm numbers by over 90% (see Figure 3).

Both the experimental and extensive data strongly suggest that use of medicated grit can dramatically reduce worm burdens in grouse. In many parts of northern England, where very high densities of grouse remain following the end of the shooting season, they still have low parasite burdens. Typically, worms drive grouse cycles in this region. The data we present suggest that the new grit may either prevent or severely dampen cycles.

Figure 3. The mean worm burdens from grouse shot in August/September at 11 sites that use medicated grit and a further 11 sites that do not

	Medicated
	Non-medicated

Dos and Don'ts for Medicated Grit

Do

• Have a gritting station per pair
  of grouse.
• Physically prevent grouse access to medicated grit one month before the shooting season and throughout the period when grouse are shot.
• Have plain grit available at sites when medicated grit is withdrawn. This keeps birds coming to gritting sites.
• Have double-sided grit boxes with sliding lids.

Don’t

• Make medicated grit available to grouse during the shooting season.
• Withdraw all grit; give non-medicated before and during the shooting season.
• Use stocks of early formulations of medicated grit; use the new formulation.