William Powell Country

Is Medicated Grit the answer?


Article written by 24 October 2013


So far as grouse are concerned, we seem to be living in an age of plenty.  Many moors have since 2007, shot large bags of grouse without the crash which used to be a regular feature after a run of good grouse years.

Grouse crashes were almost certainly caused by a massive build up of the strongyle worm (trichostrongylus tenuis).  The cycle of these years of boom and bust varied from moor to moor, with drier moors tending to hold large grouse numbers for longer before a crash occurred.  Higher, wetter moors and particularly those over deep peat, very often crashed very quickly after a good or very good year and then the cycle of building up grouse numbers started again.  It was amazing once a crash occurred, to see just how empty of grouse a previously highly productive moor could be.  Although keepers picked up relatively few grouse compared to the number that died, literally entire moors and often entire moorland areas, would be almost devoid of grouse, with only an odd pair here and there as the foundation stock, from which to start again.  Not surprisingly moorland owners, managers and keepers wanted to find a way of avoiding a crash if at all possible and instead providing a rather more constant supply of grouse for shooting parties to enjoy.  The economics of running a grouse moor are grim, without the added deficit caused by a crash.

As a result, over 25 years ago medicated grit was developed.  As grouse use grit to grind up their food (mostly heather), it seemed sensible to try and get a wormer incorporated into the grit.  As the grit consists of hard stone, the only way of doing that was to coat each individual grit piece with a wormer incorporated in a sealant.  The original coating was palm oil which proved attractive to small mammals and so a less attractive coating was developed.  The wormer used to kill the strongyle worm is derived from Flubendazole.  Originally this came in the form of Panacur but more recently Flubenvet.  Both are very similar and there are now several grit companies producing their own variations of medicated grit, usually either single or double strength.  In the early years of using medicated grit, keepers were very unsure as to how effective it was.  Over the last six or seven years, almost everyone involved in moorland management believes that “modern” medicated grit is highly effective and the results seemed to confirm this.  On certain moors in recent years, grouse records have been broken more than once.  Indeed it is doubtful whether we have enjoyed such a period of grouse production, at least since the 1930’s and some of the bags shot have been miles beyond anything we would have dreamt of, only 10 or 15 years ago.

Grouse 1

However, there are some within the grouse world who are now beginning to question firstly whether the wormers are doing what everyone thinks they are, and secondly whether large numbers of grouse living on a moor year in and year out, with no significant reduction let alone crashes, is in fact a good thing.  Although the Game and Wildlife Conservation Trust was originally involved in research on medicated grit (and the lead in manufacturing was taken by the Strathclyde Chemical Company), after the widespread crash in 2005, a Cambridge Vet, Dr. Doug Wise became very interested in the cause of the crash and how it could be avoided in the future.  He carried out extensive trials and came up with some surprising conclusions.  Initially, those conclusions were not widely accepted by the Game Conservancy as it was then or by most owners and keepers, but more recently and in conjunction with another Vet, Richard Byas from Thirsk, these two Vets are making many people reconsider their views.

Instead of the wormer used on medicated grit killing the adult worms which live in the grouse, it seems more likely that what the wormer is actually doing is in fact killing the eggs or larvae in and around the grouse faeces.  The larvae is adversely affected by both very cold and very hot and dry weather, so in the main it is only active when warm but wettish weather occurs in the spring, summer and early autumn.  However, by controlling the worm larvae intake of adult grouse, few additional worms re-infect the bird and hence the worm burden in a bird fails to increase.  If the worm burden in the bird is already low, this procedure appears to keep it at that level which poses no real threat to the bird’s health.

Evidence as to why medicated girt may not be working as we used to think, is that the Scientists now seem to accept that to kill adult worms, a grouse would need to ingest 20 milligrams of wormer per kilogram of the bird’s body weight over a 7 to 10 day period.  Given the amount of grit which a grouse body contains, it is clear that this cannot happen if the medicated grit used is single or even double strength.  Grouse not only take in grit put out by keepers, but find natural (non medicated grit) from rock edges, moorland roads and stream beds so that the actual intake of medicated grit could be fairly small.

To overcome this, Doug Wise developed a grit that has twenty times the concentration of wormer, thereby ensuring that a grouse only had to ingest a small amount of this type of super strength grit to be effective and for even adult worms to be killed.

What everyone involved in supplying medicated grit emphasises and this is thoroughly endorsed by the GWCT, is that it is absolutely essential that grouse should not avail themselves of medicated grit all the year round.  For a start, all birds entering the food chain  (i.e. after being shot), have to have had no access to medicated grit for at least 28 days, so as to remove any trace of the wormer from the body.

Equally important is that with almost every animal or bird reared or kept in large numbers, the worm develops resistance to the wormer.  This could lead to a type of super strongyle worm emerging.  This has to be avoided at all cost and the best way of ensuring this doesn’t happen, is to have both a longish period of time on every moor when medicated grit is not available, and also to ensure that wormers are only used when worm burdens are significant and increasing.  When the worm burden is low, there is little point (whatever some owners and keepers now think), of using medicated grit, not least because if you do this, then there is a real danger of producing drug resistant worms.  Similarly if a moor already has a very high worm burden, normal or even double strength medicated grit is unlikely to do much to redress this, but super strength grit probably will.

Whilst there is now much confidence in the effectiveness of wormers, we have recently seen on some moors in England and Scotland, a previously unreported disease affecting grouse.  This is called “Bulgy eye”.

No one knows whether the very recent outbreak of “Bulgy eye” first found in grouse in 2010 has anything to do with medicated grit.  However, we do know that Bulgy eye (which looks exactly as it sounds) is in fact Crypotosporidrum baileyi, a relative of Coccidiosis which in other forms is widespread in birds including pheasants and partridges.  Cryptosporidrum can ultimately kill adult grouse, but also weakens them so that they die, often in large numbers from other causes, perhaps including weather, stress or indeed by making them less resistant to predators.

Grouse on moor resize

The fact that Bulgy eye hasn’t been seen on grouse moors until very recently is a real cause for concern.  Does it’s appearance merely coincide with consistently large numbers of grouse living on many moors over the last 5 or 6 years or is it because of the now widespread use of grit trays which may harbour disease, or for some other unknown reason which might perhaps be weather related?  Coccidiosis and Cryptosporidrum are easily transmittable between birds of the same species and all diseases thrive where large numbers of birds or animals live close together.  The current very understandable trend by moor owners and their keepers to maintain large over-wintering stocks of grouse,  the almost enforced use of fixed gritting stations and the lack of the previous all too regular crashes caused by a build up of strongyle worm, could perhaps be the reasons why we are now seeing diseases in grouse that we have never previously experienced.  Whilst we know about Bulgy eye, we don’t know whether other diseases will occur in grouse that perhaps hitherto haven’t been seen. Worryingly Pox and possibly Microplasma are thought to already have been found in some grouse.


The old saying “beware what you wish for” may be increasingly relevant to those of us seeking to consistently produce large numbers of grouse.  I can well see why owners, keepers and guns would not wish to return to the old days of feast or famine.  Equally, I am not sure that our desire for grouse numbers to be as predictable as pheasants and partridges, is necessarily something we should wish for, if it brings with it, many other potentially very serious health problems for this, our last truly wild game bird.

Image courtesy of GWCT

Image courtesy of GWCT

Mark Osborne - FRICS, FAAV, MBAE

Mark Osborne is founder and Managing Director of JM Osborne & Co. and has been a qualified Chartered Surveyor and Land Agent for over 30 years. He trained on two large traditional landed estates (Chatsworth and Castle Ashby), before s.. Read more.

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