Disease control on grouse moors

Red grouse are wild game birds, but they are susceptible to population cycles where numbers peak and crash, which are often driven by disease. These cycles can make it difficult to achieve a consistent shootable surplus, which also means that income, which helps offset some of the costs of managing for grouse shooting and wider public benefits on moorland, is unpredictable. Veterinary medicine is used on grouse moors to reduce the impacts of some diseases and help stabilise grouse numbers.

This page will look at the two infections for which grouse are treated and the disease control measures applied. The first, strongylosis, is caused by a parasitic thread worm that lives in the guts of grouse and can be controlled with an anthelmintic (worming) drug administered on quartz grit, which the grouse naturally eat to aid digestion.

The second is louping ill virus, which is controlled by targeting the sheep ticks that can carry it. Louping ill is also a threat to livestock (most often sheep) on moors also through external blood-sucking ticks. Finally, the chapter will look at the impact of the disease respiratory cryptosporidiosis on grouse, for which there is no effective treatment.

Strongylosis and medicated grit

Q: What is strongylosis?
A: Strongylosis is caused by the strongyle worm Trichostrongylus tenuis. It infects the guts of red grouse, reducing gut efficiency, and therefore impacting the bird’s condition, which can ultimately cause the death of the bird if worm burdens are high enough.

Q: What effect do strongyle worms have?
A: We know from our research that high worm burdens can reduce both grouse breeding success and adult survival, including making them more vulnerable to predation. High levels of parasitic worms in some years can cause large numbers of grouse to die, leading to cyclical fluctuations with numbers crashing every 4-6 years.

Q: Is this a recent issue?
A: No. The interest in avoiding cyclical crashes is not new. The issue has concerned grouse managers since the early 20th century. The 1911 Lovat research study into red grouse ‘In Health and Disease’ was largely driven by this problem.

In recent decades, the renewed stimulus to understand grouse population cycles came following a decline in grouse abundance in the 1970s. With this came the realisation that multiple factors – more predators, moors increasingly separated from each other by forestry and farmland, tick borne disease and climate change – were at risk of exacerbating the down-turns in grouse cycles so that they might prevent grouse recovery; with subsequent financial implications for the profitability of the shoot.

Q: How was strongylosis first treated?
A: Trials conducted in the 1980s showed that catching hen grouse at night and treating them with an oral worming drug reduced worm burdens, and therefore allowed treated hens to produce and rear more young.

Q: What effect did this have on grouse population crashes?
A: Treatment with worming drugs improved bird condition and survival, and this prevented grouse population crashes. From this initial research “medicated” grit was first developed in the late 1980s.

Q: What is medicated grit?
Grit boxA: Red grouse consume naturally occurring grit to help break down and digest heather, their main food source. Grouse moor managers regularly put out boxes of quartz grit to facilitate this for grouse. Medicated grit is quartz grit coated with a thin layer of fat that is impregnated with a worming drug. When it was first developed, the drug used was fenbendazole. In 2007, an improved formulation of medicated grit was developed, using flubendazole, an alternative drug from the same family already licenced for use in game birds.

Q: Is flubendazole safe?
A: Yes. It is a licensed human medicine and veterinary drug that is routinely given to livestock and game birds to treat different intestinal worms. Sheep are regularly treated with a different group of anthelmintics throughout the year against a range of gut parasites to prevent loss of condition and poor lambing.

Q: Was the introduction of medicated grit effective?
A: Yes. The first experimental study looking at this showed that adult grouse from an area treated with medicated grit had significantly lower worm burdens than those from an untreated reference area. On the treated area, chick survival was significantly higher and hens reared more than twice as many chicks. When treatment across the two areas was reversed, grouse on the second area then performed better, illustrating that the effect was down to the medication rather than other aspects of the study sites.

Q: Do moor managers provide medicated grit all the time?
A: No. Veterinary drug use is regulated, and medicated grit is prescribed under licence by a vet. Vets should only prescribe medication when there is a demonstrable need for it. Demonstrating a need to use medicated grit involves an analysis of worm burden, either by counting the number of worms in adult grouse, or by counting the number of worm eggs in their droppings. The worm burden found at analysis will help determine a suitable course of action. In reality, grouse moor managers sometimes request, and vets sometimes provide, grit when it is unnecessary, and this inapproriate use escalates the likelihood of worms becoming resistant to the drug and the medication no longer working. Targeted usage helps reduce the risk of resistance developing within the worm population, thus allowing the drug to remain effective.

Q: How often is it likely to be needed?
A: A recently published GWCT study found that medicated grit usage varies markedly between moors. It may be needed every year on wetter blanket bog moors in the west, but may be only every 3-5 years on drier heath moors in the east. Grit is provided in trays, which can be closed as necessary to prevent or allow access as appropriate. For example, the drug must be withdrawn a minimum of 28 days before grouse are shot to prevent drug residues entering the food chain.

Q: Do we aim to remove all worms?
A: No. Current best practice advice on controlling parasites, based on management of livestock, is to leave 10% of the population untreated to help offset the build-up of resistance. For more information about best practice for using medicated grit, see both GWCT guidance and best practice advice such as the Principles of Moorland Management.

Q: What effects could medicated grit be having on the environment or other species?
A: As a licensed medication, flubendazole has passed thorough investigations into the effect on non-target species, as well as the wider environment, and toxicity data are freely available. However, it is important to continually review such possibilities and the GWCT has recently announced its funding of a three-year study with Leeds University to look at the degree of contamination of moorland soils and watercourses by flubendazole and their breakdown products.

Ticks and louping ill virus

Q: What are sheep ticks?
TickA: Sheep ticks (Ixodes ricinus) are spider-like blood-sucking parasites that feed on a wide range of host species. They have a four-stage lifecycle: egg, larvae, nymph and adult. They can transmit diseases such as Lyme disease (Borrelia) and Louping ill virus (LIV) that can have an impact on livestock, wildlife and humans.

Q: What is louping ill?
A: Louping ill is a virus transmitted by sheep ticks which can cause fevers and neurological symptoms. It is often fatal in flocks of sheep which have no immunity to it. Ticks become infected with LIV when they feed on a host that has high levels of the virus already in its bloodstream. Previously, a vaccine was available for sheep, which helped prevent the spread of disease however this is no longer in production. The main tick species that feeds on sheep or deer also feeds on red grouse, so ticks can pass LIV to grouse. LIV disease can cause around 80% mortality amongst grouse chicks in laboratory conditions, through effects on the nervous system causing loss of muscle control and lesions in the brain.

Q: Do ticks present a threat to other moorland birds?
A: Ticks also feed on other moorland birds. Although we don’t know whether waders such as curlew can become infected with LIV, high tick burdens can reduce body condition in their chicks, which may impact on survival. In one study, chicks in 91% of curlew broods carried ticks, with an average 4.5 ticks per chick, and a maximum of 64 ticks on one individual. As well as feeding on red grouse chicks, ticks were also found on the chicks of all bird species sampled on one moor in North Wales. These were black grouse, curlew, meadow pipit and Canada goose.

Q: Do ticks also affect humans?
A: Yes. Sheep ticks also bite humans. Under specific circumstances, humans can get LIV and it causes the same neurological problems, ticks are also a vector for the Borrelia parasite that causes Lyme disease, leading to flu like symptoms, joint pain and in extreme cases, paralysis. This can be very serious if not diagnosed and treated.

Q: Are tick numbers changing?
A: Tick burden and abundance appear to have risen since the early 1980s. There is some evidence that climate change and changing numbers of deer have played its part in a steady increase in tick numbers since the 1980s. Two studies of the tick burden on red grouse chicks found that between 1985 and 2003, the proportion of grouse chicks per brood carrying ticks on the study sites rose from 4% to 92%. The average number of ticks per chick also rose in this period, from 2.6 to 12.7.

Q: How can tick numbers be controlled on moorland?
A: Ticks are controlled by treating the sheep grazing on moorland, which can then kill the ticks that become attached to them. When used in this way, sheep are referred to as tick mops. Controlling the numbers of alternative wild hosts such as deer may also be beneficial, especially if combined with the use of sheep as tick mops.

Q: How can treating sheep help reduce the tick burden in grouse?
A: For farming purposes, sheep are usually treated with anti-tick medication (acaricides) twice per summer. A GWCT paper from 2012 showed that increasing this to four treatments, either by dipping the sheep or using a pour-on medication, reduced tick burdens on grouse chicks by 90%. In this study, sheep were also vaccinated against LIV, and the proportion of chicks testing positive for LIV fell throughout the study, in relation to how long the tick treatment programme had been in place on that moor.

The LIV vaccine is no longer available, leaving tick treatment as the only approach for sheep. A recent study also confirms that on sites where sheep were treated with acaricide more frequently, on average red grouse chicks had fewer ticks. When sheep were treated at six week intervals, an average of 1.7 ticks per chick was found, compared with an average 14.6 ticks per chick when sheep were treated at ten-week intervals. Where sheep are the main large animal host for ticks, this approach can reduce tick numbers very effectively as ticks which bite the sheep are killed. However, ticks also feed on other animals and if these alternative hosts are present in large numbers, this can reduce the effectiveness of treating sheep alone.

Q: What other animals can ticks feed on?
A: Ticks at different life stages can feed on a range of hosts including voles, hares, red grouse, sheep, and deer. At each life stage the tick needs a larger blood meal, which means that adult ticks generally feed on larger mammals such as hare or deer rather than grouse.

Mountain hares can be an important host for ticks at all stages of the life cycle and can be the main host for adult ticks if sheep in the area are effectively treated. If sheep are treated adequately, then tick numbers fall, and the number on hares also falls. In a trial area where mountain hare numbers were reduced, the number of ticks on grouse chicks also fell, as did the proportion testing positive for LIV. However, there is a lack of scientific evidence as to whether mountain hare culling can increase red grouse densities. One GWCT study found no evidence to suggest lower tick burdens on grouse chicks occur at sites with lower hare abundance, in fact higher hare abundance was found at sites with higher grouse brood sizes, and a higher proportion of hens with broods. Conditions which are good for grouse appear also to be good for hares without showing an increase in tick numbers. More work is needed to understand this relationship.

Q: Can controlling deer numbers reduce ticks on the moor?
A: Management of wild hosts through cull management strategies and deer fencing has been adopted on some estates. Whilst the significance of deer as tick hosts is recognised, the role of mountain hares in this disease system is still uncertain. One study showed that reducing deer density by culling, or excluding deer from an area of moorland or forestry with fencing resulted in dramatically fewer ticks. A recent GWCT study also found that sites with higher deer densities had higher grouse tick burdens, and grouse breeding success was lower.

Q: Can the grouse be treated?
A: Studies have looked at the effect of catching and treating grouse with acaricide, either using a slow-release wing tag, a pour-on treatment or a leg band. Although tick numbers can be reduced and, in some studies, chick survival improved, the results were variable. One paper used computer modelling to predict the effectiveness of treating grouse. It predicted that acaricide treatment of grouse might be effective in controlling ticks and LIV, but only if deer are at low densities (fewer than 10 deer per square kilometre), or if deer were more numerous, higher numbers of grouse would need to be treated, and that treatment must be effective for 20 weeks of the summer season. Grouse treatment was predicted to have a much smaller effect on overall tick numbers than reducing deer density. Reducing tick numbers overall seems to be more effective.

Q: What is the recommended approach for tick management?
A: As a first step, the introduction of a comprehensive and thorough acaricide treatment regime for sheep flocks throughout the tick questing period (at least from the start of April to end of October). If deer are present, we recommend assessing densities and any subsequent need for a reduction in deer numbers. The GWCT recommends that efforts to reduce the impact of ticks on red grouse should focus on a combination of deer control and an effective tick management programme for sheep. Controlling mountain hares should never be assumed to be needed or to be the first management action undertaken.


Q: What is cryptosporidiosis?
A: A disease caused by a single-celled parasitic organism from the Cryptosporidium group. There are 18 different species, with Cryptosporidium baileyi infecting birds.

It infects the sinuses of poultry, gamebirds and many other species of birds, causing swollen eyes and an excessive production of mucus; like a severe head cold. It is associated with high concentrations of birds, including captive birds in zoos and collections.

Q: When was it first reported in red grouse?
A: Clinical symptoms of Cryptosporidium were first reported and subsequently diagnosed in wild red grouse in 2010.

Q: Where did it come from?
A: Nobody knows. It is possible that it has always been present in red grouse. Since initial diagnosis in 2010, red grouse on 50% of managed grouse moors in northern England have shown signs of infection and over 80% of moors in the North Pennines. To date, apart from a small number of infected birds in the Lammermuirs (south-east Scotland), outbreaks have only been reported from grouse in northern England.

Q: How is it transmitted?
A: The spore phase (oocyst) of the  life cycle passes between birds. It is found in the droppings and mucus of infected birds, which then infects other birds. Higher densities of grouse may increase infection risk, for example congregating around communal grit trays or natural moorland drinking pools in dry weather. Generally, the highest infection rates have been associated with a period when grouse densities, especially of young grouse, have been at their highest.

Q: How can this be controlled?
A: Good hygiene at grit boxes may be important to reduce infection rates. The oocyst can survive for long periods outside the body, but does require water to persist, so making sure grit boxes are well drained can help to reduce transmission.

Q: How many red grouse show symptoms?
A: Our survey work has shown that on average about 5% of grouse on a moor may show typical symptoms of cryptosporidiosis at any one time. This incidence is highest in grouse which have no immunity, and when the disease has just arrived at the moor. Juveniles are most at risk from infection.

Q: Does infection reduce breeding success and adult survival?
A: Yes. Infected hens breed on average a week later than healthy birds and whilst they lay similar sized clutches of eggs and hatch a similar proportions of clutches, chick survival amongst infected hens is only about half that in healthy ones. A small proportion of adults have been shown to recover, but mortality rates are estimated to be twice those of uninfected birds on the same moor.

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