7 Internal and External Parasites

Diseases caused by parasites are extremely important, leading to poor welfare, reduced production and death in all ages of sheep. The main parasites, the diseases they cause, and the age groups affected, are listed below. This is not an exhaustive list, but it contains the most common and important ones. Individual cases may vary in the age group affected or in the signs shown.

Fig. 249 Ewes with wool loss: is this due to external parasites or not?

Fig. 250 A very dirty backside: is it due to internal parasites or not?

Internal Parasites

Roundworms: Issues, Diseases and Control Measures

This subject is highly complex, and advice may be modified in the light of on-going experience. It is not possible to go into great detail here, but we recommend you also look at advice given by SCOPS (Sustainable Control of Parasites in Sheep) which can be accessed at www.nationalsheep.org.uk, and take advice from your veterinary surgeon as part of a flock health plan. Where ‘safe’ grazing is mentioned, this is pasture that has not been grazed by sheep or goats during the previous twelve months; it may have been grazed by cattle or been used for arable crops or conservation.

Responsible Use of Anthelmintics and Anthelmintic Resistance

Until recently only three groups of anthelmintics were available (1-BZ, 2-LM, 3-AV), although a new one (4-AD) has now come on to the market. Resistance in roundworms in the UK has been detected to all three established groups, although in individual flocks there may be no resistance, resistance to one group, resistance to two groups, or, rarely currently, resistance to all three groups. Where multiple resistance occurs it may mean it is impossible to keep sheep on that farm, at least for several years.

Whether resistance is present has to be determined by drenching the animals correctly – this means accurate weighing and accurate dosing – and collecting faeces samples at a set time afterwards for laboratory examination. If resistance is detected, the whole group of anthelmintics identified should not be used in that flock and control has to be based on the careful use of drugs from the remaining groups.

A number of precautions can be taken to reduce the risk of anthelmintic resistance developing in a flock. Current advice includes observing the following practices:

• Do not introduce anthelmintic resistance into a flock with bought-in animals – quarantine these and treat with drugs from two different groups given separately, one after the other (see Chapter 1).
• Do not use the same drug, or a drug from the same group of compounds (check this before you buy), year after year, but rotate the groups annually to reduce the risk of resistance developing.
• Minimize drug use by having a tailor-made flock health plan including management of grazing to reduce pasture contamination where possible.
• Do not dose ewes before tupping, apart, possibly, from yearling ewes – generally this is not appropriate since most ewes have few worms at that time.
• Leave some animals untreated in order to keep a susceptible worm population on the grass – fit ewes with single lambs, and fast-growing single lambs, are the least likely to benefit from dosing since they can better withstand a worm infection.
• Do not turn freshly dosed animals immediately on to ‘safe’ pasture – they should continue grazing on contaminated pasture for a day or two so they pick up a small number of susceptible larvae, rather than potentially taking only resistant worms with them.
• Do not use the new wormer exclusively through the season. It is suggested that it is best used as a quarantine dose, and for lambs, once in mid-grazing season. Take veterinary advice for further help with this complicated subject!

THE MECHANICS OF DOSING

The following are the most important points in making sure dosing is carried out correctly:

• Anthelmintics should be stored in a safe place at room temperature (avoid freezing and excessive heat).
• Equipment such as automatic dosing guns should be well maintained and their accuracy tested before using.
• Animals must be dosed with the correct amount according to their weight; underestimation of weight and therefore underdosing is common, so use a weighing scale and set the dose to the heaviest in the group.
• Take care when inserting the drenching gun into the sheep’s mouth – drenching gun injuries as a result of careless dosing technique are common and often result in deaths.
• Make sure the dose is put carefully over the tongue so that the sheep does not spit some of it out.

Fig. 251 Make sure anthelmintics are stored correctly and equipment is well maintained. Drenching guns should be cleaned out after use and looked after according to the manufacturer’s instructions.

Fig. 252 Test the accuracy of the dosing gun.

Fig. 253 Use a set of scales to get an accurate weight when dosing.

Fig. 254 Be sure that the dose is placed over the tongue so the sheep does not spit it out, but be careful not to damage the throat by pushing the nozzle too far in.

Nematodirosis

This disease causes severe scouring, often with blackish scour, which indicates the presence of blood. Affected lambs rapidly become weak, dehydrated, and may die. The disease has a different pattern from others caused by roundworms. Lambs of up to about three months of age are mainly responsible for transmitting infection from year to year; older lambs and adult sheep develop immunity to the worm. Large numbers of eggs deposited on the ground in the faeces of infected lambs can survive on the pasture over very cold winter weather. Eggs are triggered to hatch following a period of exposure to cold weather followed by a rise in temperature to above 10°C.

The occurrence or not of disease depends on whether susceptible lambs are grazing at the time the larvae hatch. If warmer weather comes early in the season the lambs may be too young to be grazing. If cold weather is prolonged, lambs may be past the susceptible stage by the time larvae hatch. Recently the disease has been seen in older lambs later in the year, so possibly the pattern of the disease is changing.

Prevention is by not grazing young lambs on the same pasture as in the previous two years, or by strategic dosing with anthelmintic (most are effective, but take advice and read the instructions). Combined infection with nematodirosis and coccidiosis can cause severe disease.

Fig. 255 Lambs between one month and three months old are most susceptible to nematodirosis. Those affected by the disease lose weight rapidly and some die. Urgent treatment with anthelmintic is indicated, and the aim must be to prevent future outbreaks by grazing management if possible, or strategic dosing based on weather conditions and a forecast of the likelihood of the disease.

Diseases Caused by Other Roundworms

Apart from Nematodirus, most roundworms have similar life cycles so can be considered together, particularly since most disease outbreaks are due to mixed infections. Female sheep gradually gain immunity to roundworms during their first grazing season, so the diseases caused by these mainly affect growing lambs, apart from Haemonchus, which can affect any age group. Ewes lose their immunity to roundworms around lambing and during lactation, although rather than developing disease, they produce large numbers of worm eggs in their faeces. Rams do not develop a good immunity, and thin or old sheep may also carry significant worm burdens.

It is this group of worms that includes those which have developed resistance to anthelmintics. This is an extremely worrying situation, so advice on drug use, as discussed above, has changed significantly in order to slow down the spread of resistance. The new anthelmintic must be used with care, as resistance to this will inevitably develop over time.

Parasitic Gastro-enteritis (PGE)

Essentially, lambs start to pick up infective larvae of several roundworm species when they begin grazing in spring. The larvae are made up of those that have overwintered on the pasture from the previous year, and those passed as eggs by the lactating ewes and which start hatching as the warmer weather of late spring hastens their development. So the number of larvae on the pasture peaks at just the time that the growing lambs have started eating a lot of grass. The larvae develop into worms within the stomach or intestine, damaging the lining of the gut and reducing the uptake and utilization of nutrients. The end result is the characteristic picture of scouring lambs which fail to thrive.

Treatment with an effective anthelmintic is necessary if lambs have reached this stage, but if they are left on contaminated pasture they will be exposed to further damage, so moving to a safer pasture (not grazed by lactating ewes and lambs that season) is advisable.

Fig. 256 The normal consistency of faeces of most sheep, though some breeds regularly have more cow-like faeces.

Fig. 257 This scouring lamb with a rather open fleece may be carrying a significant worm burden.

Fig. 258 Profuse scouring in this age of lamb is likely to be due to roundworms, although lambs on lush grass can also scour. The appearance of the lambs and faecal egg counts will help to decide the cause.

The control of PGE is complex, and made even more so by the increasing prevalence of anthelmintic resistance in worms. We can only give the key points here, and detailed control methods need to be worked out by your vet based on the type of flock, the grazing available, the management system, and whether cattle can be used as part of a control system. For further information, see Chapter 12. The key points are as follows:

• Know your pastures and be able to assess how contaminated they are likely to be – you may need help with this, but pastures that have not had sheep on for a year, or have had only adult non-lactating ewes, or have been grazed only by cattle in the previous year, can generally be considered ‘safe’.
• Pastures that have been used for silage or hay will be safe providing they were not grazed by ewes and lambs before shutting up.
• Pastures that have had ewes and lambs grazing in the first half of the year will be contaminated.
• On contaminated ground, reducing further pasture contamination by dosing ewes around lambing time is important. Ewes start to regain their immunity about six weeks after lambing, so early lactation is the time during which they will most contaminate the ground if they are not dosed. Use of an anthelmintic with some prolonged action may be the best way, or dosing again after three to four weeks if management allows it. But do take advice!
• In lowland systems, try to get the lambs finished and away as quickly as possible before the main weight of infective larvae has time to build up on the pasture.
• Use faecal egg counting to decide whether or when dosing lambs is necessary.

Haemonchosis

The worm Haemonchus causes a different clinical picture from those causing PGE, in that all age groups can be affected if they have not been exposed to this worm previously, the main signs being anaemia, weakness and death. These worms are prolific egg layers, so infection on the fields can build up rapidly. Warm weather is required for their development, so disease is seen in late summer and autumn, although it can occur in ewes in spring as the result of worm larvae that were dormant within the ewes.

Fig. 259 This ewe has a swelling under her jaw which may indicate haemonchosis, chronic fluke or Johne’s disease. Laboratory tests will be necessary to be sure of the diagnosis

Fig. 260 A typical fluke habitat – wet, boggy ground growing rushes.

Fig. 261 A typical case of chronic liver fluke: the sheep is very thin and unwilling to get up. Closer examination shows bottle jaw, typical of advanced chronic fluke infection.

Previously confined to the south of the UK, this disease is now being seen further north as a result of changing weather patterns.

Lungworms

It is rare to see outbreaks of lungworm alone, since practically all the anthelmintics currently used are active against these. They may be found in debilitated or old sheep at post-mortem examination.

REDUCING RELIANCE ON ANTHELMINTICS

The importance of roundworms in sheep production cannot be over-emphasized, nor can the new thinking on control measures based on minimizing the risk of the development of anthelmintic resistance be ignored. In future, slightly reduced growth rates may have to be accepted as part of plans to maximize the length of effectiveness of available anthelmintics. However, it is known that different individuals and breeds vary in how well they tolerate or deal with roundworm infection, and genetic selection of those animals that cope best is likely to become available as a method of reducing reliance on anthelmintics in the future.

Liver Fluke (Fasciolosis)

Liver fluke is seen in acute, subacute and chronic forms. We have already met chronic liver fluke as a cause of thinness in sheep in Chapter 6, and have noted the marked increase in losses of sheep dying from liver fluke over recent years. The reason for this is the series of wet summers that have favoured the development and extension of the habitat of the mud snail, Galba (Lymnea) truncatula, which plays a central role in the life cycle of the fluke. Boggy ground with rushes and slow-moving streams with shallow muddy banks are typical snail habitats.

Fluke eggs passed out in the faeces of sheep develop to contain an early stage in the life cycle (miracidia), which hatch after a variable time, depending on the warmth and wetness of the environment. These find their way into the snails, where they multiply over several weeks, eventually bursting out of the snail and forming cysts (metacercariae) on grass and other plants, which are then eaten by sheep. The immature flukes hatch in the intestine, then make their way through the abdomen to the liver.

Acute disease occurs if many metacercariae are eaten over a short time, because of the damage and haemorrhage caused as the immature flukes migrate through the liver. The typical picture of acute fluke outbreaks is sudden death or weakness, anaemia, and abdominal discomfort, and occurs most commonly in autumn and early winter. Subacute fluke occurs when the metacercariae are eaten over a longer period, and is seen in winter. Anaemia and weakness are the main signs. Chronic disease occurs when adult fluke are present in the liver. Excessive thinness, anaemia and bottle jaw are the common signs, seen in the winter and spring.

There are several different drugs available to treat liver fluke. All are effective against adult fluke, but they vary in their ability to kill immature fluke, so careful selection needs to be made according to whether disease is being caused by immature or adult fluke, so check the labels and take advice. The drug with the best efficacy against immature fluke is triclabendazole, so this would be the treatment of choice for acute disease. However, recently cases of resistance to triclabendazole have occurred, particularly where this drug has been used for a number of years, so if this is the case you will need veterinary advice on how best to deal with the situation. If fluke habitats are limited in area it may be possible to fence them off or to remove the sheep from these fields at critical times. Most often, though, control of the disease is by strategic dosing, usually in late autumn and winter, though on heavily infected farms more treatments may be needed based on past weather and previous experience.

Tapeworms

Segments of adult tapeworms are often seen in lamb faeces, but these are not normally considered to cause disease. Actually, the fact that these may appear in quite large chunks shows that the animals are expelling the worms naturally, so treatment is generally not indicated.

Sheep act as the intermediate host (the cystic form of the life cycle) for other tapeworms where the adults are found mainly in dogs. These include Taenia multiceps that causes gid, Taenia hydatigena that is seen at meat inspection as cysts on the liver and peritoneal surface, and Echinococcus granulosus that causes hydatid disease in animals and man. Better control over carcase disposal in recent years has reduced the opportunities for scavenging by dogs. This, together with greater awareness of the need to dose dogs regularly with tapeworm remedies, means that these diseases are now seen less commonly than they were in the past. See Chapter 15 for more details on gid.

Fig. 262 Here a possible fluke habitat has been fenced off so the sheep are unable to get access to it.

Fig. 263 Tapeworm segments are often seen on lamb faeces, but are not usually harmful.

Fig. 264 Dogs are the intermediate host for several tapeworms that can cause disease in sheep, horses and man, so should be given regular worming treatment that covers tapeworms. They should also be kept away from the carcases of dead sheep.

Protozoa

Coccidiosis

Coccidia are microscopic parasites that live and multiply in the intestines. They are specific to a particular species – for example, coccidia that cause disease in chickens do not cause disease in sheep, and vice versa. This is a disease of intensification, in that lambs born and reared extensively do not get coccidiosis. The management system that most commonly leads to disease is where ewes lamb indoors and are kept indoors for some days or, more likely, weeks afterwards. Risk is even greater where different age groups of lambs are housed together, or where younger lambs follow older lambs through the same pens. After about eight weeks of age, lambs become more resistant to coccidia so disease is less likely after this stage.

Ewes act as the original source of infection – they harbour low numbers of the parasite, and eggs (oocysts) are passed out in the faeces. These become infective for lambs within a few days, and after being ingested, multiply massively in their intestines. The lambs born first do not generally get diseased, but increase the amount of infection on the bedding for lambs born later, which often do develop disease. Poorly bedded pens increase the risk. Sometimes the disease affects lambs outdoors where they have been kept on heavily stocked pasture. The typical picture is of lambs between four and eight weeks of age that scour and lose condition. Sometimes deaths occur before scouring is evident. Diagnosis can be complicated, so veterinary and laboratory help is needed.

Fig. 265 Lambs kept indoors are susceptible to coccidiosis. The risk can be reduced if the pens are kept well bedded, and as long as the age groups are not mixed.

Fig. 266 Dogs are involved in the life cycle of sarcocysts, which can occasionally cause vague neurological signs in growing lambs.

There are now two drenches available (containing diclazuril or toltazuril) that can be used for treatment, or by dosing strategically before disease is expected, to prevent outbreaks. Feeding an anticoccidial in the ewes’ diet is probably counterproductive, as this reduces immunity passed on from ewes to lambs via the colostrum.

Sarcocysts

These are protozoa that can get into the muscles, brain and spinal cord of sheep (they have a life cycle similar to Toxoplasma, except that dogs are involved rather than cats). The signs of disease are vague, so it is mostly diagnosed at post-mortem examination. The main way of reducing the likelihood of this disease occurring is to prevent dogs scavenging dead sheep.

External Parasites

With the decline in the use of plunge dipping and the reduced availability of chemicals to treat or control external parasites, some of the diseases caused by them have become much more common in recent years. As an added complication, treatments are sometimes given on an assumed, rather than a confirmed diagnosis, and if the diagnosis is incorrect this often means inappropriate or unnecessary use of drugs and the development of resistance, particularly in lice.

A further problem is the possible effect on the environment of dips or sprays applied to sheep as the issue of residues assumes greater importance. Treatment and control of some of these parasites has therefore become much more difficult than was the case a few years ago. Consult your vet and get a diagnosis – some itchy sheep may have more than one parasite causing their problems!

Treatment and Control Methods

The table below summarizes the effectiveness of various chemicals used in the treatment and control of external parasites. This is only a summary, so when the use of a product is being considered the full details and instructions should be consulted since these may vary as to whether treatment or control is required.

Fig. 267 This sheep is rubbing her rear against the wall: is it anything to be concerned about, or not?

Fig. 268 Several sheep in this flock have wool tags hanging from their fleeces: what are the possible causes?

Fig. 269 This field has a large amount of shed wool: is it due to natural shedding or to a parasitic disease?

Fig. 270 Sheep scab is very difficult to control where sheep are free to wander on extensive grazing, and stray sheep can easily introduce it into previously uninfected flocks.

OP: organophosphate

SP: synthetic pyrethroid

3-AV: avermectin type

IGR: insect growth regulator

Fig. 271 This handling system includes a dip, but these are used much less commonly than in the past. Dipping in an OP (organophosphate) is still the most effective means of treating and preventing most diseases caused by external parasites. Understandably, however, many people do not want to use these chemicals because of health concerns. In some areas contractors provide mobile dips.

Fig. 272 An old-fashioned dip, outdated now as it is difficult to use safely and effectively.

Fig. 273 A mobile spray is an attractive method of applying a dip product to sheep, but this method is not effective for treating sheep scab since it cannot guarantee wetting the fleece right to the skin.

Fig. 274 Application of an insect growth regulator (IGR) to sheep to prevent blowfly strike. This type of product will not treat strike, but is very effective if applied before flies lay eggs. SP (synthetic pyrethroid) pour-ons are applied in the same way.

Sheep Scab

This important disease is caused by the mite Psoroptes ovis, and has become a widespread problem affecting sheep welfare and production since dipping has declined in use. It is particularly problematic in extensively kept sheep on common grazing, since it requires a concerted effort and the cooperation of all sheep-keepers to gather and treat animals at the same time if any headway is to be made. It is also all too easily introduced into flocks by purchased sheep, whether replacement ewes or a single ram. With experience, the mites can just be seen with the naked eye (the size of sugar grains). They cause intense itching, with exudation and scabbing of the skin in the body region where they are located; in neglected cases they can spread over much of the body, and deaths can occur.

Scab infection spreads particularly rapidly when sheep are in full fleece and housed – when they are shorn the number of mites is reduced and they hide in protected areas such as folds of skin. The mites can survive off the sheep for a maximum of about sixteen days, so pens and trailers that have had sheep in them may act as a source of infection for this length of time.

Fig. 275 This localized patch of wool loss is caused by infection with scab mites. Infected sheep react strongly to being gently scratched in the area where the mites are, and often have grazes where they have been rubbing against fences or posts.

Fig. 276 A sheep with extensive wool loss as a result of scab; she will have passed the disease on to her lamb.

Only plunge dipping in organophosphate (OP) is effective for treating both lice and scab; understandably many people are now unwilling to use this product. So if sheep are itching, it is essential to get a diagnosis so that the correct treatment can be given. Applying dip by other methods – such as jetting and sprays – is not effective at eliminating all scab mites, so should not be used for this purpose however tempting it may be. Pour-on synthetic pyrethroids (SPs) available for the treatment of lice are not effective against scab mites.

The most common treatment method now is injection of one of the Group 3 products – ivermectin, doramectin or moxidectin. Instructions must be followed, as these vary for each of the products. Doramectin and moxidectin both have some residual action so can also protect sheep against reinfection for a period – again, read the instructions.

Using Injectables for the Treatment and Control of Scab

Operators using injectables for the treatment of scab should be aware of the following issues:

• Withdrawal periods can be long.
• Instructions as to injection technique vary between products.
• Repeat injections may be required to treat effectively.
• It may be at least fourteen days after treatment before itching subsides – itching is an allergic reaction to the mite faeces, which persist for a while in the fleece even though the mites themselves may be dead.
• These products are also wormers – think of the effect on the worm population at the time animals are to be treated.
• Animals that have been injected with footrot vaccine should not be given moxidectin 1 per cent injection because of possible adverse reactions.
• Worm drenches containing the same chemicals as injectables have an effect on the mite population but do not kill them all. Therefore the use of these worm drenches for scab-infected sheep may hasten the development of resistance.

Ear Mites

Ear mites are a slightly adapted strain of Psoroptes ovis that live in sheep’s ears and may be present in a flock without signs of sheep scab. The classic signs are rubbing and shaking the ears to such an extent that manyaffected sheep develop ear haematomas (cauliflower ears). Treatment with the injectables used for sheep scab is effective.

Fig. 277 A ram with foot mange; he is clearly getting relief by rubbing his leg on the wire of the fence.

Fig. 278 The typical appearance of chorioptic mange affecting the scrotal skin.

Foot Mange (Chorioptic Mange)

Foot mange is caused by another mite, Chorioptes bovis, and has become much more common in recent years – presumably it was previously well controlled with dipping, but is now becoming evident because the injectable treatments for scab are not very effective. These mites, which cause itching, live on the lower parts of the legs and the skin of the scrotum of rams and the udder of ewes. Affected animals rub their legs on troughs or fences. The only really effective treatment is the use of OP dip.

Lice

Like sheep scab, infestations with lice have become common in recent years. They also cause itching, though perhaps not as severely as in scab. Different species can be found on the body, head and legs. Lice are visible to the naked eye so a careful inspection of the skin at the base of the fleece or around the ears and legs should help in diagnosis, but remember that it is possible to have mixed infections, with scab mites being present, too.

Fig. 279 These sheep have lice: the ewe in the foreground has dry-looking wool with tags, showing she has been rubbing. However, do not assume a diagnosis without confirmation.

Fig. 180 Careful examination shows that lice are present – these are usually visible to the naked eye.

Numbers of lice build up during the winter when the sheep have full fleeces, and become much reduced following shearing. If lice infection is a problem at the end of winter, careful thought must be given as to whether to treat at this stage, or to leave and shear at the earliest opportunity – the decision has to be made based on the particular circumstances in each flock.

Treatment for body lice consists of either dipping in an OP plunge dip, or applying a synthetic pyrethroid pour-on – neither of these is desirable for full fleeced sheep, so in at least some cases it may be best to delay treatment until after shearing when the chemical will have a much better chance of being fully effective, and chemical contamination of the wool is avoided. What is best for the welfare of the sheep must be the main consideration in deciding the best plan of action.

Keds

Keds are large parasites that live in the wool and are rarely a problem, apart from being a nuisance to the sheep. They can be controlled by dipping, or by applying an SP pour-on if necessary.

Headfly

Headflies can cause great irritation around the heads of sheep in the summer months. The main effect is seen in young horned sheep, where the skin at the base of the growing horns is easily damaged. These flies are attracted to the area, and because of the irritation caused, the sheep rub their heads causing further damage. Other head wounds in sheep without horns can also be affected and an insignificant wound can quickly become much larger as a result of rubbing. Prevention is by applying an SP spot-on around the base of the horns or around the periphery of the head wounds.

Fig. 281 Young sheep with horns are most likely to be affected by headflies. These particular ones are not affected.

Fig. 282 The type of wound that can be made much worse by headfly activity in summer.

Blowfly Strike

Protection against blowfly strike in the late spring, summer and early autumn is extremely important for all sheep except those on high hill grazing where blowflies are not active. Flies are attracted by damp, soiled wool, particularly around the breech area of sheep that are, or have been scouring, but strike can also occur when the wool is apparently quite normal. Blowflies also lay eggs in diseased feet, and the resultant maggots can transfer to wool around the elbow and shoulder from these. Once a sheep is struck, other flies are attracted, more eggs laid, and the number of maggots and the area affected can increase rapidly.

At first, struck sheep look agitated, waggle their tail and try to rub or nibble the affected site. The wool is damp, and looks discoloured in that area. As the infestation worsens the sheep becomes dull, recumbent and can die within a matter of days – a very nasty death.

If a sheep is struck, the wool from the affected area must be sheared away and the maggots removed. Application of an SP pour-on will ensure that any remaining maggots are killed, and will protect against further strike. The sore area should be dressed with an antiseptic, and if the sheep is ill, antibiotics and an anti-inflammatory injection from your vet will speed recovery.

Prevention is by the application of a pour-on product: an SP will both prevent and treat strike, or a more environmentally friendly solution is to use an insect growth regulator (cyromazine or dicyclanil), but these must be applied before fly activity starts. Whichever product is used, a second application may be needed if the fly season is long, and withdrawal periods must be taken into account if the sheep are to be sold for meat. Prevention is also helped by keeping sheep clean around the rear end by good parasite control and by crutching if necessary.

Fig. 283 Typical picture of a sheep that will get fly strike unless it is cleaned up and protected.

Fig. 284 The sheep lying down has suffered fly strike to quite a large area of its hindquarters; it has been treated.

Fig. 285 Close-up of a wound with maggots in it.

Fig. 286 A foot badly infected with maggots. These often spread to the fleece when the sheep is lying down.

Fig. 287 Both these sheep have suffered fly strike down their sides before shearing. The wounds have healed, but sometimes the wool does not grow again.

Ticks and Tick-Borne Diseases

Ticks are generally a problem more for the diseases they transmit, rather than for any direct effect, although they can be irritating and can also cause anaemia in lambs with a heavy infestation. Ticks are not a problem on cultivated lowland pasture. They are found on rough grazing and hill land with bracken and heather where they spend much of their life, apart from a week or so each year when they feed on blood from a variety of animals including sheep (and people). Ticks are most active in spring and autumn when they emerge from the damp undergrowth to try to find a suitable host for a blood meal. Sheep can be protected either by dipping in an OP dip, or by applying an SP pour-on before being put on infested ground. The diseases they transmit are tick-borne fever, tick pyaemia and louping ill.

Louping ill: This is a virus that infects the brain causing neurological signs and death in a proportion of animals that are infected, though some recover and become immune. The usual method of control is by vaccination, which protects young lambs through the colostrum.

Remember that ticks can spread diseases to people – Lyme disease is the most common one, but louping ill very occasionally causes serious illness.

Fig. 288 This heather moorland is a typical tick habitat. Young lambs need to be protected by applying an SP (synthetic pyrethroid) pour-on before they are turned out.