NADIS disease forecast – sheep (August)

August 2004

By Neil Sargison BA VetMB DSHP FRCVS

NADIS Sheep Disease Forecast

Attention during August should be focussed on prevention of ill thrift and sudden death in lambs and management of blowfly strike and lameness across the whole flock.

Weather conditions during July were mostly cooler and wetter than average, becoming warmer and muggy in some areas towards the end of the month. Sheep veterinary work was generally quiet, with fewer reports of lamb ill thrift and scour caused by parasitic gastroenteritis than during recent summers, possibly associated with cooler weather. 

The main problem reported towards the end of the month was blowfly strike.  Interestingly many farmers reported lambs being struck over the shoulders rather than in the usual area around the breech.  Wet weather conditions made hay-making difficult, with some crops lying for several weeks before being salvaged by wrapping. 

Such crops seldom ferment completely and can present a risk of listeriosis when later fed to sheep. Concerns were raised about a high incidence of dog worrying, raising objections to potential ‘right to roam’ access to farm land. Lamb deaths due to pulpy kidney and pneumonic pasteurellosis were also reported.

Prevention of ill thrift in growing lambs
Production is most efficient when lambs are finished as quickly as possible.  To illustrate this point the production costs of well nourished healthy lambs with post weaning weight gains of 300 g/day can be compared with those of less thrifty lambs with weight gains of 100 g/day. 

The feed conversion efficiency of the healthy lambs is double that of the ill thrifty lambs, because the daily feed requirement for maintenance is the same, irrespective of the time taken to reach slaughter weight. 

During the last four summers, the main causes of ill thrift, involving most of the lambs in a flock, have been:

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  Parasitic gastroenteritis (worms)
  
  
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  Poor pasture nutrition
  
  
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  Cobalt deficiency
  
  
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  Selenium deficiency
  
  
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  Lameness
  
  
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  Hepatotoxic plants (bog asphodel)

Attention should now be focused on the prevention of these problems. (The diagnosis of these diseases has been discussed in previous NADIS sheep disease forecast articles.)  (Whenever only individual lambs are seen to be ill thrify, causes such as poor maternal milk production, previous joint ill, flystrike, or pneumonia should be considered.)

General nutrition
The importance of pasture management, including planning to ensure that sufficient 6 – 8 cm swards of good quality pasture are available for lambs after weaning is obvious. 

Undernourished lambs don’t grow, but are also more susceptible to the effects of parasitism and trace element deficiencies, control of which can prove difficult when nutrition is inadequate.

Parasitic gastroenteritis (worms) 
Parasitic gastroenteritis is probably the most important production limiting disease of sheep. 

PARASITIC GASTROENTERITIS IS THE MOST IMPORTANT CAUSE OF ILL THRIFT IN WEANED LAMBS

The aim of effective worm control during the spring and early summer months was to limit the exposure of susceptible lambs to significant burdens of infective worm larvae on pasture. The options for worm control now are:

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  finish lambs quickly before pasture larval burdens become production limiting
  
  
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  only graze susceptible sheep on ‘safe’ pasture
  
  
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  suppress pasture larval contamination using wormers
  
  
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  a combination of the above – integrated control

Worm eggs shed in the faeces of sheep hatch and develop to infective third stage larvae on pasture. Disease is caused following ingestion of these larvae with pasture. 

The rate of egg hatching and development to third stage larvae is highest during warm and damp weather, while survival of infective larvae on the pasture is optimum during cool and damp conditions. Thus, recent weather conditions could result in significant worm problems during the next few weeks.

Under typical UK summer conditions, few infective larvae survive on pasture for more than 9 months. In general, disease is caused in sheep and cattle by different worm species. 

Thus, pastures which have been cropped, or grazed by cattle, and not grazed by sheep for the previous 9 months, only carry very small numbers of infective sheep worm larvae. These pastures are often referred to as ‘clean’. 

Furthermore, grazing with cattle for a shorter period of time, or hay/silage cropping enables the physical removal of a high proportion of the infective sheep worm larval population, producing ‘safe’ pasture. 

When ‘safe’ or ‘clean’ grazing is available for lambs at weaning, the traditional advice has always been to dose the lambs before moving. Further anthelmintic treatments should not then be necessary until late autumn, by which time most of the lambs may have been sold. While this practice ensures good worm control in the short term, it is now recognised that it can also select for anthelmintic resistance. 

When lambs graze on contaminated pasture, most of the total worm population is present on the pasture as eggs, developing, and infective larvae, with only a small proportion in the lambs as fourth stage larvae and adults. 

Thus, when the lambs are dosed with an anthelmintic, only a small proportion of the total worm population is exposed and any surviving resistant worms only make up a tiny proportion of the remaining worm population. 

However, if lambs are dosed with an anthelmintic before moving to ‘safe’ or ‘clean’ pasture, then a larger proportion of the remaining worm population is made up of surviving resistant worms. 

While these are initially present in insufficient numbers to cause production loss, with time they are multiplied through susceptible sheep, giving rise to production limiting burdens of worms which cannot be controlled using anthelmintics belonging to the same action group as that used before the move.

This concern may be overcome by:

Not dosing lambs before moving.

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  Production loss due to Teladorsagia circumcincta, the main worm present during the summer months, is caused mostly by the effects of recently ingested larvae burrowing into the lining of the abomasum (stomach). Therefore, even if lambs have significant worm burdens before moving to clean pasture, they will thrive following the move because they are no longer exposed to incoming larvae.
  
  
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  The potential problems with this strategy are:
  • Pastures don’t remain clean, so if lambs aren’t finished quickly, satisfactory growth rates may become difficult to achieve later in the year.
  • While most problems are caused by Teladorsagia, less common worm genera such as Haemonchus and Trichostrongylus can cause severe disease in some flocks. In addition to the effects of incoming larvae, the feeding activities of adult worms of these genera can cause ill thrift. Therefore lambs may fail to thrive after a move to clean pasture if these genera are present.

Dosing 90% of the lambs before weaning and leaving about 10% untreated.

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  Within any group of lambs, individual susceptibility to worms varies, with the most resilient animals thriving despite the presence of significant worm burdens. If the heaviest 10% of lambs are left untreated before moving, they should continue to thrive, but will ensure that the previously clean pasture becomes contaminated with anthelmintic susceptible worms, as well as those resistant worms which survived dosing of the remaining 90% of the group.  Thus the sub-population of diluting resistant worms on the pasture is small compared to that of susceptible worms and the rate of development of resistance is slow.
  • The disadvantage of this strategy is that the pasture does not remain clean, although it is likely to remain safe for longer than it would have if none of the lambs had been dosed before moving.

Returning lambs to contaminated pasture after dosing, before moving to clean pasture a few days later.

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  The rationale for this strategy is that both resistant worms which survived anthelmintic dosing and susceptible worms acquired from the contaminated pasture between dosing and moving will be carried onto the clean pasture, thus slowing the rate of development of anthelmintic resistance.

When ‘safe’ or ‘clean’ grazing is not available, anthelmintic treatments will probably need to be repeated throughout the summer months. The frequency of summer anthelmintic treatments will depend on the persistence of the product used. 

Conventional wormers (white drenches, levamisole, ivermectin and doramectin) should be used at about 4 week intervals, while the dosing interval can be extended to 6 – 8 weeks if the persistent wormer, moxidectin is used. 

Depending on the level of pasture larval contamination, it may be possible to maintain satisfactory production using fewer suppressive anthelmintic treatments. However, while reducing reliance on anthelmintics reduces the rate of selection for resistance, the strategy can result in poor production if it enables pasture larval contamination to develop. 

Once pastures are heavily contaminated, satisfactory production may be difficult to achieve despite anthelmintic treatments. This problem may be overcome by regularly performing worm egg counts on faecal samples from 7 – 10 lambs. However, trends rather than absolute counts must be closely monitored, otherwise pastures may already be heavily contaminated before high counts are identified and anthelmintic treatment instigated.

Treatment of clinical parasitic gastroenteritis 
There are numerous NADIS reports every year of scour, ill thrift and lamb deaths caused by worms. The objective of worm control should be to prevent pasture contamination by worm larvae. However, once pastures have become heavily contaminated with infective worm larvae, it is difficult to achieve satisfactory weight gains in lambs that graze them. It is often reported that conventional anthelmintic drenching improved performance for about a week, after which lambs became open fleeced and started to scour again. 

This problem can be overcome by moving lambs to clean pasture after dosing with an effective conventional wormer, although in the longer term this selects for resistance. Alternatively if clean pasture is unavailable, a persistent wormer (moxidectin) should be used so that infective parasite larvae are killed before they can cause production losses. 

The meat withdrawal periods for oral and injectable moxidectin (Cydectin) are 14 and 70 days respectively. Furthermore, the persistence depends on the parasite species present. 

There are no simple recipes for the control of parasitic gastroenteritis. Your vet can provide advice about sustainable worm control in your flock.   

Cobalt deficiency
There are numerous NADIS reports every summer of ill thrift in weaned, growing lambs due to cobalt deficiency. Affected lambs appear empty, pot bellied and depressed. A watery discharge associated with a low-grade conjunctivitis is often present at the eyes. 

Severely affected animals are sometimes pale and anaemic, although in these cases the differential diagnosis of haemonchosis should be investigated.

FAILURE TO REACH TARGET WEIGHTS ASSOCIATED WITH COBALT DEFICIENCY

Cobalt is required for the manufacture of vitamin B12, which is used in the liver for energy production.  Growing animals have a higher requirement for vitamin B12 than adults, but the requirements of pre-ruminant animals are low. Consequently, clinical signs are most commonly reported in weaned lambs during the summer.

If cobalt deficiency was diagnosed in your flock last year, it would be prudent either to check the cobalt status of weaned lambs, or to assume that the problem will recur this year and supplement the lambs at weaning.

Short term cobalt supplementation of fattening lambs is usually achieved through oral drenching of weaned lambs with cobalt sulphate or vitamin B12 injections. Oral drenching with cobalt sulphate provides an effective supplement for about 7 days. 

However, weekly drenching is required to provide adequate supplementation, which is not practical in most sheep flocks. Mineralised anthelmintic drenches probably don’t provide enough cobalt to be worthwhile in most flocks. 

On many farms, vitamin B12 injections are considered to be a practical and cost effective method of short-term cobalt supplementation for fattening lambs, although where repeated injections are required, supplementation becomes expensive in terms of labour and the cost of the drug.

Long term supplementation of replacement ewe lambs or long-keep store lambs can be achieved using intraruminal cobalt bullets or glass boluses which also contain other trace elements. On most farms intraruminal cobalt bullets raise the vitamin B12 levels of sheep for over one year, although there are some reports of poorer efficacy. Care of administration is required, in order to avoid injury to the back of the throat.

Your vet can provide advice about the diagnosis and management of cobalt deficiency in your flock.

Sudden death in lambs due to pulpy kidney
There were NADIS reports during July of sudden death in lambs due to pulpy kidney. Many cases were associated with a change in diet such as movement onto silage aftermath after weaning. 

Clostridial diseases can be effectively prevented through vaccination. Previously unvaccinated ewes should be given an initial course of two vaccine injections 4 – 6 weeks apart when they enter the breeding flock, followed by an annual booster about 6 weeks before lambing. 

This pre lambing booster with a multi-component vaccine also ensures transfer to lambs of passive protection against pulpy kidney, which lasts up to 16 weeks of age. Lambs born to vaccinated dams should receive a first sensitiser dose at about 8 – 12 weeks old, followed by a second booster at least 4 weeks later. 

While an interval between sensitiser and booster of 4 – 6 weeks is recommended, an immediate response to booster vaccination has been shown in sheep sensitised as long as 18 months previously.

Vaccination should be combined with good stock husbandry, including careful introduction to improved planes of nutrition.

PULPY KIDNEY IS A COMMON CAUSE OF SUDDEN DEATH IN STORE LAMBS FOLLOWING A CHANGE IN DIET

Management of disease outbreaks
Many British sheep are inadequately vaccinated against clostridial diseases and severe outbreaks of pulpy kidney occur sporadically. In these cases it is important to immediately instigate a vaccination programme. 

Where a sensitiser, but no booster dose of vaccine has been previously used, a single injection of vaccine in the face of an outbreak usually provides immediate protection.

Even when no vaccine has been used, a single injection of vaccine offers significant protection within 48 hours, probably because most animals are naturally sensitised by the presence of small amounts of toxin in their intestines. Yarding of animals for vaccination and restricted feeding usually halt the progress of the disease until the vaccine is effective.

Administration of clostridial vaccines
The recommended dose of vaccine should be administered under the skin over the neck.  Clean needles should be used and regularly changed. Vaccines should be correctly stored in a dark place at about 50°C, but protected from freezing. 

Vaccines should be used before their expiry date and as a general rule, partially used packs should be discarded at the end of the day.

Footrot control
A few cases of footrot were seen during July. The easiest short term option at this time of year is simply to treat the few individual affected sheep. However this strategy does not prevent serious flock problems occurring later in the year. 

The most cost-effective and efficient longer term option is therefore to address the problem now in the whole flock, before a significant proportion is affected.

INFLAMMATION OF THE INTERDIGITAL SPACE AND UNDERRUNNING OF THE SOLE AND HOOF WALL DUE TO FOOTROT

Various methods can be employed for the control of footrot:

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  foot bathing
  
  
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  foot trimming
  
  
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  antibiotic injections
  
  
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  vaccination
  
  
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  selection for resistance
  
  
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  eradication

In practice, footrot control is based on a combination of the above. It is important to distinguish between those methods which are useful for treating severely affected sheep, such as trimming and parenteral antibiotics, and those which will help control disease when used correctly, such as foot-bathing and vaccination.

The objective of footrot control is to prevent the development of painful and debilitating under-running lesions, using the least arduous and most cost-effective method. This is achieved in infected flocks by reducing the incidence of new infections to maintain the prevalence of disease at a low level.

This approach utilises whole-flock control strategies such as foot-bathing and vaccination from the start of the high risk periods for disease transmission, rather than individual handling of each affected animal in order to treat advanced cases. If this objective is met, very few sheep in the flock will develop severe under-running infections and require individual treatment.

Footbathing
Application of antibacterial solutions in a foot bath is most effective for the control of footrot when practiced during the early stages of the disease, when infection in previously unaffected sheep is limited to the interdigital skin and does not involve the hoof wall. 

Footbathing usually needs to be repeated at fortnightly intervals during warm and wet weather when the risk of transmission is high, but when weather conditions are dry such treatment may achieve a 90 – 100% cure rate. Foot bathing alone is not particularly effective for the treatment of advanced footrot lesions.

Sheep should be walked through a footbath of 3% formalin or stood for up to one hour in 10% zinc sulphate solution. The standing time in zinc sulphate footbaths can be reduced by the addition of the penetrating agent, such as sodium lauryl sulphate or a squirt of washing-up liquid. Sheep should be allowed to stand in a dry area for at least an hour after footbathing to allow the chemical to dry on the feet.

Before footbathing, sheep should be run over coarse stones or slats to remove as much mud and faeces as possible. Alternatively, they should first be walked through a footbath containing only water. 

Excessive formalin footbathing in concentrations exceeding 5% can result in foot damage and should be avoided. Furthermore, zinc sulphate is less effective if formalin has been used within the previous two months. Straw placed in the footbath to prevent splashing reduces the efficacy of formalin, but has little effect on zinc sulphate. Foam rubber mats commonly used for cattle may also be effective for small groups of sheep.

The failure of regular footbathing to prevent the development of severe footrot is often caused by a combination of poor handling facilities and an ill-defined transmission period due to persistent wet weather.

Effective footbathing requires:

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  walking sheep through water or running over stones or slats beforehand
  
  
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  using the correct concentration of antibacterial solution
  
  
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  standing on dry concrete or stones afterwards
  
  
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  choosing a dry day

Foot trimming
Traditionally routine annual foot trimming has been recommended for the control of footrot. However, foot trimming has no role in preventing infection and should not be considered as a preventive method when planning control programmes. 

Furthermore, it has been suggested that unnecessary foot trimming may even facilitate the spread of infection between feet. Hoof trimming is only useful to limit the effect of the disease and assist in the resolution of the lesion after the infection has under-run the sole and hoof wall in uncontrolled cases of virulent footrot.

If hooves become severely overgrown despite effective footrot control, then the suitability of the sheep breed for the environment and management system should be reconsidered.

Antibiotic injections
The effect of parenteral antibiotics on interdigital lesions is minimal and they have no role in the prevention of footrot. However, injection of high doses of penicillin (30 mg/kg of procaine benzylpenicillin, compared to the standard therapeutic dose rate of 15 mg/kg), followed by dry standing for 24 hours can be useful for the treatment of advanced, under-running lesions, provided that sheep are kept in a dry environment for at least 24 hours after treatment. 

Vaccination can be a useful adjunct for both control and treatment of footrot. An initial course of two injections 4 – 6 weeks apart is usually recommended, followed by booster doses in advance of high-risk periods in spring and autumn. 

Vaccination provides protection against infection for about 4 – 6 months and there is some evidence that it may also enable already affected feet to heal more quickly. In some cases a single dose of vaccine administered in the face of an outbreak can be used to reduce the severity of the disease. 

The net effect of vaccination can be to reduce both the prevalence and severity of footrot in the flock. However, whole flock vaccination alone does not eradicate footrot and can prove expensive. 

Unfortunately, in many parts of the UK, the high-risk period occurs throughout the year, so vaccination cannot economically provide the duration of protection required. In many flocks, vaccination is targeted at specific high-risk groups of animals, such as rams before mating. 

Some local tissue reaction can occur at the footrot vaccination site, although lesions regress over time and don’t rupture. Furthermore, footrot-vaccinated sheep cannot be subsequently treated with injectable moxidectin because of a risk of fatal allergic reaction. 

Selection for resistance
Resistance to footrot appears to be moderately hereditable and breeding of resistant sheep by selective culling could be considered as a potential control strategy. 

However, while breeding of resistant sheep is a genuine possibility, the value of this strategy is limited by the stratified system of sheep production in the UK. To be most successful, selection should be practised at the level of the hill or longwool ram producer. 

Footrot eradication
Dichelobacter nodosus, the causative bacterium of footrot, only survives in diseased feet. Warm and moist pasture is required for the transmission of D. nodosus between animals, but the bacteria only survives in the soil for a maximum of about four days. It is, therefore, possible to eradicate footrot from affected flocks.   

Eradication of footrot is possible through a combination of regular examination and separation of affected from non-affected animals, footbathing and strict culling of persistently affected sheep. There are several pitfalls and attempts to eradicate the disease are frequently unsuccessful. 

Eradication should only be attempted during dry weather conditions when transmission of D. nodosus is slow and sheep are unlikely to be in the early stages of the infection, which could be missed or confused with scald. Unfortunately, identification of such slow transmission periods is difficult in many parts of the UK. 

Replacement animals should be quarantined for at least one month, during which time they should be inspected. However, despite treatment of any infected animals, it may be impossible to prevent the re-introduction of the disease. Stray sheep from neighbouring flocks are another common cause of re-infection. 

D. nodosus can also be carried in the feet of cattle and goats, although cattle strains are usually benign for sheep. However some goat strains of D. nodosus are particularly virulent for sheep, so goats should be included where eradication of the disease is attempted. 

The cost of eradication can be considerable when compared with the cost of management of endemic footrot in the flock. Eradication is, therefore, only suitable for closed flocks in drier parts of the UK. 

The list of other sheep disease problems reported during July 2004 includes:

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  Copper poisoning
  
  
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  Photosensitisation in hill Cheviot lambs associated with grazing pastures with large amounts of bog asphodel
  
  
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  Death of sheep after breaking into a garden
  
  
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  Polioencephalomalacia
  
  
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  Erysipelas arthritis
  
  
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  Haemonchosis in bought-in sheep

PHOTOSENSITISATION CAUSED BY BOG ASPHODEL POISONING

These diseases have been discussed in recent NADIS sheep disease focus articles. Your vet can advise you about their diagnosis and management in your flock.

• While every effort is made to ensure that the content of this forecast is accurate at the time of publication, NADIS cannot accept responsibility for errors or omissions. All information is general and will need to be adapted in the light of individual farm circumstances in consultation with your veterinary surgeon.

Copyright © NADIS 2002