NADIS is a network of 40 veterinary practices and six veterinary colleges monitoring diseases of cattle, sheep and pigs in the UK.

NADIS data can highlight potential livestock disease and parasite incidences before they peak, providing a valuable early warning for the month ahead.


NADIS disease forecasts are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare. The forecasts are based on national trends and farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

December 2004

By Neil Sargison BA VetMB DSHP FRCVS

NADIS Sheep Disease Forecast – Part 1

Part 2

Weather conditions over the past month have been mostly damp and mild. While total rainfall during November was less than usual, the water table is already high and fields have remained wet. Ploughing and muck spreading have been impossible on many farms, and not surprisingly the incidence of sheep foot lameness has been high. 

 

19TH DECEMBER – HOPEFULLY FROSTY WEATHER WILL PERSIST AND OFFER SOME RESPITE AGAINST MUDDY CONDITIONS

While ewes were mostly in moderate body condition at the beginning of November and reasonable pregnancy rates can be expected in April lambing flocks, careful nutritional management will be required for the remainder of pregnancy to ensure that they maintain adequate body condition and avoid metabolic diseases.  Many flocks will be scanned in January to determine foetal numbers and enable precise feed management.

Ultrasound scanning of a January lambing Suffolk flock showed a 20% barren rate.  Further investigation (NADIS Sheep Disease Forecast – November 2004) showed the cause of the problem to be sub acute liver fluke.  While the scanning results were disappointing, they served to draw attention to what could have proved to be a far more serious disease problem.  Had prompt treatment for liver fluke not been instigated, ewe deaths might have ensued.      

There have been few reports of significant sheep disease problems arising during the two weeks since writing the November disease forecast. This situation will inevitably change once January lambing commences and the demands of later pregnancy start to tell in spring lambing flocks. 

In a change to the format which has been developed over the past four years of writing these NADIS Sheep Disease Forecasts, this month’s report takes the form of a case quiz, describing a few of the sheep disease problems to look out for during the next few weeks. 

Case 1
At the end of January, you notice a cast-for-age Blackface ewe in your neighbour’s field nibbling at its flanks.  On closer inspection, the wool over the neck and shoulders appears to be discoloured and matted and areas of wool loss are visible over the flanks (FIG 1.1).

 

FIG 1.1

On returning to your own Halfbred ewe flock, which is due to start lambing in 3 weeks time, you notice that several ewes have similar discolouration and matting of the wool over their neck and shoulders (Fig 1.2).  Tags of wool are also seen on your fence (Fig 1.3).

 

FIG 1.2

 

FIG 1.3

  • What are the possible causes of this problem?
  • Should you be concerned?
  • How would your vet go about confirming the diagnosis?
  • How should the problem be managed?
  • How might similar problems be prevented in future years?
  • These questions are discussed at the end of this report.

Case 2
Seven of your 120 April-born Texel cross ewe lambs died suddenly during the first week of December.  You notice that another three lambs are severely depressed, with tucked up abdomens, repeated grunting and laboured breathing (Fig 2.1).  These lambs have pale ocular mucous membranes (Fig 2.2).  The remainder of the flock appears to be dull and lethargic.

 

Fig 2.1

 

Fig 2.2

  • What are the possible causes of this problem?
  • How would your vet confirm the diagnosis?
  • How should the problem be managed?
  • What sort of treatment response would you expect?
  • How might similar problems be prevented in future years?
  • These questions are discussed at the end of this report.

Case 3
During the second week of December, 8 of your 250 Dorset cross ewes, which are housed and due to start lambing at in 2 weeks time (Fig 3.1), are showing signs of: isolation from the main group; depression; apparent blindness; and fine muscle tremours of the face and ears (Fig 3.2).

 

FIG 3.1

 

FIG 3.2

The ewes are being fed hay and about 0.75 kg per day of a mixture of barley, sugar beet pulp and protein balancer mix (Fig 3.3), with an estimated metabolisable energy (ME) value of 13.5 MJ/kg DM and 18% crude protein (CP) content.

 

FIG 3.3

  • What are the possible causes of the clinical signs shown by these ewes?
  • How would your vet investigate this problem?
  • How would you treat the affected ewes?
  • What sort of treatment response would you expect?
  • How would you attempt to prevent further losses?
  • How would you prevent similar problems next year?
  • These questions are discussed at the end of this report.


Case 4
In January, you notice that several of your 120 Greyface ewe lambs are repeatedly rubbing against fences (Fig 4.1), nibbling at their shoulders (Fig 4.2) or clawing at their flanks. Small tufts of wool are displaced over the body.

 

FIG 4.1

 

FIG 4.2

  • What are the possible causes of itching in this flock?
  • How could the diagnosis be confirmed?
  • What are the potential economic consequences of this problem?
  • What are the options for the management of this problem?
  • How would you prevent similar problems in future years? 
  • These questions are discussed at the end of this report.

Case 1 – sheep scab

What are the possible causes of this problem?
The important differential diagnoses are sheep scab, chewing lice, keds and severe dermatophilosis, although the signs of discolouration and matting of the fleece over the neck and shoulders, with wool loss over the shoulders and flanks indicate sheep scab.

Should you be concerned?
Sheep scab is an important production limiting disease and serious welfare concern (Fig 1.4). The disease is highly contagious and rapidly progressive. Economic loss is incurred in the substantial cost of treatment and increased feed costs to compensate for loss of body condition. 

In young and growing lambs, sheep scab can result in rapid weight loss, debilitation and death. Slaughterhouse condemnation of carcases, practical difficulties of removing the pelts from infested sheep, downgrading of leather, and unwillingness of severely affected rams to work further add to the economic importance of sheep scab. 

 

FIG 1.4

The Animal Health Act 1981 [Sheep Scab Order 1997] prohibits movement of sheep which are visibly affected with sheep scab, requires treatment of visibly affected sheep, and gives local authorities powers to deal with sheep scab on common land. 

How would your vet go about confirming the diagnosis?
Confirmation of the diagnosis of sheep scab is relatively straightforward. The presence of Psoroptes ovis mites can be demonstrated using a piece of clear adhesive tape first applied to the skin adjacent to areas of wool loss, then stuck to a microscope slide and examined. The mites have distinctive oval bodies, pointed mouthparts and funnel-shaped suckers at the ends of three-segmented pedicels on the first and second pair of legs (Fig 1.5).

 

FIG 1.5

If microscopic examination is not possible on site, it is necessary to collect wool samples from several sites for submission to a veterinary laboratory.  However, this method is less reliable and may result in delay before treatment. 

How should the problem be managed?
All of your sheep must be treated immediately. It will also be important to ensure that your neighbour’s sheep are treated at the same time.  

The only treatment options are:

  • organophosphate plunge dipping 
  • high-cis cypermethrin plunge dipping
  • two subcutaneous injections of ivermectin, seven days apart
  • a single intramuscular injection of doramectin
  • two subcutaneous injections of moxidectin, seven days apart

Plunge dipping is undoubtedly the most reliable treatment method (Fig 1.6). However, it will be impractical in this case, where the ewes are heavily pregnant. 

 

FIG 1.6

While plunge dips kill sheep scab mites immediately, systemic endectocide injections may take several days to kill all of the mites. If treatment is delayed, this could cause problems of ewes infecting their newborn lambs. Sheep scab mites can survive off their sheep host for up to 17 days. 

However, ivermectin doesn’t achieve significant persistence, so cannot be used if the sheep are to be returned to the same fields, pens or buildings within a 17 day period.  Avoidance of these areas will probably prove impossible given the impending start of lambing.  Doramectin only persists for about 15 days. 

In most circumstances this is adequate for the treatment of sheep scab, although treatment failure has been attributed to this marginally inadequate persistence. Moxidectin persists for more than 17 days, but the data sheep requirement for two injections for the treatment of sheep scab makes this option expensive. However, the second injection may provide additional benefits for the control of the ewes’ rise in faecal worm egg output after lambing.   

How might similar problems be prevented in future years?
Control of sheep scab depends on co-ordinated, careful plunge dipping or endectocide injection, involving all flocks in a geographically defined area and taking into account the survival of the scab mite off the host, time taken to kill mites and persistence of the product used. This approach has been very successful in parts of the south-east of Scotland, where sheep scab was previously endemic. The principles of sheep scab control were described in a recent sheep disease focus article, (Prevention of sheep scab – September 2004).

Case 2 – sub acute liver fluke

What are the possible causes of this problem?
The main differential diagnoses for unexpected death of lambs during the autumn and early winter months are:

  • clostridial diseases
  • redgut
  • systemic pasteurellosis
  • grain overload
  • sub-acute liver fluke

This list is by no means comprehensive, for example in some areas, specific diseases such as louping ill may be as important as any of those listed. 

Clostridial diseases can often be ruled out on the basis of an effective vaccination programme, while redgut, grain overload and are systemic pasteurellosis are usually associated with a recent change in feed management. The clinical signs observed in the case described differ from those associated with most other causes of sudden death and are highly suggestive of sub-acute liver fluke. 

How would your vet confirm the diagnosis?
Determination of the cause of sudden death in lambs usually relies on postmortem examination of freshly dead animals. In this case, postmortem examination of two recently dead lambs revealed anaemic carcases, with extensive, acute peritonitis (Fig 2.3) and enlarged mottled livers. The livers showed irregular fibrosis, haemorrhagic tracts extending from the capsule into the parenchyma, prominent bile ducts and multifocal 1 – 10 mm abscesses (Fig 2.4).

 

FIG 2.3

 

FIG 2.4

Immature liver flukes measuring 2 – 8 mm (Fig 2.5) were recovered from cut sections of livers.

 

FIG 2.5

Postmortem examinations are not always possible, so ancillary diagnostic tests may be required to support a provisional diagnosis. Sub acute liver fluke usually results in profound changes in blood protein concentrations and up to 30 fold increases in serum concentrations of liver specific enzymes. The diagnosis of sub acute liver fluke can be also supported by real-time ultrasonographic examination.  

Sub acute liver fluke is associated mostly with the presence in the liver and peritoneum of thousands of migrating early and later immature flukes. However, a few young adult flukes are sometimes found in the bile ducts and gall bladder. Thus, the diagnosis can occasionally be supported by the identification of small numbers of fluke eggs in faecal samples (Fig 2.6).

 

FIG 2.6

How should the problem be managed?
For the treatment of sub acute liver fluke it is necessary to use triclabendazole which has good efficacy against early immature flukes. When “safe” dry pasture is available, a single treatment is given before moving the sheep. 

However, when such pasture is not available, repeated treatments at 3 week intervals are required throughout the high risk period of autumn and early winter. Alternative flukicides, nitroxynil and closantel only kill adult and immature flukes from 6 weeks, while oxyclozanide, albendazole and netobimin only kill adult flukes. These drugs are not useful for the treatment of sub acute liver fluke.

What sort of treatment response would you expect?
While some deaths may occur shortly after treatment, associated with stressful handling, the response to oral triclabendazole treatment is usually rapid. Provided that sheep don’t become re-infected, reasonable subsequent growth rates can be expected.    

Concern has been expressed in recent years that in some western areas of Ireland and Scotland, poor responses to triclabendazole treatment may be due to the emergence of flukicide resistance. 

How might similar problems be prevented in future years?
The risk of sub acute liver fluke can be predicted using knowledge of its lifecycle and weather conditions. The standard recommendation for fluke control in known fluke areas is that: all sheep and cattle are strategically treated in late spring to remove adult flukes, in October/November to remove immature flukes and in January/February to remove immature and adult flukes; in years when a high risk of liver fluke is forecast, extra strategic treatments are given 6 weeks after these standard treatments. However, in some high risk areas sheep need to be treated at 3 to 4 week intervals from October to February.

Where possible sheep should be grazed away wet areas during high risk periods.  Drainage and fencing off wet and boggy areas may help, but is often impractical. 

 

FIG 2.7

Following a recent series of wet summers, sub acute liver fluke is now common in eastern parts of the UK, where the disease was not previously seen. Sub acute liver fluke is sometimes particularly serious in these areas because it is unexpected and preventive treatments are not routinely given. 

Once liver fluke has become established on farms, the risk of disease may persist despite dry summer weather, because infected snail populations may be maintained in microhabitats such as ditches and ponds (Fig 2.7).   The need for future preventive treatments should therefore be based on assessment of the disease risk.

Where triclabendazole resistance is suspected, the treatment of sub acute liver fluke relies on the use of flukicides such as closantel or nitroxynil, which are ineffective at removing early immature flukes and may not cure the disease. In these flocks, future control of liver fluke depends on the spring treatment of all grazing livestock, with a flukicide other than triclabendazole, to remove adult fluke and thereby reduce miracidial infection of snails.

Part 2


• 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



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