NADIS disease forecast – sheep (December part 2)

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.

December 2004

By Neil Sargison BA VetMB DSHP FRCVS

NADIS Sheep Disease Forecast – Part 2

Part 1

Case 3 – pregnancy toxaemia

What are the possible causes of the clinical signs shown by these ewes?
The disease history and clinical signs suggest a diagnosis of pregnancy toxaemia, although other diagnoses such as listeriosis, early cases of hypocalcaemia and CCN should be eliminated similar clinical signs are seen in individual ewes.

Pregnancy toxaemia is a common disease of undernourished, stressed ewes carrying multiple foetuses, associated with a failure to adapt to the increasing metabolic demands of foetal growth during late pregnancy. 

How would your vet investigate this problem?
The diagnosis of pregnancy toxaemia is based on the clinical signs of depression, apparent blindness, salivation and fine muscle tremours of the face and ears (Fig 3.4). 


FIG 3.4

The diagnosis can be confirmed by blood sampling affected ewes to determine their serum concentrations of 3-hydroxybutyrate. Values greater than 3.0 mmol/l indicate pregnancy toxaemia.

How would you treat the affected ewes and what sort of treatment response would you expect?
Various treatments including oral glycerol, glucose, propylene glycol and concentrated rehydration solutions, intravenous glucose and B vitamins and insulin have been advocated; however despite intensive therapy the response in confirmed cases is poor. 

Termination of pregnancy by elective caesarian or using corticosteroid injections could be considered in valuable ewes, but viable lambs are seldom delivered. In many cases, treatment may be uneconomic and humane destruction should be considered to prevent further suffering. 

How would you attempt to prevent further losses?
The occurrence of pregnancy toxaemia usually indicates an urgent need to increase the energy nutrition of the flock. In the short term this can be achieved by introducing ad-lib treacle (Fig 3.5). 


FIG 3.5

In the longer term it may be necessary to increase the amount and/or quality of the concentrate feed, and/or change to higher quality hay or silage. The energy deficit can be established by blood sampling about 10 ewes and comparing their mean 3-hydroxybutyrate concentration with a series of reference curves.  Feeding can then be feeding adjusted accordingly.

How would you prevent similar problems next year?
Skilful nutritional management throughout the second half of pregnancy is crucial to ensure that good scanning results result in high lambing percentages. Undernutrition often results in poor lamb survival associated with low birthweights and poor milk production, while overnutrition is wasteful and can result in dystocia (birth stress) problems. 

The adequacy of dietary energy supply relative to metabolic demands can be determined in most flocks by blood sampling about 10 ewes 4 weeks before lambing and measuring serum or plasma concentrations of 3-hydroxybutyrate (Fig 3.6). 


FIG 3.6


Separation of ewes into different feeding groups on the basis of ultrasound scanning results, ram harness marks, and body condition score can also serve to ensure adequate nutrition during late pregnancy and avoid wasteful overfeeding of late-lambing or single-bearing animals. 

Case 4 – chewing lice

What are the possible causes of itching in this flock?
The important causes of itching in a group of sheep are sheep scab, chewing lice, keds and severe dermatophilosis. In this case, the displacement of small tufts of wool over the body suggests chewing lice or ked infestation.

The incidence of chewing louse infestation of sheep has risen over the past few years, associated with the deregulation of sheep scab control measures and greater use of systemic endectocides (ivermectin, doramectin and moxidectin) for the management of sheep scab. 

How could the diagnosis be confirmed?
Chewing lice can be seen with the naked eye in wool partings over the back and flanks (Fig 4.3), both on the skin surface and moving over wool fibres up to 5 cm away from the skin. Adult chewing lice are about 2 mm long with a pale brown abdomen (Fig 4.4), while nymphal stages are smaller with a whitish coloured body. 


FIG 4.4


FIG 4.4

Lice can also be detected in the vegetable matter residue following testing of wool, although there is no mechanism for transfer of this information in the UK.  

Louse burdens are assessed by parting wool to expose a 10 cm band of skin at about 6 sites over the back and sides of the sheep. Adult lice tend to move away from light as the fleece is parted, so need to be counted quickly. As a rough guide, less than one louse per parting represents a light infestation of less than 5000 per sheep, while more than 5 lice per parting represents a heavy infestation of more than 250,000 per sheep.

What are the potential economic consequences of this problem?
Louse burdens vary between individuals, but the heaviest burdens are generally associated with young or old animals in poor health and low body condition. There is, therefore, a general perception that chewing louse infestations cause ill thrift and constitute a welfare problem, although it is not certain if louse infestations lead to a reduction in body condition or if they exploit sheep that are ill-thrifty for other reasons.

There have been few controlled studies to investigate the economic effects of chewing lice in UK sheep, but based on overseas studies, it seems unlikely that moderate burdens would have any effect on bodyweight or growth rates. 

Heavy infestations can reduce the manufacturing quality of wool, although this is only likely to be economically significant in fine wooled (less than 24 microns) breeds. An immune response to even small numbers of chewing lice can cause hide damage referred to as coccle which is seen in pelts after they have been tanned (Fig 4.5).


FIG 4.5

The presence of cockle is seldom reported to individual producers, and the substantial economic losses which it incurs are borne by all. Furthermore, potential hide damage will usually have already occurred by the time that the problem is diagnosed. However, should hides become traceable to the farm of origin, then chewing louse infestation would become economically important in UK flocks.

What are the options for the management of this problem?
Management of established louse infestations is problematic and the decision to treat chewing louse-infested sheep is not straightforward, because it is not certain that they limit animal production or constitute a welfare problem in UK flocks. In some cases, the option of leaving animals untreated should be considered, although the rationale for and risks associated with this strategy should be weighed up.

Louse numbers fall dramatically during periods of hot or wet weather, which can influence populations for the subsequent 6 months or more. In some circumstances, the prospect of such weather conditions may, therefore, remove the need to treat out-wintered long-fleeced sheep. 

Shearing significantly reduces the louse burden by removing a large proportion of the lice and exposing those remaining to the lethal effects of desiccation. However, shearing is usually impractical during winter months when most chewing louse infestations are seen. 

Organophosphate, pyrethroid and amide plunge dips can provide effective louse control, but winter dipping and disposal of dipwash solution is difficult on many farms. Shower dips are only effective in short wooled sheep and thorough saturation is essential. Despite common perceptions, systemic endectocide injections are ineffective for the control of chewing lice.

Most farmers, therefore, rely on the use of pyrethroid pour-ons for the treatment of chewing louse infestations (Fig 4.6). Pyrethroid pour-ons translocate over the body in wool grease. The area of wool grease, which increases greatly after shearing, determines the effective concentration of pyrethroid, so the effective concentration of insecticide on the skin is determined by the wool length. High doses of pyrethroids are required for long-wool treatments, which are expensive, seldom remove all of the lice and may select for resistance. 

Some animals continue to show signs of rubbing, nibbling and scratching for up to 6 weeks after treatment. This may be explained by the observations that pyrethroid pour-ons take up to 6 weeks to translocate over the body and when used in full-fleeced sheep. Treatments within 6 weeks of lambing fail to prevent infection of newborn lambs.  


FIG 4.6

Pyrethroid resistant chewing lice are widespread in Australia, and have also been identified in the UK. Continued use of pyrethroids as long-wool pour-ons for the treatment of infestations is likely to select rapidly for further cases of resistance.

Pyrethroid residues are potentially hazardous to wool handlers and damaging to the environment if wastewater from scouring plants enters a watercourse. Overseas, a voluntary 60 day withdrawal period is observed before shearing or slaughter to avoid wool residue problems. 

How would you prevent similar problems in future years? 
Optimum conditions for development of chewing lice occur in long-fleeced sheep during autumn and winter months. Unlike the situation with sheep scab mites, chewing louse populations on sheep only increase slowly, so severe infestations are seldom seen until late winter. Transfer of lice between animals requires close contact and spread of infection is slow. 

Organophosphate and pyrethroid plunge dips can provide effective louse control, but shower dips are only effective in short-wooled sheep and thorough saturation is essential. No dip products are licensed for use in shower dippers in the UK.

Pyrethroid pour-ons are most effective when applied immediately after shearing. Louse numbers only increase slowly. Therefore, provided that sheep do not become debilitated for other reasons, a single off-shears application of pyrethroid pour-on during the summer can provide all year round protection.   

Finally, I would like wish everybody reading this article a happy Christmas and hope that your sheep flock will remain disease free during 2005. I would also like to thank everybody who has allowed me to take photographs and describe their sheep diseases, in order to bring the risks to the attention of others and ensure timely preventive measures.    

Neil Sargison

• 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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