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.


September 2004

By Neil Sargison BA VetMB DSHP FRCVS

NADIS Sheep Disease Forecast

Twelve months ago, the main concern of most sheep farmers was a lack of grass following the driest autumn weather for many years. 

At the same time, conditions for silage and hay making and for harvest had been ideal. In contrast, this year’s harvest proved to be one of the hardest for many years, while most farms now have more grass than usual.

Parasitic gastroenteritis (worms)
While there were few NADIS reports of parasitic gastroenteritis during August 2003, there were numerous reports during August 2004 of scour and ill thrift in lambs caused by worms.

SCOUR AND ILL THRIFT DUE TO HELMINTH PARASITISM (WORMS)

Once pastures have become heavily contaminated with worm larvae, it is difficult to achieve satisfactory production in lambs which graze on those pastures. It is often reported that lambs improve for a few days after dosing with a conventional wormer, after which they start scouring and appear ill thrifty again. 

This is not necessarily caused by anthelmintic resistance, but results from damage to the stomach lining caused by newly acquired larvae from pasture. The problem may be overcome if lambs are moved to safe pasture after worming, although this is often impractical on sheep-only farms. 

In practice safe grazing means pasture which has not been grazed by sheep or goats during the previous 12 months. Most cattle worms do not survive in sheep and vice-versa. Safe pasture can, therefore, be generated through the annual rotation of grazing by the two species, although to be sustainable both species should occupy a similar proportion of the grazing area. 

The inclusion of cereal crops, brassicas and silage in the rotation can also be useful. However, grazing management has implications for the whole farm and few commercial sheep farms are able to provide sufficient safe pasture for the purpose of nematode parasite control in sheep, without compromising the efficiency of crop or cattle production.

When it is necessary to return lambs to heavily contaminated pasture after dosing, the use of a persistent wormer, moxidectin, should be considered, to ensure that freshly acquired larvae are killed before they can cause damage. Unfortunately, the persistence of moxidectin differs depending on the helminth parasite (worm) species present. 

Oral moxidectin persists for at least 3 weeks against Teladorsagia circumcincta, the commonest cause of parasitic gastroenteritis in lambs during the summer and autumn, but only achieves limited persistence against Trichostrongylus vitrinus (or T. colubriformis in southern England) which can become important in some flocks during the autumn and early winter.

It is therefore necessary to monitor the performance of lambs after dosing and repeat treatments may be required in some flocks after a few weeks. The meat withdrawal period for oral moxidectin in lambs is 14 days.    

Susceptible sheep grazing on helminth worm-contaminated pasture invariably suffer from production losses through reduced growth rates. The aim of effective nematode parasite control during spring and summer months was to limit the exposure of susceptible lambs to significant burdens of infective larvae on pasture.

If lambs were weaned on to clean grazing at weaning, further anthelmintic treatments should not be necessary at this stage, but if safe grazing has not been available, further treatments will probably be needed during September. At this time of year monitoring of faecal egg counts can be useful to determine the timing of anthelmintic treatments. 

Bulked freshly voided faecal samples from 7 to 10 animals can provide useful information. Samples of about 2 grams (a teaspoon full) can be easily collected when sheep are disturbed and moved into the corner of a field. Your vet will be able to arrange for the egg counting and provide advice on the basis of the results.

Anthelmintic resistance
New cases of benzimidazole, levamisole and ivermectin resistance were identified during August. This is a good time of year to check for the presence of anthelmintic resistance in your flock. 

Bulked freshly voided faecal samples should be collected 7 – 14 days after worming (except where a persistent-acting wormer such as moxidectin has been used) and submitted to your vet for egg counting. 

Positive post-treatment faecal egg counts indicate that the drench was not 100% effective, although an assumption is made that adult egg-laying parasites were present before drenching.

 

FAECAL WORM EGG COUNTS 7 – 14 DAYS (FOR LEVAMISOLE AND BENZIMIDAZOLE/IVERMECTIN RESPECTIVELY) AFTER WORMING PROVIDES AN INDICATION ABOUT THE EFFICACY OF THE DRENCH

Health status of purchased animals
Autumn sales are now well underway and so far prices have been little changed from last year. 

Good market prices provide no guarantee of freedom form infectious diseases, so it is important to check the health status of purchased animals and avoid the introduction of new problems to your flock.

The following diseases are commonly introduced with purchased animals:

  • sheep scab
  • footrot
  • enzootic abortion
  • anthelmintic resistant worms
  • Haemonchus contortus

This list is by no means comprehensive and your vet can advise you about specific problems to be aware of in your local area. Introduced sheep should be checked for the presence of footrot, orf and caseous lymphadenitis. If these problems are identified, your vet can advise you about their treatment and control. 

Furthermore, the management of problems which may already be present in the flock such as clostridial diseases, abortion, pneumonia, footrot, liver fluke and parasitic gastroenteritis should be addressed in these purchased animals.

The clostridial vaccination status of replacement animals should always be checked, and a vaccination course started immediately if required. The need for other vaccination programmes depends on the disease status of the flock.
 
Quarantine treatment for anthelmintic resistant worms
The report during August 2004 of multiple resistance to benzimidazole, levamisole and ivermectin wormers in a terminal sire flock illustrates the need for quarantine anthelmintic treatment of all introduced sheep. 

The management practices which led to the development of resistance in this flock differ little from those practised in most pedigree terminal sire flocks. Sheep from these flocks are dispersed throughout the country after the autumn ram sales, potentially spreading multiple anthelmintic resistance.

 

ALL PURCHASED SHEEP SHOULD BE WORMED ON ARRIVAL

All purchased sheep (and goats) should be treated with moxidectin or a combination of a group 3 (ivermectin, doramectin or moxidectin) and group 2 (levamisole or morantel) wormer administered sequentially. Injectable moxidectin has the added benefit of controlling sheep scab, provided that the basic principles of scab prevention (NADIS Sheep Disease Focus – September 2004) are adhered to. 

Animals should be yarded or placed in an area which will not be used by sheep for the next 9 months for at least 24 hours after treatment. Ideally, they should then be moved to worm-contaminated pasture rather than ‘clean’ or ‘safe’ grazing. Your vet can advise about the best quarantine anthelmintic treatment regime for your flock.   

Enzootic abortion
Enzootic abortion-free flocks should only introduce sheep from flocks of similar status. In flocks which have an endemic enzootic abortion problem, optimal control of the disease can be achieved through the purchase of Premium Health Scheme accredited enzootic abortion-free replacements and their vaccination at least 4 weeks before mating.

 

CHLAMYDIAL (ENZOOTIC) ABORTION CAN BE MANAGED BY VACCINATION

Unfortunately, the major live enzootic abortion vaccine, ENZOVAX, is not currently available.  However, the alternative live vaccine, CEVAC CHLAMYDOPHILA, and the killed vaccine, MYDIAVAC, are still currently available. 

Your vet can advise you about the use of these vaccines in your flock, including discussion of any complications, such as possible incompatibility with toxoplasmosis vaccine.

Ram breeding soundness 
Last autumn there were several NADIS reports of abnormalities in the scrotal contents of purchased replacement rams. These cases serve as a reminder to palpate the scrotal contents of all replacement rams to ensure that they are sound for breeding purposes. 

This is especially important where groups of ewes are to be single sire mated, or where a high ratio of rams to ewes is required following synchronisation with intravaginal sponges.

 

EPIDIDYMITIS IN A SHEARLING RAM

Sound rams during the breeding season should have two large, firm testes, which are similar in size. The tails of the epididymes should be firm and smooth and heads of the epididymes and spermatic cords should be free from nodular defects or hard swellings.  Mature Suffolk rams should have a scrotal circumference of 30 – 35 cm. 

Rams with large symmetrical scrotal contents free of defects are likely to produce large quantities of good quality semen, whilst those with small soft testes or other palpable defects are likely to produce poor quality semen.

 

BRISKET SORES SELDOM HEAL AND OFTEN RESULT IN PERMANENT USOUNDNESS FOR BREEDING. WHENEVER HARNESSES ARE USED, CARE SHOULD BE TAKEN TO ENSURE THAT THEY ARE CORRECTLY FITTED.  THEY SHOULD BE REGULARLY CHECKED AND ADJUSTED.

Vaccination against toxoplasmosis
Unfortunately in many cases TOXOVAX was again unavailable this year for the prevention of high barren rates and abortions due to toxoplasmosis in replacement hoggs or gimmers. In these flocks alternative strategies such as feed medication with decoquinate could be considered to mitigate against late toxoplasma abortions. 

Following exposure, ewes develop a life-long immunity, therefore if exposed to a contaminated environment prior to first pregnancy they don’t abort. This situation is difficult to reproduce in practice, where on infected farms, ewes are 4 years-old before 90 percent have T. gondii antibody. 

Vaccination is, therefore, the most obvious method of control. TOXOVAX is a live vaccine composed of strain 48 of Toxoplasma gondii which following vertical transmission through several generations of mice, has lost the ability to form bradyzoites in tissue cysts and is, therefore, unable to complete its lifecycle. 

Replacement breeding ewes should be vaccinated no less than 3 weeks before mating.  TOXOVAX is claimed to provide protection for 2 seasons, but in practice subsequent boosters are not required. 

Ideally sheep should not be vaccinated with a live enzootic abortion vaccine within 4 weeks of TOXOVAX vaccination, although in practice it is acceptable to administer both products on different sides of the neck at the same time. 

Severe orf following plunge dipping
An unusual skin disease problem was reported in a flock of about 1600 Blackface and Texel cross Blackface ewes and their lambs, about two weeks after they had been plunge dipped in diazinon for the control of blowfly strike. The sheep had all been dipped in one day in a 2000 litre dipper, which was sited at the end of a series of muddy pens. 

The sump volume of the dipper had been checked beforehand and the dipper initially charged with the correct concentration of diazinon. The dip bath had been regularly replenished, but had not been emptied and refilled during the session.

 

UNUSUAL PROLIFERATIVE SKIN LESIONS OVER THE FLANK OF A TEXEL CROSS LAMB

About 10 percent of a group of about 200 crossbred ewes and their 220 lambs were severely affected and many were also badly fly struck. 

This worst affected group had been dipped last, and the fact that many were flystruck indicated that despite accurate charging and replenishment of the dip bath, dipping had been ineffective.

 

MANY SHEEP WERE FLYSTRUCK, ONLY 2 WEEKS AFTER PLUNGE DIPPING IN DIAZINON

The problem had arisen as a result of heavy soil and faecal contamination of the dip bath, which would have bound the diazinon, thus reducing its effective concentration. 

Furthermore, the contamination of the dip would have made the sheep more attractive to blowflies than before they were dipped.

 

THE SKIN DISEASE WAS CAUSED BY A COMBINATION OF ORF AND DERMATOPHILLOSIS, INFECTION HAVING BEEN SPREAD IN THE DIPPER

The list of other sheep diseases reported during August 2004 includes:

  • Ringworm in Suffolk and Cheviot shearling rams
  • Pyoderma in Suffolk rams
  • Johne’s disease in ewes
  • Pneumonia in lambs
  • Polioencephalomalacia (CCN) in ewes and lambs
  • Rape scald (photosensitisation) in Texel cross lambs
  • Lameness in sheep of all ages due to scald and footrot

The diagnosis and management of all of these diseases has been described in recent NADIS sheep disease forecasts.


• 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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