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Friday 22 June 2018

Don’t get hit with a hidden disease - National sheep flock facing 'iceberg' of diseases

  • Don’t have your head in the sand about these diseases – know your own flock status
  • Talk about these diseases – especially with your vets and the people you buy sheep from
  • Check your flock security and don’t unwittingly buy in someone else’s problem
A winning pen of sheep at the Mayo Mules and Greyface Breeders Show and Sale at Ballinrobe Mart
A winning pen of sheep at the Mayo Mules and Greyface Breeders Show and Sale at Ballinrobe Mart
FarmIreland Team

FarmIreland Team

The threat to the national sheep flock from ‘iceberg diseases’ might be considered an amusing add-on to this evening’s Teagasc National Sheep Conference, given that Irish stock are unlikely to find themselves marooned on such floating glacial entities.

However, the subject of the paper is far from light-hearted and addresses the serious challenges posed by such diseases as OPA (Jaagsiekte), Ovine Johnes Disease, CLA (Caseous Lym­phadenitis), Maedi Visna (MV), Border Disease, TB, and Scrapie.

These conditions are termed ‘iceberg diseases’ be­cause the number of identi­fied clinical cases is dwarfed by the large number of undiagnosed, or submerged, cases in the flock.

The importance of these emerging diseases will be addressed by Dr Fiona Lo­vatt, a sheep vet consultant from England at the confer­ence this evening which is being held in the Loughrea Hotel, Loughrea, Co Galway.

Maedi Visna

Is currently not thought to be present in Ireland, however its prevalence is increasing in UK so some brief information about it is included here.

In Maedi vets are seeing respiratory signs & heavy lungs in older sheep as well as mastitis.

In the Visna form of the disease, we see progressive nervous signs (classically dragging a hind leg). MV is caused by a lentivirus (a non-oncogenic RNA retrovirus) and spread by oro-nasal infection (like a common cold).

The lentivirus becomes latent in the genome of white blood cells so it evades the host immune system. In the UK the control of MV has been by an accreditation scheme though there has been an increase in the numbers of cases throughout the country in recent years.

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It is important that Irish producers are aware of the disease and the need to strictly comply with importation requirements to keep it out of Ireland.

OPA

Symptoms: Increased respiratory rate with a soft cough. Sheep remain bright with a good appetite unless a secondary bacterial pneumonia develops (this becomes more common in affected flocks). Later in disease, lots of clear frothy fluid flows from nostrils when head down.

Survival: Jaagsiekte sheep retrovirus (JSRV) will survive several weeks in the environment

Spread: By respiratory aerosol & via contaminated troughs & water. Transmitted via milk & colostrum. Uterine spread low if at all.

Pathology: Oronasal infection leads to a cancer in the lung with lots of white foaming fluid. Incubation is generally 3 years long but it can be seen from 2 mths to 11 yrs old

Diagnosis: No immune response so no blood serology possible. PCR is possible in the respiratory system & peripheral blood lymphocytes but it is not commercially available and it has a low sensitivity as virus levels fluctuate. Thoracic ultrasound can used to identify infected animals if the lesions are greater than 2cm and on the surface of the lung.

Control options

  1. Identify & cull infected ewes and their offspring.
  2. House & manage in single age groups – especially keep young sheep separate from adults
  3. Reduce close contact – consider housing, stocking density, trough feeding.
  4. Snatch lambing & rear artificially – shown to be successful in a German case study

Context: Prevalence of OPA in Ireland appears to be similar to the UK with JSRV-positive sheep identified in the counties of Donegal, Kerry, Kilkenny, Offaly, Tipperary, Waterford and Wicklow, often clustered within the same flocks. Nearly 2000 adult sheep were investigated and 1.6% were found to be infected with JSRV and 0.5% with OPA.

Ovine Johnes Disease

Symptoms Sheep get very thin and emaciated but do not scour like cattle with Johnes do. Increase in n and emaciated but do not scour like cattle with Johnes do. Increase in culling rates due to poor condition

Survival 9 months in water, 11 months in slurry, 48 months in soil

Spread Ingested orally. Invades & multiplies in small intestine lymphatic tissue & mesenteric lymph nodes. Shed in faeces of clinical & pre-clinical cases. Found in milk & placenta of clinical cases. Any age is susceptible but especially if young.

Pathology 70% of cases are multibacillary – lots acid-fast bacilli in mucosal macrophages. Villous fusion, gut thickening, humoral antibody response 30% of cases are paucibacillary –few or no bacilli. Cell-mediated response but little gut change or antibody response.

Diagnosis ELISA test detects 3-6 months post infection or AGIDT test – both have good specificity but poor sensitivity (as low as 50% for paucibacillary cases) – so lots of false-negatives. PCR on 50% for paucibacillary cases) – so lots of false-negatives. PCR on faeces works well and can pool faeces from up to 10 animals Most reliable diagnosis by postmortem histopathology of small intestine and mesenteric lymph nodes

Control options

1. Limit spread by early culling of clinical cases & remove offspring.

2. Test & cull. Blood testing, faecal culture or PCR two to four times a year. Expensive

3. Management – limit lambing in wetter areas. Keep young stock away from adults. Use automatic water feeders & strict hygiene inside.

Vaccine Gudair (Virbac) is now available in UK - suggest vaccinated everything below one year old & all bought-ins. Vaccinates will still shed Johnes in faeces & have detectable Ab so not suitable for export. Review of several papers indicates: Vaccination reduces culture +ve sheep >70%, lesions by 96%, clinical cases by >50%

CLA - Caseous lymphadenitis

Symptoms Discrete swellings around the head and neck which can burst to discharge a thick pus

Survival This bacterium with a lipid coat survives for 8months in soil, 24h in dip Spread Incubation is 6 weeks - 4months as it spreads from infected wound to the draining lymph node.

Spreads by close contact – at clipping, dipping, especiallyshowering and post-dipping. Also respiratory spread in droplets. Very low prevalence in sheep up to 1 year old and little spread to pre-weaned lambs

Diagnosis Difficult to diagnose (especially once the abscess becomes walled off) Blood test ELISA (Sensitivity=87%, Specificity= 99%) which means a few false positives & lots of false negatives. Confirm a positive by Western Blot test. SAC Monitoring Scheme tested pre-sale (Isolate for 12 weeks & then required 2 negative ELISA taken 6 weeks apart)

Control options Restrict movement & contact with infected flock. Avoid any nose-to-nose contacts at sale. Check flock regularly. Handle young sheep first. Keep ram lambs separate. Investigate suspicious cases. Avoid puncturing abscesses unnecessarily. Don’t handle closely in dusty yards. Quarantine infected premises for a long time.

Vaccine Glanvac available from Zoetis Australia – VMD reports that approximately 25000 doses are imported into UK each year & it is estimated that this is half the total imported Research at Moredun has developed a DIVA vaccine but not commercially available.

Border Disease

Is caused by a pestivirus with very similar epidemiology to BVD in cattle. It is transmitted via the oro-nasal route with the persistently infected (PI) animals as important reservoirs of infection in a flock.

Pregnant ewes are at most risk as they either abort or produce PI lambs that are either weak, normal or ‘hairy shakers’ that continually shed virus.

Border Disease was considered fairly important in Ireland with a prevalence of 5.6% of individuals and 46% of flocks though many of these were found to actually be BVD virus and suggested to be a ‘ spill-over’ from cattle2 .

Hopefully these figures may have reduced as a result of the cattle BVD eradication scheme.


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