Iceberg diseases in sheep Flashcards
What are the 5 iceberg diseases in sheep?
- MV
- OPA
- CLA
- OJD
- BD
How do iceberg diseases often present? (& therefore why they are on many ddx lists)
- vague signs of ill thrift
- CS are often mild initially and occur late in the dz process
- typically occur in older sheep
- none are treatable and diagnostic tests need careful interpretation
Maedi Visna
- Lentivirus
- Long incubation period (months to years)
- Regional variation in prevalence (Leicestershire and Gloucestershire high)
– In high prevalence areas 15% of flocks may have MV, and within flock prevalence can be up to 85% in some flocks - There may be some genetic susceptibility
– Hill breeds appear less likely to carry the susceptibility gene
MV presentation
- Can be variable
- Often chronic wasting and laboured breathing
- Can present as an increase in mastitis, ill thrift or swollen joints
- Often see weakness on back end, altered gait around back legs as well
MV transmission
- Oronasal is the main route of transmission
- But can also be spread by:
– colostrum/milk
– semen
– trans-placental
– fomites
MV diagnosis
- Serological diagnosis (antibodies produced within weeks to months but tend to wax and wane)
MV tx/control
- No tx or vaccine
- Control methods include:
– purchasing from accredited flocks (or isolate and test)
– monitoring and culling positive animals (if practical)
– reducing stocking density
– preventing contact with neighbouring flocks
MV - what to do if high level of infection
- Cull flock & restock with accredited sheep
- Try and reduce losses through management:
– keep flock young
– split into older & younger sheep for management
– cull thin/suspect cases
- Run a less intensive system to reduce spread
Ovine pulmonary adenocarcinoma
= OPA
= Jaagsietke
- Retrovirus
- Long incubation period – 6 months to several years
- Prevalence in the UK is thought to be around 1-5.6%
OPA presentation
- Causes neoplastic proliferation of lung cells = adenocarcinoma
- This results in laboured breathing, increased respiratory rate, ill thrift and sudden death
- Resp signs due to fluid accumulation on the lungs and reduced gaseous exchange in the lungs
- Half of all cases of OPA present as sudden death (but not because OPA is an acute disease process – this is more to do with how stoic sheep are)
- Also see reduced reproductive performance, immune suppression and reduced milk yield
- Sheep with clinical signs are usually 3-4 years old, but lambs are infected at a young age
It’s thought that if a sheep has OPA and MV, OPA can increase the spread of MV (as have all the extra resp secretions coming from the adenocarcinoma)
OPA transmission
- Transmission mainly aerosol, but can be spread through milk and colostrum
OPA diagnosis
- No blood test commercially available for diagnosis
- Lung ultrasonography has been used but should be interpreted with caution
- Post-mortem the only definitive way to diagnose
OPA tx
- no tx or vaccine
OPA control
- Identify & cull infected & offspring – very difficult without test. Need to PM any sudden deaths/those showing ill thrift.
- Manage in single age groups – keep young separate from adults
- Reduce close contact – housing, stocking density, trough feeding. Need good hygiene. Housing for less time
- Snatch lambing & rear artificially – successful in German case study
Biosecurity also important with all infectious dz
Caseous lymphadenitis
= CLA
- Corynebacterium pseudotuberculosis
- Zoonotic (although rarely reported in people)
CLA transmission
- Transmission through skin abrasion, inhalation or ingestion – often when sheep are grouped
- Shearing is a common transmission time
