6.2.4: Iceberg diseases in sheep

Question 1

What are 5 common iceberg diseases in sheep?

Answer 1

• Maedi Visna
• Ovine Pulmonary Adenocarcinoma (OPA)
• Caseous Lymphadenitis (CLA)
• Ovine Johne’s Disease (OJD)
• Border Disease (BD)

Question 2

Maedi Visna causative agent and relevance of this

Answer 2

• Lentivirus
• Has long incubation period (months to years)

Question 3

How is Maedi Visna transmitted?

Answer 3

• Oronasal - mainly
• Colostrum/milk
• Fomites

Question 4

Describe regional variation in Maedi Visna

Answer 4

• (See up to date figures but) high prevalence in Leicestershire and Gloucestershire
• Tend to see more MV in lowland flocks compared to upland -> lowland have lower stocking density/housed more often or for longer

Question 5

Diagnosis of Maedi Visna

Answer 5

• Serology - but antibodies wax and wane so can’t rule it out even if negative result

Question 6

Treatment of Maedi Visna

Answer 6

No treatment

Question 7

Control of Maedi Visna

Answer 7

• Purchase from accredited flocks
• Test, monitor and cull positive animals
• Reduce stocking density
• Prevent contact with neighbouring flocks (e.g. double fencing/high fencing)

Question 8

If you have a flock with a high prevalence of Maedi Visna, what control measures could you implement?

Answer 8

1. Cull flock and restock with accredited sheep - may be economically not viable
2. Try to reduce losses through management i.e.
   * Keep flock young
   * Split older and younger sheep for management
   * Cull thin ewes/suspect cases
   * Run a less intestive system to reduce spread

Question 9

Ovine pulmonary adenocarcinoma causative agent

Answer 9

Retrovirus
Can have long incubation period of 6 months to several years

Question 10

Pathogenesis and clinical presentation of OPA

Answer 10

• Causes neoplastic proliferation of lung cells (=adenocarcinoma)
• Presents with increased RR, laboured breathing, ill thrift, sudden death

Question 11

Transmission of OPA

Answer 11

• Mainly aerosol
• Can be spread through colostrum/milk

Question 12

Diagnosis of OPA

Answer 12

• No blood test commercially avaiable
• Interpret lung ultrasonography with caution (anything that causes consolidation will look similar)
• No definitive diagnosis possible in live animals, only on PME

Question 13

Control of OPA

Answer 13

• Identify and cull infected and offspring - difficult without test
• Undertake PM of sudden deaths/those showing ill-thrift
• Manage in single age groups with youngstock separate from adults
• Reduce close contact -> consider housing, stocking density, adequate trough space, hygiene
• Snatch lambing is theoretically good but not practical for most flocks

Question 14

Causative agent of Caseous Lymphadenitis (CLA)

Answer 14

Corynebacterium pseudotuberculosis

Question 15

Transmission of CLA

Answer 15

• Through skin abrasion, inhalation or ingestion

Question 16

Clinical presentation of CLA

Answer 16

• Abscesses in LNs with characteristic green pus
• Typically around head and neck, sometimes inguinal/scrotal in tups, or potentially internal
• If abscesses on internal LNs, may only see weight loss

Question 17

Diagnosis of CLA

Answer 17

• Bacteriology - requires draining abscess so should quarantine this sheep
• Serology - high specificity, low sensitivity so cannot rule out just because test negative. Antibodies wax and wane and presence of antibody doesn’t necessarily mean bacterium has caused clinical disease.
• Repeat testing and examining at flock level is recommended

Question 18

Treatment and control of CLA

Answer 18

• Antibiotics not effective
• Vaccine not available in UK, can be imported on special licence but efficacy variable
• Control: buy from trusted sources, boundary biosecurity, separate infected animals

Question 19

True/false: CLA is zoonotic.

Answer 19

True
But it is rarely reported in people

Question 20

You suspect a sheep to have CLA. Is it a good idea to lance the abscess to aid diagnosis? Why/why not?

Answer 20

• Can lance abscess for bacteriology
• However this sheep could then spread infection from the draining abscess so you must ensure it is quarantined away from the rest of the flock

Question 21

If you choose to vaccinate CLA, which animals should you vaccinate?

Answer 21

• Vaccinate the lambs that are going to be reared as replacements
• Hopefully this will protect the flock in future

Question 22

If you choose to vaccinate for CLA, what should you be aware of?

Answer 22

• Vaccine not available in UK and has to be imported under Special Import Licence
• Vaccine efficacy is variable especially against UK strains
• Vaccine interferes with testing/eradication programmes that use serology (not DIVA)

Question 23

Causative agent of Ovine Johne’s Disease

Answer 23

Mycobacterium avium spp. paratuberculosis (MAP)
* There are cattle and sheep strains (C and S) and sheep are susceptible to both

Question 24

Breed? Disease?

Answer 24

• Suffolk
• Shows low BCS -> suspicious of any chronic wasting disorder
• This animal has Ovine Johnes Disease (OJD)