The iceberg phenomenon is a metaphor coined in human medicine to describe a disease in which, for every visibly affected individual, the population will contain numerous others that are sub-clinically infected, carriers or undiagnosed clinical cases. What is seen clinically is a small representation of the overall infection.
In sheep, the term is used to describe chronic infectious conditions that are insidious in onset and/or spread and have a huge and increasing impact on the productivity and welfare of the national flock. They may affect ewe longevity and health as well as impacting lamb viability and growth. Examples in the UK include: ovine pulmonary adenomatosis, ovine Johne’s disease, border disease and maedi visna.
Ovine pulmonary adenomatosis/Jaagsiekte (retrovirus)
- Spread via aerosol, contaminated troughs and water, colostrum and milk
- Can survive for several weeks in the environment
- Infection causes neoplastic proliferation in lungs
- Incubation can be up to three years
- Clinical picture includes weight loss, dyspnoea with sheep tending to remain bright, alert and have good appetite, often frothy nasal discharge and many (but not all) showing positive with the “wheelbarrow test”
- Invariably fatal
- Can be diagnosed with transthoracic ultrasound; with experience, the operator will identify tumours greater than 1 to 2cm in size
- Cull ewe and perform fallen stock post-mortem examination screens
Ovine Johne’s disease (Mycobacterium avium paratuberculosis)
- Spread via oral ingestion from faeces, milk and placenta
- Mostly young animals below six months of age
- Survives in water, slurry and soil for up to 48 months
- Clinical picture shows weight loss and bottle jaw
- Increased susceptibility to gastrointestinal parasites may mask underlying problem
- Generally, clinical signs are seen in animals over two years old
- Can be diagnosed with ELISA Ab serology in clinical cases – but has poor sensitivity as flock screen; PCR for MAP in faeces may be pooled and combined with culture in flock screening
Border disease (pestivirus BDV and BVDV)
- Spread via horizontal transmission from ewe to lamb and via vertical transmission in saliva, milk, semen and by aerosol
- Short lived in the environment, the disease persists in a flock as a result of persistently infected (PI) lambs
- Causes higher than expected barren rates (above 5 percent); hairy shaker lambs (poorly grown, rough hair coat) can become infected with cytopathogenic virus and die between two and four months of age; survivors shed virus and are likely to have poor growth rates
- Diagnosis made via ELISA Ab serology, PCR antigen in rams and in a PI search
Maedi visna (lentivirus)
- Spread can be oro-nasal, respiratory by aerosol or via milk, colostrum and faeces
- Survives for approximately 14 days in environment
- Clinical picture includes weight loss, indurative mastitis, progressive neurological deficits in hind limbs (classic dragging of a hind leg), heavy lungs and dyspnoea. Poor performance in lambs due to loss of ewe condition
- Diagnosis can be made via AGIDT or ELISA – repeated blood sampling is required in animals over 12 months old since antibody production is delayed. SRUC cull ewe testing package is available
Contagious lymphadenitis (Corynebacterium pseudotuberculosis)
- Spread via close contact with an infected wound; bacteria infect lymph nodes which drain, and bacteria shed onto clippers, into dip, via direct contact etc
- Potentially viable in the environment for up to eight months
- Clinical picture includes abscessed superficial lymph nodes with characteristic thick pus, also present as a visceral form with internal abscessation and associated weight loss
- Diagnosis by clinical signs, Ab ELISA on blood and Western blot confirmatory
On-farm monitoring and management
On-farm monitoring is key since clinical cases may not be seen until disease is disseminated throughout the flock. Production data is crucial to give early warning of potential disease incursion, allowing control programmes to be implemented while still financially viable to do so.
Routine testing of thin ewes post weaning is a cost effective way of monitoring, particularly for maedi visna and Johne’s disease. On-farm post-mortems (Figures 1 to 3) as part of a flock health plan are also cost effective for monitoring infectious disease. Many local disease surveillance centres offer subsidised cull ewe post-mortems (Figure 4), which can provide a wealth of information on farm health status.
Regular stock examination is important. Thin ewes should be separated and fed accordingly – and those which don’t gain condition as expected should be culled and investigated.
Health scheme participation is recommended; in pedigree flocks, consider maedi visna, contagious lymphadenitis and Johne’s accreditation (SRUC premium sheep and goat health schemes). Though individual animal testing on a large scale is unlikely to be economically viable, subsidised maedi visna testing is available for commercial monitoring.
The industry is slowly waking up to the potentially devastating impact of these diseases and pressure from farmers buying replacement ewes is likely to mean that in the future, those farms which have invested in testing and management strategies will be at a commercial advantage.
Given the difficulties with many of the tests and potential risks from bought-in animals together with co-grazing/ contact from neighbouring flocks, it is sensible to introduce strategies to minimise spread within the flock and prevent incursion, even when disease hasn’t been confirmed.
- Run a closed flock where possible once flock health has been established.
- Consider running shearlings/ewe lambs as a separate flock. This enables monitoring of older ewes and protecting replacements for as long as possible before mixing.
- Fence off water courses and limit lambing in wet areas (Johne’s disease).
- Avoid mixing sheep close to tupping or during pregnancy (border disease).
- Handle young sheep first at scanning/shearing/vaccination (contagious lymphadenitis).
- Be vigilant with regards to fomites; consider trailers, shearing equipment, clothes, feed troughs (contagious lymphadenitis/ovine pulmonary adenomatosis).
- Consider fencing – escapee sheep from infected neighbours are a real threat.
- Dagging/lambing shed hygiene is important (Johne’s disease).
- Take care with regards to pooled colostrum (maedi visna/Johne’s disease/ovine pulmonary adenomatosis) or cow colostrum.
- Demand health status of purchased sheep – forge relationships between testing producer flocks and purchasers. Buyers should be willing to pay a premium for high health status sheep.
- Avoid buying through markets or dealers mixing sheep from multiple sources.
The diagnosis and management of these diseases provides a real challenge to the profession. With little information on production impact in the UK, and little understanding among farmers (and some vets!) regarding how, when and why to test effectively, there is an urgent need for education as well as industry pressure on producers to take greater responsibility for health status.