Iceberg Diseases in Sheep Flashcards
What are the characteristics of Maedi Visna? How is it controlled?
- Lentivirus – long incubation period (months to years)
- Presentation can be variable, but often chronic wasting and laboured breathing.
- Oronasal is the main route of transmission, but can also be spread through colostrum/milk and by fomites.
- Regional variation in prevalence (Leicestershire and Gloucestershire high).
- Serological diagnosis (antibodies produced within weeks to months but tend to wax and wane)
- No tx or vaccine. Control methods include purchasing from accredited flocks (or isolate and test), monitoring and culling positive animals (if practical) and reducing stocking density, preventing contact with neighbouring flocks.
Control
If high level of infection:
1. Cull flock & restock with accredited sheep
2. Try and reduce losses through management: keep flock young split into older & younger sheep for management cull thin/suspect cases
3. Run a less intensive system to reduce spread
What are the characteristics of ovine pulmonary adenocarcinoma? How is it controlled?
- Retrovirus
- Causes neoplastic proliferation of lung cells = adenocarcinoma. This results in laboured breathing, increased respiratory rate, ill thrift and sudden death.
- Transmission mainly aerosol, but can be spread through milk and colostrum.
- Long incubation period – 6 months to several years.
- 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.
- No tx or vaccine.
OPA control
1. Identify & cull infected & offspring – very difficult without test. Need to PM any sudden deaths/those showing ill thrift.
2. Manage in single age groups – keep young separate from adults
3. Reduce close contact – housing, stocking density, trough feeding. Need good hygiene
4. Snatch lambing & rear artificially – successful in German case study
What are the characteristics of caseous lymphadenitis? How is it controlled?
- Corynebacterium pseudotuberculosis
- Transmission through skin abrasion, inhalation or ingestion – often when sheep are grouped.
- Forms abscesses in lymph nodes with characteristic green pus, typically around head and neck (inguinal/scrotal in tups).
- Diagnosis by bacteriology/serology.
◦ Bacteriology requires draining abscess
◦ Serology has high specificity but low sensitivity Antibodies tend to wax and wane and presence of antibody doesn’t necessarily lead to disease. - Control methods include buying from trusted sources, boundary biosecurity and separating infected animals.
- Treatment with antibiotics not effective. Vaccine not available in UK can be imported on special licence (efficacy variable).
- Zoonotic (although rarely reported in people)
Vaccination
* CLA prevalence in Australia (abattoir survey) decreased from 26% in 1995 to 5% in 2009 largely attributed to successful vaccination
* Glanvac (6-in-1; Zoetis)
◦ Can only use in UK under Special Import License (VMD)
◦ Made up of formalin-killed bacteria & toxoid
◦ Flock protection varies from 25%-90%
◦ Needs 2 doses of vaccine a month apart & annual booster
◦ MDI affects lamb immune response up to ~10 weeks old
◦ Affects any eradication program using serology as not DIVA
What are the characteristics of ovine Johne’s disease? How is it spread? How is it controlled?
- Mycobacterium avium spp. Paratuberculosis (MAP)
- Causes inflammation in the gut and reduces metabolic efficiency leading to reduced fertility and weight loss. Don’t see scouring, but thin sheep/high flock culling rates/poor lamb performance.
- Cattle and sheep strains (C and S) – sheep susceptible to both (cattle fairly resistant to S strain).
- Transmission mainly faeco-oral, but can be passed via milk/colostrum. Lambs are infected early on in life (first 3-4m usually), but clinical signs are usually seen in animals over 2-3y.
- Diagnosis can be by serology, but antibody levels often remain low until clinical disease. Faecal PCR – unclear how much bacteria correlates with disease. PME most definitive.
What are the characteristics of border disease? How is it spread? How is it controlled?
- Caused by virus very similar to BVD in cattle (pestivirus) See hairy shaker lambs, poor ewe fertility, high levels of abortions or weak lambs.
- Proportion of UK flocks infected estimated to be between 30-38%
- Transmission via respiratory secretions, transplacental, from dam to offspring during peri-parturient period, or via semen of PI rams.
- Intensive flocks at increased risk - transmission in extensive flocks appears to be moderate.
- Diagnosis by detection of antibody or antigen. As in cattle, PI animals will be negative for antibody (persistently), but will be positive for antigen.
Control
* PI hunt may not be practical (as you would with cattle)
* BVD ‘Tag and Test’ not validated for border disease
* Natural immunity through exposure of NON PREGNANT animals to PI (but transmission can be slow).
* BVD vaccines? No published evidence of this
* Don’t retain breeding females from lamb crops with affected cases/test all ewes be retained for breeding (antigen).
* Optimise nutrition and parasite control
What are control strategies for iceberg diseases?
- Biosecurity – most risk from animal movement.
- Testing/accreditation
◦ Premium Sheep and Goat Health Scheme - Screen cull ewes and fallen stock
◦ BD exception here – screening lambs is more useful - Screen bought-in animals
◦ Ideally pre-purchase - (Quarantine)
◦ Keep purchased females separate from pregnant ones until after lambing
