Respiratory disease in sheep Flashcards
5 production limiting diseases in sheep (Iceberg dz)
- Maedi Visna
- Ovine Pulmonary Adenocarcinoma
- Caseous Lymphadenitis
- Johne’s
- Border disease
Ddx of LRT dz in sheep
- Bacterial pneumonia
- Viral pneumonia
- Lung abscess
- Chronic viral resp dz
- Parasitic pneumonia
- Other (e.g. inhalation pneumonia, mycotic pneumonia [Aspergillus spp])
Maedi Visna
- 1st detected in Iceland in 1954 and was introduced to the UK through infected sheep
- MV is caused by a retrovirus (Lentivirus non-oncogenic RNA retrovirus)
- Capable of significantly reducing fertility and productivity
- A chronic wasting disorder which also includes laboured breathing
- All ages of sheep can be affected
- Due to its long incubated period, infected sheep can appear healthy for long periods of time (potentially years)
- The infection is lifelong, however antibodies are produced which can de detected through blood testing
- Predominantly affects the lungs, causing chronic inflammation
- Oro-nasal infection of the slow replicating lymphoid tissues causes hyperplasia, targeting the monocytes, macrophages and dendritic cells
- Can be latent in the genome of monocytes; acting as a trojan horse and evading the immune system
- Increased stocking density increases spread
- The virus doesn’t live long in the environment
- Ultimately a fatal condition, for which there’s no vaccine available
- It’s unlikely a vaccine will be developed as the virus can mutate around the sheep immune systems
MV spread
- The disease is spread through aerosol transmission, increased by close contact
- Can also spread through colostrum and milk, indirect contact from fomites, and potentially through the placenta or semen
MV presentation
- variable
- Il thrift
- Increase pneumonia and mastitis incidence
- Low milk production leading to poor lamb growth and increased young lamb loss
- Swollen joints and lameness
MV diagnosis
Due to variable clinical signs, blood testing is the only reliable method of detection
MV - risk reduction
○ Sourcing animals from accredited flocks
○ Monitoring the flock and testing new stock
○ Reducing stocking density
- Increasing biosecurity to prevent contact with neighbouring flocks
- In some heavily infected flocks it’s likely that eradication through testing wouldn’t be possible, and the only way to achieve disease free status would be to cull the flock and then re-populate from an accredited source
OPA
= ovine pulmonary adenocarcinoma
* Also known as Jaagsiekte
* Beta retrovirus which infects cells in the lungs
* It replaces normal tissue with tumours through the neoplastic proliferation of Clara cells and type II pneumocytes producing adenocarcinoma
* The tumours can produce high levels of a white foaming fluid increasing the risk of secondary infections such as Pasteurella
- This fluid is also highly infective and may remain infective on surfaces too
OPA spread
- typically through aerosol transmission and can also be passed through milk and colostrum
OPA risks
- As with many similar diseases higher stocking density increases risk, and the virus can survive for several weeks in the environment
- Younger stock are more susceptible but older animals can also be infected
OPA clinical signs
- Due to a long incubation period and the slow development of tumours, animals can appear healthy for months to years post infection
- Infection typically peaks in ewes ~4y/o and tups at 1-2y/o, but can appear in animals as young as 2-11 months
○ Weight loss
○ Laboured breathing
○ Increased rr
○ Some animals may cough or produce a nasal discharge - Sudden death - up to 50% of cases submitted for PM
OPA diagnosis
- No immune response so no blood test available for diagnosis
- Small tumours may not be detected using ultrasound and it can be difficult to view all of the lung area
- A ‘scan and cull’ system can be used at 6-12m intervals in infected flock
- Caution should be exercised when interpreting images, particularly in flocks where OPA prevalence may be low
OPA preventation
predominantly through:
- investigation of sudden deaths (esp in purchased stock)
- to buy from trusted sources
- investigate incidence of ill thrift
- minimise common trough use
- reduce stocking density
- increase boundary biosecurity
- ultrasound of the lungs for tumours
CLA
= caseous lymphadenitis
* A bacterial infection (Corynebacterium pseudotuberculosis) which enters the body through skin abrasions (may be microscopic), inhalation or ingestion
* Typically, an infected wound will spread the infection into the draining lymph node where the bacteria multiplies and forms a micro-abscess
* This will coalesce into a larger abscess filled with characteristic green pus (bacteria, cellular debris and eosinophils)
* The abscess will then caseate or ‘wall off’ which leads to further multiplication giving an ‘onion ring’ effect and the release of an exotoxin (phospholipase D) which can damage cell membranes
* Bacteria may also spread to other sites from lymph
- Antibiotics aren’t effective inside the abscess and aren’t an effective treatment
- Shearing is a risk time for spread
* The bacteria can survive for up to 8m in the environment and up to 24h in sheep dip, although it is killed by sunlight
* Incubation can vary from 42d-4m
CLA spread
- Often introduced through new infected stock, but potentially via fomite transmission through contact with infected pus
- If the abscess is in the lungs, aerosol transmission is also possible
