3.1.2: Respiratory disease in sheep Flashcards
Differential diagnoses of LRT disease in sheep
- Bacterial pneumonia
- Viral pneumonia
- Parasitic pneumonia
- Mycotic pnuemonia
- Other causes e.g. aspiration pnuemonia
- Lung abscesses e.g. TB, Fusobacterium necrophorum
- Chronic viral resp disease e.g. Maedi Visna, OPA
Upper respiratory tract disease differentials
- Laryngeal chondritis
- Nasal myiasis
- Nasal foreign bodies
- Nasal tumours
- Salmonella arizonae rhinitis
What are some causative agents of bacterial pneumonia in sheep?
- Mannheimia haemolytica
- Mycoplasma species
- Bibersteinia trehalosi
- Pasteurella multocida
- Bordetella parapertussis
-
Histophilus somni
And others
What are some causative agents of viral pneumonia in sheep?
- Parainfluenza virus type 3
- Ovine adenovirus
- Respiratory syncytial virus
- Reovirus types 1, 2 and 3
- Herpesviruses
Laryngeal chondritis in a Texel shearling ram
It died soon after developing acute-onset resp distress
OPA
Lungs have extensive neoplastic changes in caudal lobes due to jaagsiekte retrovirus infection
What is the wheelbarrow test?
- Lift the sheep’s hindlegs
- See if fluid drains from the nose
Adult Dictyocaulus filaria worms in a lamb’s trachea during PM
True/false: Maedi Visna is always fatal.
True
There is currently no cure or treatment
It is a chronic wasting disorder
Clinical signs of Maedi Visna
- Pneumonia (shows as laboured breathing)
- Progressive weakness leading to paralysis
- Ill-thrift
- Arthritis/lameness
- Chronic mastitis
- Low milk production leading to poor lamb growth
Tends to present in sheep older than 3 years of age.
Transmission of Maedi Visna
- Direct contact between infected sheep
- Aerosol spread - droplets from the nose and mouth
- Through milk
- Through contaminated needles/blood
- Potentially through semen/placenta
- Indirect contact through fomites
How likely is it that a vaccine for Maedi Visna will be developed?
Unlikely
The virus can mutate around the sheep’s immune system
Pathogenesis of Maedi Visna
- MV predominantly affects the lungs, causing chronic inflammation
- Oro-nasal infection of lymphoid tissue causes hyperplasia, targeting monocytes, macrophages, dendritic cells.
- MV can be latent in the genome of monocytes (Trojan horse; allows it to evade the immune system)
Control of Maedi Visna
- Difficult; condition = fatal and no vaccine available
- Blood test -> serology -> ELISA for antibody
- Source sheep from accredited flocks
- Monitor the flock and test new stock
- Reduce stocking density
- Increase biosecurity to prevent contact with neighbouring flocks
- In heavily infected flocks: may need to cull the flock and re-populate from an accredited source
Pathogenesis of OPA
OPA: Ovine Pulmonary Adenocarcinoma
* Beta retrovirus which infects cells in the lungs
* Replaces normal tissue with tumours
* Specifically, neoplastic proliferation of Clara cells and Type II pneumocytes producing adenocarcinoma
* Tumours can produce high levels of foaming fluid which increases the risk of secondary infections e.g. Pasteurella
Transmission of OPA
- Typically through aerosol transmission
- Can be passed in colostrum and milk
- Fluid is highly infective and remains infective on surfaces. Virus can survive in environment for several weeks.
Clinical signs of OPA
- Weight loss
- Laboured breathing
- Increased RR
- Some animals may cough
- Nasal discharge
- Sudden death
True/false: OPA may present as sudden death.
True.
Describe the incubation period of OPA and relate this to its typical presentation
- Long incubation period: animals may appear healthy for months-years post infection
- Infection typically peaks in ewes around 4 years, tups around 1-2 years.
- However, may present in younger animals (2-11 months) too
True/false: severe coughing is a sign of OPA
False.
Some animals may cough but severe coughing is not considered consistent with OPA
Diagnosis of OPA
- No immune response so no blood test is available
- Can detect tumours through ultrasound, but may still miss some small ones. It can be difficult to view all of the lung area.
Control and prevention of OPA
- Investigate sudden deaths esp. in purchased stock
- Buy from trusted sources
- Investigate incidence of ill-thrift
- Minimise common trough use
- Increase boundary biosecurity
- Ultrasound lungs for tumours; caution when interpreting images. Could operate a ‘scan and cull’ system at 6-12 intervals.
Describe the predisposition to laryngeal chondritis among some sheep
- Texels, Beltex, Southdowns typically affected
- More common in rams than ewes
- Aetiology unclear but probably associated with short thick necks of these breeds, genetics, and high concentrate feeding.
Pathogenesis of laryngeal chondritis
- Suppurative lesions form within the arytenoid cartilage (associated with E. coli, A. pyogenes, F. necrophorum)
- This narrows the lumen of the larynx
- There is severe dyspnoea with laryngeal stridor which can often be fatal
