3.1.2: Respiratory disease in sheep

Question 1

Differential diagnoses of LRT disease in sheep

Answer 1

• 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

Question 2

Upper respiratory tract disease differentials

Answer 2

• Laryngeal chondritis
• Nasal myiasis
• Nasal foreign bodies
• Nasal tumours
• Salmonella arizonae rhinitis

Question 3

What are some causative agents of bacterial pneumonia in sheep?

Answer 3

• Mannheimia haemolytica
• Mycoplasma species
• Bibersteinia trehalosi
• Pasteurella multocida
• Bordetella parapertussis
• Histophilus somni
  And others

Question 4

What are some causative agents of viral pneumonia in sheep?

Answer 4

• Parainfluenza virus type 3
• Ovine adenovirus
• Respiratory syncytial virus
• Reovirus types 1, 2 and 3
• Herpesviruses

Question 5

Answer 5

Laryngeal chondritis in a Texel shearling ram
It died soon after developing acute-onset resp distress

Question 6

Answer 6

OPA
Lungs have extensive neoplastic changes in caudal lobes due to jaagsiekte retrovirus infection

Question 7

What is the wheelbarrow test?

Answer 7

• Lift the sheep’s hindlegs
• See if fluid drains from the nose

Question 8

Answer 8

Adult Dictyocaulus filaria worms in a lamb’s trachea during PM

Question 9

True/false: Maedi Visna is always fatal.

Answer 9

True

There is currently no cure or treatment

It is a chronic wasting disorder

Question 10

Clinical signs of Maedi Visna

Answer 10

• 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.

Question 11

Transmission of Maedi Visna

Answer 11

• 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

Question 12

How likely is it that a vaccine for Maedi Visna will be developed?

Answer 12

Unlikely
The virus can mutate around the sheep’s immune system

Question 13

Pathogenesis of Maedi Visna

Answer 13

• 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)

Question 14

Control of Maedi Visna

Answer 14

• 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

Question 15

Pathogenesis of OPA

Answer 15

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

Question 16

Transmission of OPA

Answer 16

• 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.

Question 17

Clinical signs of OPA

Answer 17

• Weight loss
• Laboured breathing
• Increased RR
• Some animals may cough
• Nasal discharge
• Sudden death

Question 18

True/false: OPA may present as sudden death.

Answer 18

True.

Question 19

Describe the incubation period of OPA and relate this to its typical presentation

Answer 19

• 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

Question 20

True/false: severe coughing is a sign of OPA

Answer 20

False.
Some animals may cough but severe coughing is not considered consistent with OPA

Question 21

Diagnosis of OPA

Answer 21

• 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.

Question 22

Control and prevention of OPA

Answer 22

• 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.

Question 23

Describe the predisposition to laryngeal chondritis among some sheep

Answer 23

• 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.

Question 24

Pathogenesis of laryngeal chondritis

Answer 24

• 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