ENZOOTIC PNEUMONIA OF SHEEP

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 ENZOOTIC PNEUMONIA OF SHEEP

Nature of the disease
Enzootic pneumonia is an acute infectious disease of sheep characterised by fever, nasal discharge, pneumonitis and pleuritis. The aetiology is complex and variable between regions involving interactions between bacteria, chlamydiae, mycoplasmas, viruses and stress. Thus ‘enzootic pneumonia’ is more of a clinical term describing a range of respiratory diseases in sheep. In Australia, agents most commonly implicated are:
  • Mycoplasma ovipneumoniae 
  • Parainfluenza 3 virus
  • Chlamydia spp.
  • Pasteurella haemolytica and P. multocida 
Classification
SPC List D disease
Susceptible species
Sheep of all breed and sexes with the highest incidence in lambs.
Distribution
Ovine pneumonia occurs in all major sheep-raising areas of the world including southern and northern Africa, South America, Europe, the Middle East, Russia, India, Mongolia, Australia and New Zealand.
Clinical signs 
The first signs of an outbreak may be sudden deaths in lambs. The number of cases increases rapidly. Affected animals show:
  • Fever
  • Refusal to eat
  • Depression
  • Loss of condition
  • Mucopurulent nasal discharge
  • Ocular discharge
  • Dyspnoea, often accompanied by coughing

When mycoplasmas are involved, there may be haematogenous spread to the joints and lameness associated with arthritis occurs.
A morbidity rate of around 50% may be seen, with mortality rates up to 10%.
Because permanent damage to the lungs may occur (e.g. adhesions to the chest wall), recovered sheep may tire easily when driven.

Post-mortem findings 
Findings are largely restricted to the respiratory system and joints:
  • Ventral parts of lungs show varying degrees of consolidation
  • Variable amounts of fluid in the pleural cavity
  • Bronchial and mediastinal lymph nodes are swollen and hyperaemic
  • Haemorrhages over serosal surfaces
  • In chronic cases there may be adhesions between lung lobes and chest wall
  • Infected joints contain increased synovial fluid and flakes of fibrin
Differential diagnosis 
Pneumonia can also occur with other diseases:
  • Caseous lymphadenitis
  • Lungworms or lungworm larvae
  • Hydatid cysts
  • Septicaemic pasteurellosis
  • Polyarthritis of lambs
  • Enterotoxaemia
  • Contagious agalactia
Specimens required for diagnosis 
Pneumonia can be suspected based on the clinical findings and can be confirmed by post-mortem examination.

A range of samples of tissues including lesions, lung , lymph nodes, joint fluid, blood should be submitted for histological, bacteriological and virological investigation to ascertain the aetiology.

 However, given the multifactorial nature of pneumonia in sheep, a single cause may not be found.

Transmission   
Enzootic pneumonia is transmitted by direct contact between infected and susceptible sheep. Spread is aided by the inclination of sheep to crowd together.

Some sheep normally carry chlamydiae in the gastro-intestinal tract, while others carry them in the upper respiratory tract. During crowding the micro-organisms contaminate the air, feed, water and equipment and are transmitted from carrier sheep to susceptible sheep by inhalation and ingestion.

There appears to be a positive relationship between exposure to stress and the onset of clinical disease.

Risk of introduction   
Introduction of infected sheep into a flock is the most likely way of introducing the disease.
Control / vaccines  
There is no highly effective method of preventing outbreaks of pneumonia in sheep. The disease can be reduced by limiting stress as much as possible e.g. overcrowding.

Infected flocks can be treated with antimicrobial agents where this is economic. Oral dosing with sulphadimidine is often recommended for this purpose as it is cheaper than injectable antibiotics.

References
  • Chronic Enzootic Pneumonia of Sheep, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 1070-1071