Subacute Rumenal Acidosis

• Like bloat, rumenal acidosis is associated with mismanagement of feeding and involves cessation or depression of rumenal movements with development of a degree of tympany.
• More common in cattle and goats than in sheep due to differences in diet.

Clinical

• Animals display signs of abdominal discomfort, but usually recover if treated.
• Acute disease can result in death if acidosis is severe.

Pathogenesis

• Occurs following feeding of highly fermentable material, usually carbohydrate, in the form of concentrate or cereal
  • e.g. in barley beef feeding systems.
• An overgrowth of Gram positive bacteria results, breaking carbohydrate down very quickly.
  • Increased volatile fatty acid production decreases the rumen pH to about 5.
    • Rumen epithelium is damaged- ruminitis.
    • Osmotic effects of rumen contents produce severe dehydration.
    • Death may ensue when the rumen pH drops to 4.5 or below.
• Normal flora may invade the damaged wall of rumen e.g. Fusiformis necrophorum.
  • Results in areas with mushy necrotic papillae.
  • Especially on pillars and ventral parts of rumen.
• Excess gas production may occur
  • Concentrate does not contain soluble proteins, therefore foam does not develop.
  • Only mild to moderate free gas bloat is likely- much of the rumenal distension which occurs with rumenal acidosis is due to fluid sequestration in the rumen.

Pathology

Gross

• Gross PM findings may be non-specific.
• Ruminal stasis/bloat leads to rumen distension
• Rumen contents are porridge-like.
• Rumenitis may be present
  • Normal flora invading wall of rumen may enter blood as thromboemboli and travel in the portal flow to liver
  • Focal abscesses are produced, which heal with sunken star-shaped scars.
• Overgrowth of Clostridia may cause endotoxic shock with widespread petechial haemorrhages.

Histological

• Microvesicles may be present in the epithelium of the rumen papillae.
  • Contain mainly neutrophils
• May be focal erosions/ulceration.

Sequelae

• Scarring- necrotised papillae are replaced by smooth epithelium closely adherent to sub mucosa.
• Has economic significance since many barley beef livers are condemned at meat inspection.
• More persistent organisms can invade rumen wall producing chronic infection.
  • These are often fungi, e.g. Mucor (see Mucormycosis).