Bovine Lactic Acidosis Study Guide

Overview and Clinical Importance

Ruminal lactic acidosis (also called grain overload, carbohydrate engorgement, or rumen overload) is one of the most economically significant digestive disorders in cattle production. It results from the ingestion of excessive amounts of rapidly fermentable carbohydrates by animals not adapted to such diets, leading to profound disturbances in rumen microflora, systemic acid-base balance, and multiple organ function.

The condition exists on a continuum from subacute ruminal acidosis (SARA) to acute ruminal acidosis (ARA), with severity depending on the quantity of grain consumed relative to the animal's adaptation status. Understanding this disease is essential for the NAVLE as it represents a high-yield topic involving pathophysiology, diagnosis, treatment, and prevention.

Etiology and Risk Factors

The primary cause of ruminal acidosis is the ingestion of highly fermentable carbohydrates (grains such as wheat, barley, and corn) by animals not adapted to such diets. The key point is that ruminants must be gradually adapted over 2-3 weeks to high-grain diets; otherwise, acidosis commonly occurs.

Common Scenarios

• Accidental grain access: Cattle escaping pens and accessing grain storage
• Abrupt diet changes: Rapid transition from forage to high-concentrate diets
• Feed mixing errors: Incorrect formulation or inadequate mixing of TMR
• Feed sorting behavior: Preferential consumption of concentrate over forage
• Inadequate fiber: Diets deficient in physically effective neutral detergent fiber (peNDF)

Risk Factors by Production System

Pathophysiology

The pathophysiology of ruminal acidosis involves a cascade of events beginning with excessive fermentation of rapidly digestible carbohydrates. Understanding this cascade is essential for both diagnosis and treatment.

Microbial Shifts

Normal rumen pH is maintained between 6.0-7.0 through a balance of acid production, buffering by saliva, and absorption across the rumen wall. When large amounts of rapidly fermentable carbohydrates enter the rumen:

• Initial VFA surge: Amylolytic bacteria rapidly ferment starch, producing volatile fatty acids (VFA) and causing initial pH drop
• Gram-negative bacteria decline: Lactate-utilizing bacteria (Megasphaera elsdenii, Selenomonas ruminantium) are pH-sensitive and begin to die at pH less than 5.5
• Streptococcus bovis proliferation: This Gram-positive bacterium thrives in low-pH environments and shifts from VFA to lactic acid production
• Lactobacillus dominance: At pH less than 5.0, Lactobacillus spp. become dominant, producing even more D-lactate
• Protozoa death: Ciliated protozoa are eliminated, further destabilizing the rumen ecosystem

D-Lactate vs L-Lactate: Clinical Significance

Rumen bacteria produce both D-lactate and L-lactate isomers. While mammals can efficiently metabolize L-lactate via L-lactate dehydrogenase, D-lactate metabolism is much slower because D-lactate dehydrogenase activity is limited. This leads to:

• Systemic D-lactic acidosis: D-lactate accumulates in blood, causing metabolic acidosis
• Neurological signs: D-lactate crosses the blood-brain barrier via monocarboxylate transporters, causing CNS depression, ataxia, and depressed reflexes
• Palpebral reflex depression: The extent of palpebral reflex depression correlates with plasma D-lactate concentration and disease severity

Classification: Acute vs Subacute Ruminal Acidosis

Clinical Signs

Acute Ruminal Acidosis

Clinical signs develop within 12-24 hours of grain engorgement and progress rapidly. The interval between overeating and onset is shorter with ground feed than whole grain.

Early Signs (0-12 hours)

• Anorexia and decreased rumination
• Rumen hypomotility progressing to atony
• Mild bloat (dorsal distension)
• Increased heart rate

Progressive Signs (12-24 hours)

• Rumen atony: Complete absence of primary contractions; gurgling fluid sounds on auscultation
• Hydrorumen: Fluid splashing sounds on ballottement; rumen feels resilient due to excessive fluid
• Profuse diarrhea: Watery, gray-green, sour-smelling; may contain visible grain particles
• Dehydration: Sunken eyes, prolonged skin tent (greater than 4-6 seconds), tacky mucous membranes
• Cardiovascular: Tachycardia (greater than 100 bpm), weak pulse, hemoconcentration

Severe/Terminal Signs (24+ hours)

• Neurological: Ataxia, staggering, bumping into objects, recumbency
• Depressed reflexes: Sluggish or absent palpebral reflex (correlates with D-lactate levels)
• Hypothermia: Subnormal body temperature in terminal cases
• Cardiovascular collapse: Shock, weak thready pulse, cold extremities

Subacute Ruminal Acidosis (SARA)

SARA is characterized by intermittent, self-correcting episodes of decreased rumen pH. Clinical signs are subtle and often delayed from the actual low-pH episode, making diagnosis challenging.

Herd-Level Indicators

• Milk fat depression: Fat:protein ratio less than 1.0 in early lactation cows
• Cyclic feed intake: "Slug feeding" behavior with decreased DMI followed by overconsumption
• Poor body condition: Despite adequate feed availability
• Loose, inconsistent feces: Variable consistency, may contain undigested grain, mucin casts
• Increased lameness: Sole hemorrhages, white line disease (delayed by weeks to months)
• Liver abscesses at slaughter: Indicator of previous SARA episodes

Diagnostic Approach

Rumen Fluid Analysis

Rumen fluid collection and analysis is the cornerstone of diagnosis. Two methods are available:

Rumenocentesis (Gold Standard for SARA)

Technique: Puncture the ventral ruminal sac at the level of the stifle joint using a 13-16 gauge, 10 cm needle. The site is in the left paralumbar fossa, at the intersection of a vertical line through the midpoint between the last rib and tuber coxae, and a horizontal line at the level of the stifle.

• More accurate pH measurement (no saliva contamination)
• Sensitivity 0.74, Specificity 0.79 for SARA detection
• Risk of local peritonitis (rare with proper technique)

Oro-ruminal Tube

• Less invasive but saliva contamination elevates pH
• Discard first 200-500 mL of fluid
• Sensitivity 0.68, Specificity 0.84
• Modern sealed probes reduce contamination

Rumen Fluid Parameters

MBRT = Methylene Blue Reduction Test

Blood Work and Additional Diagnostics

• Blood gas analysis: Metabolic acidosis (decreased pH, decreased HCO3-, negative base excess)
• PCV/TS: Hemoconcentration indicates dehydration severity
• BUN/Creatinine: Azotemia indicates prerenal kidney failure
• Serum lactate: Elevated D-lactate (specific assay required)
• CBC: Neutropenia with left shift (severe cases)

Sequelae and Complications

Even animals that survive acute acidosis may develop serious long-term complications. These sequelae result from rumenitis and bacterial translocation.

Treatment

Acute Ruminal Acidosis Treatment Protocol

Treatment goals are: (1) Remove ruminal contents and neutralize remaining acid, (2) Correct systemic acidosis and dehydration, (3) Restore rumen function, and (4) Prevent complications.

1. Rumen Evacuation

Rumenotomy (preferred for severe cases): Indicated when rumen pH is less than 5.0, heart rate greater than 100 bpm, greater than 8% dehydration, and/or animal is recumbent. Allows complete evacuation and visualization of rumen wall for assessment of damage.

Rumen lavage (alternative): Use large-bore stomach tube (2.5 cm diameter, 3 m long). Add water to distend rumen, then drain by gravity. Repeat 15-20 times. Risk of aspiration in recumbent animals.

2. Fluid Therapy

Important: Avoid lactated Ringer's solution as it contains lactate. Monitor for urination resumption to confirm renal perfusion.

3. Additional Treatments

• Oral antacids: Magnesium hydroxide 400g in 10L water PO (if IV bicarbonate not given)
• Thiamine: 10-20 mg/kg IM to prevent/treat polioencephalomalacia
• Transfaunation: Fresh rumen fluid (8-16L) from healthy donor to restore microflora
• Antibiotics: Procaine penicillin G to prevent bacterial rumenitis and liver abscesses
• NSAIDs: Flunixin meglumine for endotoxemia and pain
• Dietary management: Good-quality hay only for 2-4 days, then gradual grain reintroduction

Subacute Ruminal Acidosis Management

SARA is self-correcting and there is no specific treatment for the low-pH episodes themselves. Management focuses on prevention and treating secondary conditions.

• Address dietary factors (see Prevention section)
• Treat lameness and other secondary conditions as needed
• Improve bunk management to reduce slug feeding

Prevention Strategies

Dietary Adaptation

The cornerstone of prevention is gradual adaptation to high-grain diets over 2-3 weeks. This allows time for:

• Rumen papillae growth to increase absorptive surface area
• Proliferation of lactate-utilizing bacteria (M. elsdenii, S. ruminantium)
• Adaptation of VFA absorption mechanisms

Feed Management

• Adequate fiber: Maintain appropriate forage:concentrate ratio; include physically effective NDF (peNDF) to stimulate rumination and saliva production
• TMR mixing: Ensure proper mixing to prevent sorting; avoid finely ground concentrates
• Bunk management: Adequate bunk space (24 inches/cow minimum); consistent feeding times; avoid feed deprivation followed by overfeeding
• Dietary buffers: Sodium bicarbonate (0.75-1% of DMI) or potassium carbonate

Ionophores

Monensin (Rumensin) is the most commonly used ionophore for acidosis prevention. It acts by:

• Selectively inhibiting Gram-positive bacteria (lactate producers) including S. bovis
• Favoring Gram-negative bacteria (lactate utilizers)
• Increasing propionate:acetate ratio (more efficient energy metabolism)
• Decreasing lactic acid production and reducing methane output

Dosage: Feedlot cattle: 5-40 g/ton complete feed (providing 50-480 mg/head/day). Used in over 90% of US feedlots.

Memory Aids for NAVLE

"GRAIN" Mnemonic for Acute Acidosis Signs

• G - Gut (rumen) atony with fluid splashing
• R - Runs (profuse diarrhea)
• A - Ataxia and depressed reflexes
• I - Increased heart rate (tachycardia)
• N - No appetite (anorexia)

"FOUL" Mnemonic for Sequelae

• F - Fusobacterium liver abscesses
• O - Omasum/rumen parakeratosis
• U - Ulcerations (mycotic rumenitis)
• L - Laminitis

pH Quick Reference

• Normal: 6.0-7.0 (forage diet) or 5.5-6.0 (grain-adapted)
• SARA: 5.2-5.6 (remember: "SARA = 5 Something")
• Acute: Less than 5.0 (remember: "Acute = Acidic = less than 5")