Equine Neonatal Septicemia Study Guide

Overview and Clinical Importance

Neonatal septicemia is defined as a systemic inflammatory response syndrome (SIRS) in response to bacterial infection in foals typically less than 14 days of age. It represents the leading cause of morbidity and mortality in equine neonates, accounting for nearly one-third of all foal deaths. Sepsis is the second most commonly diagnosed problem in equine neonates, superseded only by failure of passive transfer (FPT) of maternal antibodies.

Survival rates for septic foals have improved significantly over the past 30 years, from approximately 25% to 50-70% with intensive care treatment. However, early recognition and aggressive intervention remain critical for successful outcomes. Understanding the pathophysiology, clinical presentation, diagnostic approach, and treatment principles is essential for the NAVLE examination.

Etiology and Pathogenesis

Common Bacterial Pathogens

Gram-negative bacteria remain the most common isolates (60-70%) from septic neonatal foals. Escherichia coli is the most frequently isolated organism, followed by Actinobacillus species.

Common Pathogens in Neonatal Foal Sepsis

Routes of Infection

• Gastrointestinal tract: Oral inoculation prior to colostrum intake (most common route)
• Umbilicus: Traditional "Navel Ill" or "Joint Ill" - omphalophlebitis leading to bacteremia
• Respiratory tract: Inhalation of pathogens, aspiration of infected amniotic fluid
• In utero: Placentitis, ascending infection

Risk Factors for Septicemia

Foal Factors

• Failure of passive transfer (FPT): IgG less than 400 mg/dL = complete FPT; 400-800 mg/dL = partial FPT
• Prematurity/Dysmaturity: Gestational age less than 320 days; immature immune system
• Perinatal asphyxia: "Dummy foal" syndrome; reduced nursing ability
• Meconium staining: Indicates in utero stress
• Twins: Smaller birth weight, often premature

Maternal Factors

• Placentitis: Ascending bacterial infection; premature udder development
• Premature lactation ("running milk"): Loss of colostrum quality before foaling
• Retained fetal membranes: Source of bacterial contamination
• Maternal illness/endotoxemia: Compromised colostrum quality
• Mare-foal bonding issues: Rejection, aggression preventing nursing

Environmental Factors

• Unsanitary foaling conditions: Overuse of foaling box, poor cleaning, poor ventilation
• Dystocia: Trauma, delayed first nursing
• Red bag delivery: Premature placental separation
• Cesarean section or induced parturition

Clinical Presentation

Clinical signs of neonatal sepsis are often subtle and nonspecific in early stages, making early detection challenging. The progression from early compensated sepsis to decompensated septic shock can be rapid in neonates due to limited metabolic reserves.

Progression of Clinical Signs

Organ System Manifestations

• Musculoskeletal: Septic arthritis ("Joint Ill"), osteomyelitis, physitis - lameness, joint effusion, may develop weeks after initial sepsis
• Respiratory: Pneumonia (50% of septic foals at necropsy), tachypnea, nasal discharge
• Umbilical: Omphalophlebitis, patent urachus, umbilical abscess
• CNS: Meningitis - stiff neck, altered mentation, seizures
• Ophthalmic: Anterior uveitis - blepharospasm, miosis, aqueous flare, hypopyon
• GI: Enterocolitis, ileus, gastric ulceration
• Cardiovascular: Endocarditis (rare) - tachycardia, new murmur, jugular pulsation

Diagnosis

IgG Assessment (Critical First Step)

Assessing passive transfer is essential in any sick neonate. Testing should ideally occur at 8-12 hours of age to allow time for intervention if FPT is detected.

Testing Methods: SNAP Foal IgG test (semi-quantitative, stall-side), Brix refractometry, glutaraldehyde coagulation test (field test), single radial immunodiffusion (SRID - gold standard but slower).

Modified Sepsis Score (Brewer-Koterba)

The sepsis scoring system uses 14 historical, clinical, and laboratory variables to predict the likelihood of sepsis. A score greater than 11 is considered indicative of sepsis. Original sensitivity was 93% and specificity 86%, though more recent studies show lower values (sensitivity 56-67%, specificity 73-86%).

Key Sepsis Score Variables

Laboratory Findings

Complete Blood Count

• Neutropenia with left shift: Classic finding; high band:seg ratio
• Toxic neutrophils: Döhle bodies, cytoplasmic basophilia, toxic granulation
• Note: Neutrophil count can also be normal or increased - do not rule out sepsis based on normal WBC

Serum Chemistry

• Hypoglycemia: Common due to depleted glycogen reserves; can cause seizures
• Elevated creatinine: May indicate poor renal perfusion, placental insufficiency, or perinatal asphyxia
• Hyperbilirubinemia: Hepatic endotoxin-related damage
• Hypoalbuminemia: Common in sepsis

Arterial Blood Gas

• Hypoxemia, hypercapnia
• Mixed metabolic and respiratory acidosis
• High anion gap (greater than 20 mEq/L)

Lactate

Blood lactate concentration is an important prognostic indicator. Admission lactate is associated with survival - each 1 mmol/L increase in L-lactate concentration increases odds of death. A lactate concentration of approximately 5.0 mmol/L can be used as a prognostic cutoff. Persistence of hyperlactatemia despite treatment indicates poor prognosis.

Serum Amyloid A (SAA)

SAA is an acute phase protein that increases rapidly with infection. Septic foals have significantly higher SAA (median ~1080 mg/L) compared to sick non-septic foals (median ~312 mg/L). While sensitivity is low for diagnosing sepsis, SAA has good specificity and can help rule out sepsis when low. SAA is reliably elevated in bacterial sepsis unlike WBC count which may vary.

Blood Culture

Blood culture remains the gold standard for confirming bacteremia and provides crucial antimicrobial susceptibility information. However, it has limitations: results take 48-72 hours, sensitivity is only fair (30-50%), and false negatives occur with low circulating bacterial numbers or prior antibiotic administration. Do not delay treatment while awaiting culture results!

Treatment

Treatment of neonatal sepsis requires an intensive care approach focusing on early goal-directed therapy. The cornerstones are: broad-spectrum antimicrobials, IV fluid resuscitation, and optimal nursing care. Treatment should begin as soon as sepsis is suspected - do not wait for culture confirmation!

Antimicrobial Therapy

Initial empirical therapy should provide broad-spectrum coverage against both Gram-positive and Gram-negative organisms. The combination of ampicillin + amikacin remains the first-line choice with excellent efficacy (~91.5% of bacteria susceptible).

Duration: Treatment should be long-term (at least 10-14 days) to prevent localization to joints, bones, or other organs.

Fluid Therapy

Early goal-directed IV fluid therapy is critical to restore tissue perfusion, attenuate cytokine response, and reverse cellular injury.

• Crystalloids: Balanced electrolyte solutions (Plasmalyte, Normosol, LRS); Bolus 20-80 mL/kg for shock; Maintenance ~100 mL/kg/day
• Colloids: Plasma (20 mL/kg) preferred - provides immunologic support; Hydroxyethyl starch (10 mL/kg) if cost-prohibitive
• Dextrose supplementation: Add 2.5-5% dextrose to fluids for hypoglycemic foals; Monitor glucose frequently

Plasma Transfusion

IV plasma transfusion (1-2 L) is indicated to raise IgG greater than 800 mg/dL and provide colloidal oncotic support. Foal blood volume is approximately 10% body weight; plasma volume of a 45 kg foal is approximately 3 L. Give maximum 1 L/day to avoid volume overload. Recheck IgG 12-24 hours post-transfusion. Septic foals may consume IgG rapidly and require repeated transfusions.

Additional Supportive Care

• Nutritional support: If not nursing adequately, feed mare's milk or substitute at 15-25% body weight per 24 hours via nasogastric tube
• Oxygen supplementation: Intranasal insufflation (2-10 L/min) for hypoxemic foals
• GI ulcer prophylaxis: Historically recommended, but recent evidence suggests NOT routinely indicated - improved outcomes without anti-ulcer medication with better intensive care
• Nursing care: Padded stall to prevent pressure sores, frequent turning, maintaining normothermia
• Septic arthritis management: Joint lavage, regional limb perfusion with antimicrobials
• Umbilical infections: Local treatment for minor cases; surgical resection for severe abscessation

Prognosis

Recovery from neonatal sepsis depends on severity and specific manifestations of infection. Currently reported survival rates at referral centers are 50-81% depending on the underlying disease. Severe neonatal pulmonary disease has been associated with higher mortality (35-50%).

Prognostic Indicators

Long-term athletic outcome: Surviving bacteremic Thoroughbred foals are as likely to start races as their siblings, although their earnings may be lower. Early intensive treatment is key - if the foal survives the initial illness, it has potential to become a healthy adult horse.

Prevention

• Ensure adequate colostrum intake: Foal should nurse within 1-2 hours of birth; 1-3 L of high-quality colostrum (specific gravity greater than 1.060)
• Test IgG at 8-12 hours: Allows time for intervention with oral colostrum if needed
• Colostrum banking: Freeze high-quality colostrum; Thaw in warm water (not microwave); Use within 12 months
• Umbilical care: Apply 0.5% chlorhexidine solution (preferred over iodine) 2-4 times daily until dry
• Clean foaling environment: Avoid overuse of foaling boxes, ensure good ventilation
• Monitor high-risk mares: Those with placentitis, premature lactation, previous abnormal foals
• Note on prophylactic antibiotics: NOT recommended - encourages resistance; Focus on early recognition instead