NAVLE Hemic and Lymphatic

Equine Failure of Passive Transfer Study Guide

📅 March 28, 2026 ⏱ 25 min read 👁 2 views

Failure of Passive Transfer (FPT) is the most common immunodeficiency disorder in horses, occurring in 3-24% of newborn foals.

Overview and Clinical Importance

Failure of Passive Transfer (FPT) is the most common immunodeficiency disorder in horses, occurring in 3-24% of newborn foals. Due to the epitheliochorial nature of the equine placenta, no immunoglobulin transfer occurs in utero, making foals entirely dependent on colostral antibody absorption for early immune protection. FPT is defined as inadequate transfer of maternal immunoglobulins (primarily IgG) from colostrum to the foal's bloodstream, resulting in increased susceptibility to life-threatening infections including septicemia, pneumonia, and septic arthritis.

FPT represents a critical NAVLE topic because early recognition and treatment dramatically improves survival rates. Understanding the pathophysiology, diagnostic testing, and treatment protocols is essential for any equine practitioner managing neonatal foals.

Time Post-Birth	Absorption Capacity	Clinical Significance
0-6 hours	Maximum absorption (peak efficiency)	Critical window; oral colostrum most effective
6-12 hours	Rapidly declining	Test at 9-12 hours; oral supplementation still possible
12-18 hours	Minimal absorption	Oral supplementation has limited value
Greater than 18-24 hours	"Gut closure" - negligible absorption	IV plasma transfusion required for FPT treatment

Pathophysiology of Passive Transfer

Equine Placentation and Immunoglobulin Transfer

The mare has an epitheliochorial placenta with six tissue layers separating maternal and fetal blood. This prevents transplacental transfer of immunoglobulins, making foals born agammaglobulinemic (essentially no circulating antibodies). While there is minimal IgM production in utero, this is insufficient for protection against environmental pathogens.

Foals must acquire immunity through colostrum ingestion, the thick, antibody-rich first milk produced by the mare in the final 2-3 weeks of gestation. Colostrum contains high concentrations of IgG (primary), IgA, and IgM, along with other immune-modulating factors.

High-YieldThe equine placenta is epitheliochorial (6 layers), preventing immunoglobulin transfer. In contrast, humans have a hemochorial placenta (3 layers) allowing IgG to cross. This is why human babies are born with maternal IgG, but foals are born agammaglobulinemic.

The Window of Absorption

Specialized enterocytes in the neonatal small intestine possess the ability to absorb intact immunoglobulin molecules through pinocytosis. This "open gut" state is temporary and critical to understand.

Immunoglobulin Absorption Timeline

NAVLE TipRemember the "1-2-3 Rule" for normal foal behavior: Stand by 1 hour, nurse by 2 hours, pass meconium by 4 hours (mare passes placenta by 3 hours). Deviation from this timeline should prompt FPT evaluation.

Classification	Serum IgG at 24h	Clinical Recommendation
Adequate Transfer	Greater than 800 mg/dL	No intervention required; routine monitoring
Partial FPT	400-800 mg/dL	Consider plasma if high-risk; close monitoring; treat if concurrent illness
Complete FPT	Less than 400 mg/dL	IV plasma transfusion required; high sepsis risk
Severe FPT	Less than 200 mg/dL	Multiple plasma transfusions likely required (2-4 L)

Classification of Passive Transfer Status

Quality Grade	IgG (mg/mL)	Brix %	Specific Gravity
Very Good	Greater than 80	Greater than 30%	Greater than 1.080
Good	50-80	23-30%	1.060-1.080
Fair	28-50	15-22%	1.040-1.060
Poor	Less than 28	Less than 15%	Less than 1.040

Etiology and Risk Factors

FPT can result from mare factors, foal factors, or management issues. Understanding these categories helps identify at-risk foals for early intervention.

Mare-Related Factors

Premature lactation (placentitis, twin pregnancy): Colostrum is depleted before foaling; most common cause of agalactia in North America is fescue toxicosis
Poor colostrum quality (IgG less than 3000 mg/dL): Young maiden mares and older multiparous mares may produce lower quality colostrum
Failure to produce adequate colostrum volume: Mares produce colostrum only once per pregnancy; premature dripping depletes supply
Foal rejection or poor maternal behavior: Mare prevents nursing or exhibits aggression toward foal

Foal-Related Factors

Prematurity or dysmaturity: Immature enterocytes reduce absorption capacity; stress accelerates gut closure
Weakness or inability to stand/nurse: Neonatal maladjustment syndrome ("dummy foal"), musculoskeletal abnormalities
Poor suckle reflex: Cleft palate, neurological disorders, birth trauma
Gastrointestinal disease or malabsorption: Concurrent illness impairs immunoglobulin uptake

Management Factors

Delayed nursing (greater than 2 hours post-foaling): Critical absorption window missed
Late-season foaling (October-December): Higher FPT incidence reported in late-season foals
Dystocia or assisted delivery: Foal stress accelerates gut closure; delays nursing
Failure to assess colostrum quality: Routine testing identifies poor quality colostrum before nursing

High-YieldStress in the neonate (traumatic birth, illness, hypothermia) causes endogenous glucocorticoid release, which hastens gut closure and reduces the window for immunoglobulin absorption.

Test	Sensitivity/Specificity	Time to Result	Notes
SRID (Gold Standard)	High/High; Quantitative	18-24 hours	Accurate but impractical for stall-side use
SNAP Foal IgG Test	High sensitivity; Semi-quantitative	10 minutes	Most popular stall-side test; gives ranges
Zinc Sulfate Turbidity	Moderate; Semi-quantitative	1 hour	Inexpensive; hemolysis causes false high
Glutaraldehyde Coagulation	Good sensitivity (85%)	10-60 minutes	Inexpensive; practical field test
Total Serum Protein (Refractometer)	Variable; TSP less than 4.5-5.0 g/dL suggests FPT	1 minute	Screening only; not IgG-specific
Turbidimetric Immunoassay	High; Quantitative	5-10 minutes	Point-of-care; provides actual IgG values

Colostrum Quality Assessment

Evaluating colostrum quality before the foal nurses allows early identification of potential FPT and intervention with supplemental colostrum. Good quality colostrum is thick, yellow, and sticky; poor quality is watery and white.

NAVLE TipFor NAVLE, remember that equine colostrum should have a Brix value of at least 23% to be considered good quality. This is slightly higher than bovine colostrum (22% Brix cutoff).

Foal Age at Diagnosis	Treatment Option	Details
Less than 6 hours	Oral Colostrum (preferred)	1-2 L good quality colostrum via bottle or nasogastric tube; 250-500 mL increments
6-12 hours	Oral Colostrum (may work)	Absorption declining; recheck IgG after supplementation; may need IV plasma
12-18 hours	IV Plasma Transfusion	Gut nearly closed; oral supplementation has limited value; IV plasma required
Greater than 18-24 hours	IV Plasma Transfusion (only option)	Gut closed; oral colostrum ineffective; plasma is the sole treatment

Diagnosis of Failure of Passive Transfer

All foals should have IgG levels assessed at 18-24 hours of age, though high-risk foals may be tested as early as 6-12 hours to allow time for oral supplementation if needed. The gold standard is Single Radial Immunodiffusion (SRID), but practical stall-side tests are used in most clinical situations.

Diagnostic Tests Comparison

High-YieldThe SNAP Foal IgG test is highly sensitive but has moderate specificity in hospitalized foals. A positive SNAP test (indicating FPT) should prompt treatment, but be aware that false positives can occur in sick foals.

Parameter	Details
Plasma Source	Commercial hyperimmune plasma (IgG greater than 1200 mg/dL); fresh frozen plasma from donor horse
Expected IgG Increase	1 L plasma increases serum IgG by 200-300 mg/dL in a 50 kg foal
Typical Volume Needed	2-4 L for complete FPT (IgG less than 200 mg/dL) to reach greater than 800 mg/dL
Initial Infusion Rate	0.5 mL/kg over 10-20 minutes (20-30 mL for 45-50 kg foal) while monitoring for reactions
Maintenance Rate	If no reaction, increase to 30 mL/kg over 60-90 minutes (1 L over 1 hour for 45 kg foal)
Administration	Thaw slowly in warm water (not microwave); administer via aseptic jugular catheter with blood/plasma filter
Post-Transfusion	Recheck IgG 12-24 hours after transfusion; repeat if less than 800 mg/dL

Treatment of Failure of Passive Transfer

Treatment approach is determined primarily by foal age at diagnosis and severity of FPT. Early diagnosis allows oral supplementation, while late diagnosis requires IV plasma.

Treatment Algorithm by Age

Oral Colostrum Supplementation

Dosing: Minimum 1.5-2 L (ideally 5-12% body weight) of good quality colostrum over first 12-18 hours, administered in 250-500 mL increments every 1-2 hours.

Sources of colostrum: Frozen banked colostrum from donor mare (can be stored at -20°C for up to 2 years without IgG loss); commercial colostrum substitutes (less effective, may increase plasma reaction risk).

Administration: Bottle feeding only in foals with strong suckle reflex; nasogastric intubation for weak foals to prevent aspiration.

Important: Donor mares should be tested for anti-RBC antibodies to prevent neonatal isoerythrolysis (NI) in recipient foals.

Intravenous Plasma Transfusion

IV plasma is the treatment of choice for foals greater than 12-18 hours old with FPT and for all foals with complete FPT regardless of age.

Transfusion Reaction Signs

Monitor closely during initial infusion for: muscle fasciculations, tachycardia, tachypnea, fever, respiratory distress, laryngeal swelling, abdominal pain, pale mucous membranes, or collapse. If reaction occurs, stop transfusion immediately and provide supportive care.

High-YieldTyler's Disease (serum hepatitis/Theiler's disease) is a rare but serious complication of plasma transfusion caused by equine pegivirus. Commercial plasma is tested to minimize this risk.

Component	Details
Antimicrobials	Broad-spectrum: Ampicillin (15-30 mg/kg IV q6-8h) or Penicillin (22,000 IU/kg IV q6h) PLUS Amikacin (20-25 mg/kg/day IV); Ceftiofur (4.4-6 mg/kg IV q6-12h) for renal compromise; Add Metronidazole (10-15 mg/kg PO/IV q12h) if anaerobes suspected
IV Fluids	Crystalloids with dextrose supplementation (hypoglycemia common); goal-directed resuscitation to restore tissue perfusion
Plasma Transfusion	Correct FPT; provide antibodies and coagulation factors; antiendotoxin effects
Nutritional Support	Mare's milk or replacer at 15-25% body weight/day; small frequent feedings; parenteral nutrition if not tolerating enteral
Supportive Care	Oxygen therapy; anti-ulcer prophylaxis; NSAIDs (flunixin) for inflammation; intranasal oxygen if hypoxic

Complications of FPT: Neonatal Sepsis

FPT is the primary risk factor for neonatal sepsis, the leading cause of morbidity and mortality in foals under 4 weeks of age. The "open gut" that allows immunoglobulin absorption also permits bacterial translocation if IgG levels are inadequate.

Common Organisms in Neonatal Sepsis

Gram-negative bacteria (most common): Escherichia coli (most frequent isolate), Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella spp., Enterobacter spp., Actinobacillus equuli

Gram-positive bacteria (increasing): Streptococcus spp., Staphylococcus spp., Enterococcus spp., Clostridium spp.

Clinical Signs of Neonatal Sepsis

Depression, lethargy, weakness, decreased suckle reflex
Fever (may be hyperthermic or hypothermic)
Petechiation of ears, mucous membranes, sclera
Injected/muddy mucous membranes, prolonged CRT
Diarrhea, colic, abdominal distension
Joint swelling, lameness (septic arthritis)
Respiratory distress, cough (pneumonia)
Uveitis, hypopyon (ocular involvement)

Sepsis Score

The Brewer Sepsis Score combines historical, clinical, and laboratory parameters to assess likelihood of sepsis. A score greater than 11 indicates high probability of sepsis.

Key laboratory findings: Neutropenia with left shift, toxic neutrophil changes, hypoglycemia, metabolic acidosis, hyperfibrinogenemia, hypoalbuminemia, increased L-lactate

Treatment of Neonatal Sepsis

Prognosis

Survival rates for septic foals in intensive care units range from 55-75% with aggressive early treatment. Poor prognostic indicators include: pneumonia, severe depression, septic arthritis (especially multiple joints), profound neutropenia, and failure to respond to initial therapy. Treatment typically requires 1-4 weeks of intensive care.

NAVLE TipFor NAVLE: E. coli is the most common organism in neonatal sepsis. Gram-negative sepsis leads to endotoxemia. The combination of penicillin/ampicillin + amikacin provides excellent initial coverage while awaiting culture results.

Prevention of Failure of Passive Transfer

Pre-Foaling Management

Vaccinate mares 4-6 weeks before foaling: Core vaccines (tetanus, EEE/WEE, WNV, rabies) plus risk-based vaccines ensure high antibody levels in colostrum
Move mares to foaling location 30+ days prior: Allows mare to develop antibodies to local pathogens that will transfer to foal
Monitor udder development: Early "waxing" or dripping colostrum signals potential loss of quality colostrum
Maintain colostrum bank: Collect 250-500 mL from healthy donor mares; freeze at -20°C; test donor for NI antibodies

Post-Foaling Management

Observe nursing behavior: Foal should stand by 1 hour, nurse by 2 hours; intervene if benchmarks not met
Test colostrum quality immediately: Brix refractometer or colostrometer; supplement if quality poor
Routine IgG testing at 12-24 hours: All foals should be tested; early testing (9-12 hours) in high-risk cases
Assist weak foals to nurse: Bottle or tube feed colostrum if foal cannot nurse independently
Umbilical care: Disinfect stump with dilute chlorhexidine or iodine solution to reduce infection risk

Practice NAVLE Questions

Test your knowledge with 10,000+ exam-style questions, detailed explanations, and timed exams.

Start Your Free Trial →