Equine IgG / Passive Immunization (Foals)

Biology / Background

: Foals are born immunologically naive and with virtually no circulating antibodies of their own. The mare’s diffuse epitheliochorial placenta prevents the transfer of maternal immunoglobulins to the unborn foal. The protection of the newborn foal during the first weeks of life therefore relies exclusively on the intake of immunoglobulins—primarily immunoglobulin G (IgG)—from colostrum (first milk). Healthy foals begin suckling within the first one to two hours; maternal antibodies are detectable in the blood as early as a few hours later.

Pathophysiology / Epidemiology

The absorption of intact immunoglobulins through the intestinal epithelium is only possible during the first few hours of life and declines rapidly; with the onset of so-called intestinal closure (gut closure) within about 24 hours, the ability to absorb colostrum largely ceases, so that oral administration of colostrum thereafter is of little benefit. If colostrum is ingested too late, in insufficient quantities, or of inadequate quality, or if absorption is impaired, this results in insufficient passive immune transfer (Failure of Passive Transfer, FPT). FPT is the most common immune system disorder in foals; contributing factors include premature milk let-down in the mare, poor-quality colostrum, insufficient suckling, and prematurity.

Course & Consequences

Failure of passive transfer does not cause any symptoms on its own, but it is a major risk factor for the foal’s health: Affected animals are significantly more susceptible to infections, particularly neonatal sepsis (septicemia), which is one of the leading causes of illness and death in the first weeks of life. Adequate transfer is a prerequisite, but no guarantee, that a foal will remain healthy.

Diagnostic Relevance

The success of passive transfer in foals can be assessed by measuring the serum immunoglobulin G (IgG) concentration, typically about 18 to 24 hours after birth; at-risk foals may be tested earlier. As a reference, a serum IgG concentration above 800 mg/dl is considered sufficient, a value between 400 and 800 mg/dl indicates partial failure, and a value below 400 mg/dl indicates complete failure of passive transfer. An early assessment of IgG status allows for targeted intervention, such as supplemental colostrum administration within the absorption window or intravenous plasma administration after the window has closed.

Rapid Test Component

The Fassisi® Equine IgG enables a semiquantitative assessment of the immunoglobulin G status in foals using serum or whole blood, thereby supporting bedside evaluation of passive immune transfer.

References can be found here.