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Understanding NI Foals: Are We Ignoring a Preventable Problem?
- 3 hours ago
- 6 min read
In today’s horse world, especially in breeds where blood typing has largely been replaced by DNA parentage testing, Neonatal Isoerythrolysis (NI) has quietly faded from everyday breeding conversations. Unfortunately, the risk itself did not disappear just because the testing became less common.
Over the last several years, many breeders have stopped blood typing their horses altogether. With only a very limited number of blood typing laboratories still available, it has become easier to simply “play the odds” and hope for the best.
But is that good enough?
For many breeders, the answer is probably no.
What Is Neonatal Isoerythrolysis (NI)?
Neonatal Isoerythrolysis is a condition where a mare’s antibodies attack her newborn foal’s red blood cells after the foal nurses colostrum.
The foal is not born sick.
The problem begins after nursing.
The mare’s first milk, called colostrum, contains antibodies that are critical for protecting the foal from disease. However, if the mare has developed antibodies against certain blood factors carried by the foal, those antibodies can destroy the foal’s red blood cells once absorbed into the bloodstream.
This can lead to:
Severe anemia
Weakness
Jaundice (yellow gums or eyes)
Rapid decline
Death if untreated
The most common blood factors associated with NI are:
Aa
Qa
These are part of the horse’s blood group systems.
Understanding Horse Blood Typing
Unlike human blood types (A, B, O, etc.), horses have multiple blood group systems. Within those systems are many individual factors that can either be present (+) or absent (-).
A horse may test:
Positive (+/+) or (+/-) for a factor
Negative (-/-) for a factor
When breeders discuss “double negative” horses, they are usually referring to horses that are:
Aa negative
Qa negative
These horses are often written as:
Aa-/Qa-
or simply “double negative”
A mare that is double negative can develop antibodies if exposed to a foal carrying Aa or Qa positive blood factors.
That exposure typically occurs:
During foaling
Through placental leakage
After abortion
Following blood transfusion
Occasionally after previous pregnancies
Once sensitized, the mare may produce antibodies that can affect future foals.
How Do You Get an NI Foal?
An NI foal occurs when:
The mare lacks a blood factor (often Aa or Qa)
The stallion carries that factor
The foal inherits the positive factor from the stallion
The mare has developed antibodies against it
The foal nurses colostrum containing those antibodies
Importantly:
Not every incompatible breeding creates an NI foal
Not every double negative mare will become sensitized
A mare can have several healthy foals before suddenly producing an NI foal
That unpredictability is exactly why the issue becomes dangerous.
Many breeders assume:
“She’s crossed fine before.”
Until she doesn’t.
Do Other Blood Factors Matter?
While Aa and Qa are the blood factors most commonly associated with severe NI foals, they are not the only blood group antigens horses possess.
Horses have several blood group systems with many individual factors. Other antigens can occasionally be involved in NI reactions, but Aa and Qa remain the primary concern because they are the factors most likely to produce severe, life-threatening disease.
Other blood factors that have occasionally been implicated include:
Qb
Ac
Pa
and others within the equine blood group systems
For most breeders, understanding Aa and Qa status provides the most practical and useful information when evaluating NI risk.
Why Blood Typing Still Matters
As an industry, we shifted heavily toward DNA testing because it is excellent for:
Parent verification
Genetic disease testing
Color genetics
Identity confirmation
But DNA testing does not replace blood typing for NI risk assessment.
Blood typing tells us about the blood factors involved in NI risk.
Without blood typing:
Breeders often do not know whether mares are double negative
Stallion stations may not know the blood factors of stallions they stand
Crosses are made blindly
NI risk becomes guesswork
For many breeding operations, especially those heavily using shipped semen, frozen semen, embryo transfer, or valuable foals, that may not be a risk worth ignoring.
Should You Blood Type Your Breeding Stock?
For many breeders, the answer is yes — especially mares.
Knowing your mare’s blood type can:
Identify double negative mares
Help evaluate breeding risk
Allow planning before foaling
Prevent emergency situations
Knowing your stallion’s blood type also helps mare owners make informed breeding decisions.
The good news is:
Not every horse is high risk
Not every breeding requires panic
Many breedings are perfectly safe
The key is simply understanding where the risk exists.
What You Need to Worry About — And What You Don’t
Higher Concern Situations
Double negative mares (Aa-/Qa-)
Mares with a prior NI foal
Mares with unknown blood type bred to unknown stallions
Valuable foals where risk reduction matters
Mares that received blood transfusions
Mares with multiple pregnancies and unknown sensitization history
Lower Concern Situations
Crosses where incompatibility is ruled out
Mares that are not sensitized
Breedings where blood typing confirms compatibility
Foals monitored appropriately after birth
A prior healthy foal does not guarantee future safety.
And a mare with a previous NI foal should always be considered high risk in future pregnancies.
What About Embryo Transfer (ET) and ICSI?
Embryo Transfer and Intracytoplasmic Sperm Injection do not eliminate the risk of an NI foal.
This is a very common misunderstanding.
The foal’s blood type genetics still come from:
The donor mare
The stallion
The recipient mare does not genetically influence the foal’s blood type.
However — and this is the important part — the foal will nurse the recipient mare’s colostrum, not the donor mare’s.
That means the recipient mare’s antibodies are what matter at foaling.
In Embryo Transfer (ET)
The important pairing becomes:
Recipient mare
Foal
Not donor mare and recipient mare.
Even though the donor mare genetically produced the foal, the recipient mare’s colostrum is what could potentially attack the foal’s red blood cells.
That means:
A recipient mare can produce an NI foal
Even if the donor mare herself never had NI issues
Because the foal may inherit blood factors incompatible with the recipient mare
In ICSI
With Intracytoplasmic Sperm Injection, the situation is essentially the same.
ICSI only changes how fertilization occurs in the laboratory. The resulting embryo still carries genetics from:
The donor mare
The stallion
Once transferred into a recipient mare:
That recipient carries the pregnancy
Produces the colostrum
Becomes the NI consideration at foaling
So Who Needs Testing for ET and ICSI?
For higher-value ET and ICSI pregnancies, it is ideal to know:
The donor mare’s blood type
The stallion’s blood type
The recipient mare’s blood type
Especially if recipient mares have:
Unknown histories
Carried multiple pregnancies
Unknown sensitization status
The recipient mare is the horse providing colostrum and antibodies to the foal — making her an important part of NI risk evaluation.
As reproductive technologies advance, awareness of NI risk probably needs to advance with them.
What Testing Options Are Available?
Blood Typing
One of the primary laboratories still performing equine blood typing in the United States is:
They offer:
Blood typing
NI risk evaluation
Parentage testing
Additional genetic testing
Some specialty equine reproductive veterinarians may also coordinate testing through reference laboratories.
Pre-Foaling NI Testing
For mares considered at risk, additional testing can be done late in pregnancy.
Mare Antibody Screening
A veterinarian can submit blood samples from the mare in the weeks before foaling to determine whether harmful antibodies are present.
This is often performed:
During the last 2–4 weeks of gestation
Especially in mares with prior NI history or known incompatibility risk
Colostrum Testing
After foaling, the mare’s colostrum can be tested against the foal’s red blood cells.
This helps determine whether the foal can safely nurse.
Testing methods may include:
Jaundice agglutination test
Crossmatch testing
Laboratory compatibility testing
If the colostrum is unsafe:
The foal may need to be muzzled temporarily
Alternative colostrum provided
Plasma supplementation given
Nursing delayed until antibodies are no longer absorbed
Timing matters tremendously because foals absorb antibodies most efficiently during the first several hours after birth.
Questions Every Mare Owner Should Ask
Before breeding, consider asking:
Has this stallion been blood typed?
Is he Aa or Qa positive?
Has this mare been blood typed?
Has she ever had an NI foal?
Should this pregnancy be monitored differently?
Many stallion stations may already have blood typing on file — breeders simply are not asking for it.
As a Breed, Are We Paying Attention?
Modern breeding technology has advanced rapidly. We can freeze semen, perform ICSI, transfer embryos, and genetically test for countless traits with incredible precision.
Yet in many ways, basic blood typing knowledge has quietly faded into the background.
NI is not necessarily common — but when it happens, the consequences can be devastating. And unlike many breeding risks, this is one where education and preparation can genuinely prevent loss.
The goal is not fear or overreaction.
The goal is awareness.
Knowing the blood type of breeding horses, understanding where risk exists, and having a plan before foaling can make all the difference.
Sometimes the most important breeding decisions happen long before a foal ever stands to nurse.
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