Severe Combined Immunodeficiency in Horses: SCID in Arabian Foals
- See your vet immediately if an Arabian or part-Arabian foal develops fever, diarrhea, cough, nasal discharge, or trouble breathing in the first 2-8 weeks of life.
- SCID is an inherited, autosomal recessive immune disorder seen mainly in Arabian and Arabian-cross foals. Affected foals are born looking normal but cannot make a functional immune response.
- Foals often become sick as maternal antibodies fade, then develop severe bacterial, viral, fungal, or protozoal infections. The condition is considered fatal.
- A DNA test can identify affected foals and healthy carriers. If both parents are carriers, each foal has a 25% chance of being affected.
- Typical diagnostic and supportive-care cost range in the US is about $300-$2,500 for farm exam, bloodwork, IgG testing, infectious disease testing, and genetic testing. Hospitalization or intensive foal care can raise the total to $2,000-$8,000+.
What Is Severe Combined Immunodeficiency in Horses?
Severe combined immunodeficiency, or SCID, is a fatal inherited immune-system disorder seen primarily in Arabian and part-Arabian foals. These foals are usually born appearing normal, but they lack functional T cells and B cells, the white blood cells needed to fight infection and make antibodies. Without those defenses, everyday germs can become life-threatening.
Many affected foals seem healthy for a short time because they absorb protective antibodies from colostrum after birth. As those maternal antibodies fade over the first weeks of life, the foal becomes increasingly vulnerable. Clinical signs often begin between 2 and 8 weeks of age, and many foals die or are euthanized by 6 months of age because the disease cannot be cured.
This is different from failure of passive transfer, where a foal does not receive enough antibodies from colostrum. In SCID, the problem is genetic and affects the foal's own immune system. That distinction matters, because the long-term outlook and breeding implications are very different.
For pet parents and breeders, SCID is as much a breeding-management issue as it is a medical one. DNA testing has made it possible to identify carriers and reduce the risk of producing affected foals.
Symptoms of Severe Combined Immunodeficiency in Horses
- Fever
- Coughing or nasal discharge
- Rapid, labored, or noisy breathing
- Diarrhea
- Poor nursing or weak suckle
- Poor growth or failure to thrive
- Repeated or hard-to-clear infections
- Depression or lethargy
See your vet immediately if a young Arabian or part-Arabian foal has fever, diarrhea, cough, nasal discharge, weakness, or breathing trouble. SCID foals can decline quickly because even routine infections become dangerous. Signs often start after a foal seemed normal at birth, which can make the change feel sudden.
These symptoms are not specific to SCID, so your vet will also consider sepsis, pneumonia, rotavirus, Rhodococcus, and failure of passive transfer. The combination of breed background, age, repeated infections, and poor response to treatment often raises concern.
What Causes Severe Combined Immunodeficiency in Horses?
SCID is caused by an inherited genetic mutation in the DNA-dependent protein kinase (DNA-PK) gene. This mutation prevents normal development of immune-system diversity, so the foal cannot produce functional T and B lymphocytes. The result is a profoundly weakened immune system from birth, even though the foal may look healthy at first.
The condition is inherited in an autosomal recessive pattern. That means a foal must inherit two copies of the mutation, one from each parent, to be affected. Horses with one copy are called carriers. Carriers are usually healthy and show no clinical signs, but they can pass the mutation to their offspring.
If both parents are carriers, each pregnancy has a 25% chance of producing an affected foal, a 50% chance of producing a carrier foal, and a 25% chance of producing a foal that is clear of the mutation. This is why SCID prevention focuses heavily on pre-breeding genetic testing rather than waiting for a sick foal to appear.
SCID has been reported mainly in Arabian horses and Arabian crosses. Published and laboratory data suggest the mutation is now less common than it once was, likely because breeders have used DNA testing more consistently.
How Is Severe Combined Immunodeficiency in Horses Diagnosed?
Your vet will usually start with the foal's history, breed background, age, and pattern of illness. A young Arabian or part-Arabian foal with recurrent pneumonia, diarrhea, fever, or poor growth raises concern, especially if the foal seemed normal at birth and then worsened over the next few weeks.
Initial testing often includes a physical exam, complete blood count, chemistry panel, and infectious disease testing based on the foal's signs. Your vet may also check IgG levels to evaluate passive transfer, because failure of passive transfer can look similar early on. These tests do not confirm SCID by themselves, but they help rule in or rule out other common causes of severe illness in foals.
A DNA test is the most specific way to confirm the known SCID mutation. UC Davis Veterinary Genetics Laboratory lists testing for Arabian and Arabian-cross horses, with hair-root samples commonly used for submission. Genetic testing can identify whether a horse is clear, a carrier, or affected, and it is also useful for testing the sire and dam when breeding decisions are being made.
In some cases, a diagnosis is strongly suspected during life and then supported by necropsy after death or euthanasia. Because SCID is fatal and has breeding implications, a confirmed diagnosis can help families, breeders, and your vet make informed decisions for the current foal and future matings.
Treatment Options for Severe Combined Immunodeficiency in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm or clinic exam
- Basic bloodwork such as CBC/chemistry
- IgG testing to assess passive transfer
- Targeted infectious disease testing based on symptoms
- SCID DNA test for the foal or parents
- Short-term supportive care such as fluids, nursing support, and antibiotics if your vet feels they are appropriate
- Quality-of-life discussions and humane end-of-life planning if SCID is strongly suspected
Recommended Standard Treatment
- Full veterinary exam and repeat monitoring
- CBC/chemistry and additional infectious disease workup
- IgG testing and assessment for concurrent failure of passive transfer
- SCID genetic confirmation
- Broad supportive care for pneumonia, diarrhea, dehydration, or sepsis risk as directed by your vet
- Isolation or biosecurity steps to reduce infectious exposure
- Clear prognosis counseling and breeding guidance for the parents
Advanced / Critical Care
- Referral-hospital or neonatal ICU care
- Continuous monitoring and oxygen support if needed
- IV fluids, plasma or other blood-product support when indicated by your vet
- Advanced imaging or repeated lab monitoring for pneumonia and systemic infection
- Aggressive treatment of secondary infections
- Isolation nursing and intensive supportive care
- Necropsy after death or euthanasia if the diagnosis remains uncertain or breeding records need confirmation
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Severe Combined Immunodeficiency in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Based on my foal's age, breed, and symptoms, how concerned are you about SCID versus other causes of infection?
- What tests do you recommend today, and which ones are most important if I need to prioritize the cost range?
- Should we test this foal for the SCID mutation now, and should the sire and dam be tested too?
- Are we also checking for failure of passive transfer or sepsis, since those can look similar early on?
- What supportive care options are realistic at home, and when would referral or hospitalization make sense?
- What signs would mean my foal is suffering or declining despite treatment?
- If SCID is confirmed, what is the expected prognosis and what humane care options should we discuss?
- How should future breeding plans change if one or both parents are carriers?
How to Prevent Severe Combined Immunodeficiency in Horses
The most effective way to prevent SCID is pre-breeding DNA testing of Arabian and Arabian-cross breeding horses. Because SCID is autosomal recessive, the goal is to avoid carrier-to-carrier matings. A horse that is a carrier is not sick, but breeding two carriers together creates a 25% risk of an affected foal in every pregnancy.
Prevention does not mean every carrier must automatically be removed from breeding. In some programs, your vet and breeding team may discuss using a valuable carrier only with a horse that has tested clear, then testing offspring before future breeding decisions. This approach can reduce disease risk while still protecting genetic diversity within a bloodline.
It also helps to keep clear records of test results, pedigrees, and mating plans. If a foal is born sick and SCID is suspected, confirming the diagnosis can guide future breeding choices for related horses. Testing is widely available, and UC Davis lists SCID testing for Arabians and Arabian crosses using hair-root samples.
Good newborn care still matters, but it cannot prevent SCID in an affected foal. Prompt colostrum intake, early veterinary exams, and monitoring for fever, diarrhea, or respiratory signs may help your vet identify problems sooner, yet only genetic screening before breeding can prevent producing affected foals.