Severe Combined Immunodeficiency in Arabian Foals: SCID Explained

By Dr Duncan Houston

A foal that repeatedly develops infections or fails to grow normally needs a thorough veterinary investigation. In Arabian and part-Arabian foals, one possible explanation is severe combined immunodeficiency, commonly called SCID or, in older descriptions, combined immunodeficiency.

SCID is a devastating inherited disease, but genetic testing gives breeders a practical way to avoid producing affected foals. Understanding the difference between a healthy carrier and an affected animal is essential.

Quick Answer

SCID prevents an affected foal from developing normal immune defences, leaving it vulnerable to severe infections. Sick foals need urgent care while the cause is investigated; do not wait for a genetic result before seeking help. Testing both prospective parents and avoiding carrier-to-carrier matings can prevent affected offspring for the known Arabian SCID variant.

What goes wrong with the immune system?

Affected foals do not make functional B and T lymphocytes, important immune cells involved in protection against infection. Antibodies received through the mare’s colostrum can initially provide some protection, so a foal may appear normal at birth and become ill later.

The UC Davis SCID overview describes recurrent or severe infections, poor growth and a grave prognosis. Respiratory illness, diarrhoea and weight loss may occur during the first weeks or months. These signs also have other causes, so breed and symptoms alone are not a diagnosis.

When does a foal need urgent help?

Emergency steps while arranging examination

1. Tell the vet the foal’s age, breed, nursing pattern and how quickly signs developed.
2. Provide the mare’s and stallion’s genetic reports if available, plus previous blood tests and treatment details.
3. Keep the mare and foal in a clean, quiet, safe space while following the vet’s instructions.
4. Do not force-feed a weak foal or give leftover antibiotics or human medicines.
5. Ask whether transport is safe and whether the receiving hospital needs special arrangements.

Keep a short record of episodes rather than relying on memory: dates, signs, medicines, apparent improvement and relapse. This can help the vet recognise a pattern without delaying care for the current illness.

How vets confirm SCID

Blood counts and immune testing may reveal persistent abnormalities, while a DNA test identifies the known inherited variant. A laboratory report must match the correct foal’s identity and be interpreted alongside the examination.

Important alternatives include failure to receive adequate antibodies from colostrum, sepsis and other causes of immune dysfunction. A foal with inadequate passive transfer does not automatically have SCID. Some alternative explanations have a different treatment outlook, which is why a careful work-up matters.

Understanding genetic results

The UC Davis Veterinary Genetics Laboratory SCID guide explains the autosomal recessive pattern: an affected foal inherits a copy from each parent. Both male and female foals can be affected.

For two carriers, each pregnancy has a 25% chance of an affected foal, a 50% chance of a carrier and a 25% chance of a clear foal. These are probabilities for every mating, not a promise that four foals will fall neatly into those groups. A carrier-to-clear mating avoids affected foals for this variant but can still produce carriers.

Treatment and difficult decisions

Antibiotics and supportive care may address individual infections, but they do not correct the inherited immune defect. Routine clinical care does not offer an established cure, and affected foals usually die or require euthanasia early in life.

Once the diagnosis is confirmed, ask the treating team to explain comfort, likely response and the burden of further interventions. A compassionate plan may include euthanasia when recovery is not realistic. This discussion should be based on the foal’s confirmed condition, not solely on a worried owner’s suspicion.

Common mistakes to avoid

• Assuming a healthy-looking newborn cannot have an inherited disorder.
• Waiting for laboratory paperwork while a foal is critically ill.
• Calling every carrier sick or unsuitable for all breeding.
• Relying on an unverified pedigree statement instead of an identifiable test report.
• Believing extra supplements can rebuild the missing immune function.

Frequently asked questions

Can carriers develop SCID later?

A carrier of this recessive variant does not develop SCID from having one copy.

Should all carriers be removed from a breeding programme?

Informed pairing with tested clear horses can avoid affected foals while preserving valuable genetic diversity. Discuss the broader breeding plan with an experienced adviser.

Does a clear SCID test guarantee a healthy foal?

No. It addresses this specific inherited variant, not all infections, immune disorders or other genetic conditions.

Accurate records, transparent disclosure and planned matings can prevent avoidable suffering. For a foal that is already unwell, prompt veterinary care remains the first priority.

For help understanding your animal’s care and preparing questions for your veterinarian, visit ASK A VET™.