Arabian Horse Genetic Diseases: SCID, CA & Lavender Foal Syndrome

Key point: a carrier horse looks completely healthy. A DNA test is the only reliable way to know a horse's status before breeding [1][2][3][4].

Three inherited conditions are closely tied to Arabian horses and their crosses: SCID, cerebellar abiotrophy (CA) and lavender foal syndrome (LFS) [4]. Each is caused by one known gene change, and each can be avoided through informed breeding choices [1][2][3][4].

How are these diseases inherited?

All three follow the same pattern, called autosomal recessive [1][2][3][4].

• A foal is only affected if it gets one changed copy of the gene from its sire and one from its dam [1][2][3][4].
• A horse with just one changed copy is a carrier. It is healthy and shows no signs, but can pass the copy to its foals [1][2][3][4].
• You can't tell a carrier from a clear horse by looking at it [1][2][3][4].

What happens if two carriers are bred?

Each parent passes on one copy of the gene, chosen by chance. So when two carriers are mated, on average [1][2][3]:

These are averages over many foals, not a promise for any single foal [1][2][3]. If only one parent is a carrier and the other is clear, no foal can be affected, though some may be carriers [1][2][3].

What is SCID in horses?

SCID (severe combined immunodeficiency) is an immune system disorder. Affected foals can't make the white blood cells they need to fight infection [1][4][5]. It is described in Arabian and part-Arabian horses [1][5].

• At birth: the foal looks normal, because antibodies from its dam's first milk (colostrum) still protect it [1].
• At around 2 to 3 months: as that protection fades, repeated serious infections appear, such as coughing, nasal discharge and noisy breathing. Diarrhoea, lameness and poor growth are also possible [1].
• Outlook: there is no cure. Most affected foals die of infection by about 5 months of age, even with veterinary care [1].

What is cerebellar abiotrophy (CA)?

CA is a neurological condition. Nerve cells slowly break down in the cerebellum, the part of the brain that controls balance and movement [2][4][5]. It affects Arabian horses and Arabian crosses almost only [2].

• Signs usually start between about 6 weeks and 4 months of age [2][4][5]. They include a head tremor, unsteady movement, a high-stepping walk, a wide stance, stumbling or falling, trouble getting up, and not blinking when something moves toward the eye [2][4][5].
• Signs can be severe or subtle, and a few affected horses show no clear signs at all [2].
• Outlook: there is no cure, and affected horses can't be ridden safely. Depending on severity, they are kept as pasture or companion horses, or humanely put to sleep [2].

What is lavender foal syndrome (LFS)?

LFS joins an unusually pale coat, described as lavender, pale pink or silver, with severe neurological signs [3][4][5]. It is seen mainly in horses with Egyptian Arabian bloodlines [3][5].

• Signs are present from birth. The foal can't stand or nurse normally, and may have seizures, stiff over-straightened legs, neck and back, paddling leg movements and involuntary eye movements [3][5].
• Outlook: sadly, LFS is fatal. Affected foals die, or are humanely put to sleep, within hours to a few days of birth [3][4][5].

When should you call a vet?

All three conditions are urgent. Call a licensed vet straight away if a young Arabian or part-Arabian foal shows [1][2][3]:

• Repeated or worsening breathing illness at around 2 to 3 months old.
• A head tremor, wobbly walking, a high-stepping gait or frequent falls from about 6 weeks old.
• As a newborn: trouble standing or nursing, seizures, stiff paddling legs, or a pale, lavender-tinged coat with signs of distress.

How common are carriers?

These figures come from specific study groups outside Saudi Arabia. They are not Saudi or Gulf figures, and carrier rates vary a lot between countries and bloodlines [5].

• The South African SCID drop came after routine DNA testing began there in 2005. The study's authors see this as evidence that testing plus informed breeding can make a disease gene less common [5].
• The same paper cites earlier US estimates of about 19.7% for CA, and about 10.3% for LFS in Egyptian-blood Arabians (about 1.8% in other lines) [5].

How does a DNA test help breeders?

For each condition, a DNA test places a horse in one of three groups: clear, carrier or affected [1][2][3].

• If the mare and stallion are both clear for a condition, their foal can't be affected by it [1][2][3].
• If one parent is a carrier, choosing a mate that tested clear for the same condition avoids affected foals [1][2][3].

In the US, the Arabian Horse Association, a private breed association, has an internal Code of Ethics rule for its own members. Since 31 December 2017, members must disclose a breeding horse's known SCID, CA or LFS status when selling, leasing or offering it for breeding [6]. This is one association's rule, not a law, and it doesn't apply in Saudi Arabia.

Is testing available in Saudi Arabia?

We did not find an official Saudi or Gulf source on local testing for these conditions, or any Saudi carrier figures. Ask your vet about current testing options. For registration questions, the King Abdulaziz Arabian Horse Center (KAAHC) is named on Saudi Arabia's Ministry of Environment, Water and Agriculture website as the body that registers Arabian horses in the Kingdom [7]. This is a separate matter from genetic testing. Nothing here means KAAHC, the Ministry, or any other Saudi authority endorses, recommends or requires SCID, CA or LFS testing.

FAQ

Can a carrier horse look healthy?

Yes. Carriers show no signs at all, so only a DNA test can reveal their status [1][2][3][4].

What is the chance of an affected foal from two carriers?

On average 1 in 4 for each foal. Two in 4 are expected to be carriers and 1 in 4 clear [1][2][3].

Is there a cure for SCID, CA or LFS?

No. None of the three has a cure [1][2][3].

Do these diseases affect part-Arabian horses?

Yes. SCID and CA are described in Arabian crosses too [1][2][5].

Is lavender foal syndrome linked to a bloodline?

It is seen mainly in horses with Egyptian Arabian bloodlines [3][5].