SCID fully known as severe combined immunodeficiency disease, is an autosomal recessive gene that has been identified in Arabian horses or Arabian cross horses. It is interesting to note that a similar condition exists in mice and humans, as well as several other species. An autosomal recessive gene is a gene pair that has to be inherited one from the dam and one from the sire, resulting in both parents contributing the recessive gene. Once the foal has a recessive gene for SCID from both parents, he or she will have the condition. The parents, each having only one copy of the gene, are considered to be carriers, even though they will show no signs of the genetic condition. It is estimated that 1 of every 567 Arabian foals will have SCID and approximately 82.9 percent of the population of Arabians are clear, which means they do not carry the recessive gene for SCID. SCID affects both male and female foals equally, so it is not sex linked in nature.
SCID is not identified at birth simply because there are no obvious visible signs. The foal will appear normal and will often continue to do well for one to three months. What the inherited disease does is prevent the foal from developing a normal immune system, meaning that the foal will not be fight off viral and bacterial infections. Even mild infections that are normal for foals to easily fight off will be fatal to an SCID Arabian.
Since the symptoms of SCID will be the symptoms of the viral or bacterial infection that affects the foal, each case will have individual symptoms. High fevers, colic, discharge from the nose, lack of energy, weakness and coordination problems are seen in many of the viral infections that are prominent in foals. SCID foals will not respond to treatments and therapies and will succumb to the viral infection very quickly, usually not living longer than four to six months.
The wonderful news is that the gene mutation that causes the fatal SCID in Arabians has been identified, as well as a gene marker test to either diagnose the horse as a carrier or as clear, thereby allowing even carrier horses to be used in careful breeding programs to always produce non-affected foals. Any foals of crosses of clear and carrier Arabians would then need to be checked to determine if they were clear or carriers.
In the future Arabian breeders will need to make the decision about testing requirements before registration and well as the role of the carrier in breeding programs. Some breeders, since the estimated number of carriers is between 18-25% of the total number of purebreds, recommend a careful program designed to gradually decrease the number of carriers over time, eventually breeding out the recessive condition entirely.