Posted 05 June 2007 - 04:43 AM
WHAT TO DO WITH CA CARRIERS?
I have been recently asked on how I would handle dealing with the potenial CA carriers? Should they not be bred at all (both mares & stallions), should all stallions deemed carriers be gelded? As of right now without testing one must have produced an affected foal to know if the parents are carriers. So how to handle this both before and after testing?
In my opinion good breeding practices consist of a program based on quality Foundation Mares for which individual stallions are used ( in the plural) to complement the mare's phenotype and genotype.
Within the Spanish bloodlines (with which I am most familiar), it has been a long accepted axiom to avoid doubling up on the mare lines. Whereas it is common to find line breeding to a specific stallion and in many cases an inbred factor to an individual stallion, repetitive usage of inbreeding or line breeding to a specifc mare is not recommended. There are many reasons for this approach. The major one being that the stallion lines have already been reduced to three in direct male descent. Further reduction of an already small genetic pool by reducing the female lines with inbreeding would reduce the genetic variation even more and jeopardize the future of the Spanish Arabian. Reducing the genetic variation not only enhances and sets the positive desired traits; it also does the same for those not so desirable traits. This applies to physical attributes as well as temperment and any hidden neurological disorders such as SCIDS and CA.
This selective form of line breeding ( ie. avoiding it in the female lines) would be a wise approach to breeding in any of the Arabian bloodlines.
Not always, but in general, Cerebellar Abiotrophy is found in Arabians with repetitive crosses to one or more carriers. Obviously the more confirmed and suspected carriers in the background of a horse, the more likely the odds are that it also will be a carrier and/or affected if bred to another carrier. At this point in time a confirmed carrier is a mare or a stallion which has produced/sired a foal with clinical signs of Cerebellar Abiotrophy and has had a histopathic diagnosis completed on the status of the Purkenje cells.
As all affected horses will produce 100 % carriers no matter if bred to a clear or carrier mate, no affected horse should be used for breeding at all. Confirmed Carrier mares should not be bred to known carrier stallions or even stallions with suspected carriers in their background. Confirmed Carrier stallions should be stated as such and bred only to non-carrier mares whenever possible. These breeding practices will not eradicate CA from the genetic pool but they will reduce the occurrance of CA and provide some sort of protection for future generations.
SCID testing has had a beneficial effect on lowering the rate of scid foals produced. Keep in mind that the owner of a scid foal knows that it will die. In order to avoid financial loss and time loss, mare owners have become more selective in their choice of stallions and in testing their own mares. With CA, even when there is a test, affected foals do not die. If only slightly affected many will not be recognized and may be used for breeding as is the case at present. A normal looking foal will mature to breeding age and will usually be bred, even if a carrier. Testing will only provide guide lines for the responcible breeder.
What is happening now, is that confirmed carrier horses are being used as breeding stock. Any affected foals are quietly euthanized. The carrier mare is rebred; produces a normal looking foal and both are sold onwards. Or, in some more responcible establishments, because carrier mares can also produce clear foals, selected daughters are kept , bred and if no affected foal is produced, will be retained in the breeding herd and the original dam sold onwards or simply retired from breeding. Testing will eliminate such a long drawn out and expensive process of identifying carrier progeny. However, at the moment that is the only way to ascertain if a mare line is a CA Carrier line or not.
Short term breeders are the cause of much of the spread of undesirable genetic conditions. They tend to breed for a quick turn over. These breeders can have very few or numerous breeding stock. Numbers are not a qualification of the description. Short term breeders do not test their own progeny's bloodlines through breeding before selling. Far too often they will continue to breed their carrier mare for a number of years before keeping a "replacement" daughter, without ever having bred any of the previous daughters themselves or test any of the sons..
A concientious breeder will want to see how the progeny breed onwards and if the correct breeding selections are being made to further their breeding goals. The short term breeders are also the ones who usually sell off their confirmed carrier stock without a word to an unsuspecting buyer that the mare or the stallion has had wierd head shaking foals. It comes down to keeping the profit/loss margin in favor for the breeder. A horse with a neurological condition is pretty hard to sweep under the rug.
It will all be up to the breeders themselves, but I do feel that horses with suspected genetic defects should be required to have a disclosure clause in any breeding or sales contract. That is just my opinion to be sure.