The DNA tests for hydrocephaly and dwarfism have been available for members since May last year. Every animal species and each breed has its own hereditary disorders. The DNA tests are a strong instrument for a significant reduction of these problems. Last year the DNA tests received a lot of attention on the KFPS website, in Phryso and during lectures and on how this issue should be targeted in practical terms. Now that a large number of mares have been tested for these innate defects we are confronted with queries from the field, particularly about stallion choices for mares who have been tested positive. With this article we once more aim to clarify what steps should be taken.
Carriers retain their value for breeding: The fact that a mare is a carrier does not mean that she has lost her value for breeding. The KFPS supports this view because carrier status is no reason to reject a stallion for the purpose of stallion selection. Now that risky matches can be excluded thanks to DNA tests, carrier status has lost most of its magnitude. The exclusion of carriers (stallions and mares) would not only lead to a (renewed) significant increase of inbreeding, it will also stand in the way of further improvement of quality.
What is the ratio of carriers? The ratio of horses with the dwarfism- or hydrocephaly gene is more or less equal. Both disorders have a rate of 1 out of 7 affected horses. This is in line with preconceived expectations and we now see this confirmed on the basis of the number of horses tested so far. The dedicated efforts to prevent risky matches even before DNA tests became available have paid off, resulting in less than 1% of foals born with these disorders.
How do we define a risky match? A risky match is defined as a pairing between a stallion and mare who are both carrier of the same disorder. Risky matches will produce 1 out of 4 foals (25%) with the same disorder. It is a matter of plain common sense that such risky matches must be avoided. If the mare is not a carrier the chances of a foal with dwarfism or hydrocephaly are zero, regardless of the carrier status of the stallion in question.
How best to prevent risky matches? As long as data about carrier status is only available for stallions approved starting in 2014, the best strategy to avoid risky matches is by testing of the broodmares and by subsequently discussing the choice of stallion with the stallion keepers. It should be a matter of principle to have all broodmares tested, but especially so when a stallion is used which has been tested positive for carrier status. The reason being that such a stallion can only be used on mares which have been tested negatively.
Why are not all stallions tested? The KFPS policy is aimed at the exclusion of risky matches, in other words, the exclusion of the birth of dwarfism- or hydrocephaly foals. If carrier status of all stallions would be public knowledge then there is a fair chance that the affected stallions might become disregarded for breeding. This would be an undesirable situation because in spite of their carrier status, these stallions can still present a positive contribution to breeding. Besides, the exclusion of two times one out of seven stallions (nearly a third) will lead to an irresponsible increase of inbreeding, together with the attached risk of creating a basis for new hereditary disorders. This is why the KFPS has chosen to introduce a transitional period. Carrier status of all stallions approved from 2014 onwards will be publicised. During this transitional phase breeders and stallion keepers can work together to prevent further occurrence of dwarfism and hydrocephaly.
Is it advisable to breed with unaffected animals only? The odds that a carrier will pass on the anomalous gene to one of its offspring is 50%. Meaning, that matches between a carrier and an unaffected animal will yield an average of 50% of foals with carrier status. Should we strive to breed from unaffected animals only? The KFPS is very much opposed to breeding with unaffected animals only because:
1 The exclusion of all animals with carrier status will lead to an increase of inbreeding.
2 Excluding animals with high breeding values will cause a delay in the fabric of genetic evolution.
3 With the availability of tests risky matches can be ruled out. Any animal´s positive carrier status is therefore no longer a valid reason to exclude it from breeding.
4 The policy to avoid risky matches will lead to a drop in the use of stallions with positive carrier status (a manifest tendency even before the development of DNA tests) which will trigger a gradual decline in the percentage of carriers and the ultimate phase-out of the disorder.
5 The KFPS is working towards the development of tests for other disorders. In the long-term it will therefore be no option to breed from unaffected animals only.
How best to choose a stallion for mares with positive carrier status? In the case of mares who have turned out to be carriers of dwarfism and/or hydrocephaly after DNA testing, the right strategy is to discuss the safety of the planned pairing with the stallion keeper where the intended stallion is standing stud. In the case of disorders this information should be reported back to the stallion keeper in question so that he will be made aware that the stallion is in fact a carrier. If in doubt, the other option is to consult with the KFPS about the risk of particular match for a mare with carrier status (mail@kfps.nl).
What is the procedure for testing mares? From the point of view of animal welfare the KFPS holds the view that it is immoral to breed from untested mares. This is the reason why the KFPS charges a low rate for testing. Horses can be tested for carriership of dwarfism, hydrocephaly or chestnut factor at a rate of € 46,10 (ex VAT). Tests can be ordered online via My Horse. For the purpose of testing, the KFPS already has a data bank with hair samples of a great number of horses. This means that there is no need to collect hair samples and the test can be ordered simply by a couple of mouse-clicks. Members can also get in touch with the KFPS by phone or email to apply for tests.
As posted in the KFPS Newsletter March 2015
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