Hereditary Defects

Policies regarding hereditary defects, 6/18/07.

KFPS has been working hard the past years to address the issue of hereditary defects in the horse population. First an inventory has been made of the problem and then it was looked at how effective policies would have to be formulated. To begin with the emphasis is on water on the brain and dwarfism. Regularly articles have been published in Phryso about the issue. Here is an overview of the matter.

Origin

Literature shows that we can assume the hereditary defects are as old as the studbook itself. In the Stallion Book various stallions from the early years are mentioned that were carriers of dwarfism. The origins of water on the brain is less clear. The defects have always been, be it dormant, present in the population, but have increased in numbers because a number of very influential stallions were carriers. For the passing on of dwarfism the preferent stallion Ritske 202 has most likely played a big role. For the gene for water on the brain it was especially the (also preferent) stallions Wessel 237 (Ritske dam) and Oege 267, through Wessel’s sire Jarich 226. All three of the latter stallions were by the way most likely both carriers of the gene for water on the brain and dwarfism.

Inventory

Indicating which of the current stallions are carriers is seriously hampered because it’s estimated that less than 10% of the foals with defects is reported to the studbook by the breeders. To illustrate this: in 2005 and 2006 combined, e.g., only 3 foals with dwarfism were reported to KFPS. In spite of that, based on the (limited amount of) information KFPS has, an estimation has been made of the size of the problem. The estimate is that per year about 30 foals with dwarfism are born and about 50 foals with water on the brain. Of the foals with water on the brain only part is carried full term by the way. It is known that many foals with water on the brain are aborted early. The inventory has moreover rendered the estimate that the frequency with which the deviating genes occur is increasing. It is estimated that of the current studbook stallions 12 are carrier of the gene for dwarfism and 16 are carrier of the gene for water on the brain. It is emphasized that this is an estimate.

Singular recessive

It has been determined from stam/mareline research that the characteristic is passed on as a socalled singular recessive characteristic. This means that both water on the brain and dwarfism are cause by one gene pair. It does concern two different gene pairs however. With such characteristics there are three variants: free animals (AA), carriers (Aa), and patients (aa). Carriers have a normal gen and a deviating gene, and pass this deviating gene on to 50% of the offspring but do not show any symptoms of the deviation themselves. A combination of two carriers results in 1 of 4 cases in a patient. If both sire and dam’s sire are a carrier then the chance is 1 in 8 and if the sire and the grand dam’s sire are both a carrier then the chance is 1 in 16 for a defective foal.

DNA test

The KFPS has concluded from the inventory that combating the problem needs to receive the highest priority. Given that both characteristics are determined by one gene pair it is possible to develop a DNA test. With a DNA test it can be reliably determined which horses (especially stallions) are carriers. For the development of a test DNA material is needed from patients, so from dwarf foals and foals with water on the brain. The KFPS called in help for this from all veterinarians in The Netherlands. Meanwhile an attempt has already been made to develop a DNA test for dwarfism (there already is enough material available for this), but unfortunately not yet with result. Now that a new, better analyzing technique is becoming available (SNP technique) it can be expected that rather soon a test for both defects will become available.

Policies

Developing policies is not as simple as it may seem at first sight. The first objective is to prevent defective foals from being born. This can be done by preventing combinations with high risk. The most simple way would be to disapprove all carrier stallions. KFPS is, however, of the opinion that this is not desirable. Not only would this mean that a number of very valuable stallions could be lost for breeding but also the gene pool would again be diminished with an extra increase in inbreeding as a result. KFPS is of the opinion that carrier stallions have to be maintained for breeding. Moreover young carrier stallions have to, if they possess extra qualities, continue to be eligible for approval. If we do not do this then 1 in 3 young stallions will based on the DNA test not be eligible right off the bat. Of course the evaluation report will, in case of approval, include if a stallion is a carrier so that risky combinations can be prevented. To this end there will be two possibilities in the future:

• evaluation if there are carrier stallions in the dam line of the dam and
• a DNA test can be performed on the mare.

By setting higher demands for carriers when inspecting young stallions, the number of carriers with the gene frequency of the deviating genes will ultimately diminish in the population. This is next to preventing defective foals as a second objective.

Transitional measure

As long as there is no DNA test available, it will be impossible for KFPS to publish a list of carriers as a consequence of the minimal announcement of defective foals by the members. The chance that, based on the current information, stallions would be incorrectly singled out as a carrier or that based on such a list stallions would be incorrectly listed as a non carrier, is too big. To still minimize the number of defective foals as much as possible up to the moment that a DNA test becomes available, a meeting took place between the KFPS breeding council and the board of the VFH (stallion owners/managers). At this meeting it was agreed that the stallion owners/managers will take care of making as few as possible risky combinations. At the end of this year KFPS will review to which degree this is an effective approach and will adjust the policies accordingly.

Conclusion

KFPS realizes that by taking both defects out of the taboo atmosphere the discussion is open to all. This is inevitable. KFPS is, however, of the opinion that there absolutely no reason for panic. The number of defective foals is relatively limited. Moreover it genetically concerns a relatively simple problem. With the progress of the techniques and the policies based on that both defects can therefore quickly become a problem of the past. The policies are therefore exactly and especially focused on preventing damage caused by side effects (breeding-technically taking valuable horses out, diminishing the gene pool). After all, the remedy cannot be worse than the problem.

For more information: Phryso June 2006, pg. 42 and on. (This was translated by Anneke and published in The Friesian Jan/Feb 2007). Source: KFPS website.