Testing for Intervertebral disc disease (IVDD) in Kooikers?

An intriguing mystery has been slowly developing within the Kooikerhondje world involving body proportions, recently-discovered genetics, and possible disc disease. While the research and discussions are only in their infancy, currently nothing comes up on a Google search for IVDD in Kooikers. So while this article is highly speculative and will likely need future revision, I wanted to combine some of the discussions Kooiker enthusiasts are having with the known research.

What is IVDD?

Intervertebral disc disease, or IVDD, is a painful and debilitating disease of the spine, similar to what in humans we might call a slipped, ruptured or herniated disc. It is most classically seen in Dachshunds and other long-backed, short-legged dogs – called a chondrodystrophic phenotype, or body type. The development of the chondrodystrophic body type predisposes these dogs to spinal issues, although IVDD can occur in any dog. Symptoms include weakness or refusal to walk, pain over the herniated disc, a “drunken” walk, scuffing of toes due to loss of sensation, and occasionally urinary incontinence. (See IVDD articles by ACVS and on VIN.)

Recent Genetic Findings

In 2017, a paper was published identifying the genetic connection between the long, low chondrodystrophic body type and IVDD (Brown et al, 2017). It was followed by another 2019 study (Batcher et al, 2019). To simplify, these studies found a mutation in a gene called CDDY that was associated with the chondrodystrophic body type as well as the higher incidence of clinical IVDD. In some breeds like Cavaliers, the CDDY gene is “fixed” so all dogs of the breed have two copies. In many breeds, however, it appears appears that there is variation between individuals. IVDD associated with this gene can develop in dogs with one OR two copies of the CDDY mutation, and is inherited as an autosomal dominant trait. Dogs with two copies of the CDDY gene are shorter in stature and more likely to develop clinical IVDD.

These studies shook up some other breeds and spurred a push in some to add genetic testing for these IVDD-involved genes to breed health testing standards. While the UC Davis Veterinary Genetics Laboratory initiated testing for this CDDY gene and supports ongoing research on it, genetic testing for this can also be done through labs such as Embark, Pawprint Genetics, and Animal Genetics. In a brief survey of known affected breeds on the OFA website, none had added this genetic test as a CHIC requirement at the time of writing, but it is a “recommended” test among breeders of other dog breeds.

What about Kooikers?

Critically, NONE of the above-named research has ever included a Kooikerhondje in their analyzed data. Thus when discussing this issue in Kooikers, we must extrapolate in a way that is currently unfounded by science. But there is evidence enough to support discussing it in the breed:

  • There are anecdotal reports of possible IVDD in the breed that appear correlated with dogs with longer bodies and shorter legs (this would possibly imply a dog with two copies of the CDDY mutation).
    • The breed’s Register contains very few mentions of IVDD, disc, spine, or back issues. A search for “Laagesteld” which is defined by the Register as “too short on legs compared to the length of the body” returns 35 individual dogs with that descriptor, and there appears to be at least a loose familial relationship between these dogs. Whether this is describing chondrodystrophic dogs is unclear. A search for “wervel” (vertebra), “wervelkolom” (spine), and “schijf” (disc) had 11 results with nonstandardized and sometimes vague descriptions including related pain and malformations, and only one dog born in 2005 was listed with an unequivocal diagnosis of “tussenwervelschijf problemen” (intervertebral disc problems). However, the Register is limited by its dependence on owner-reported data, which may be inaccurate, incomplete, or completely unreported.
  • Until the late 20th century, the Kooiker standard mandated a height limit but made no mention of proportion. This meant that dogs that were short due to abnormally short, possibly chondrodystrophic legs were included in the show and breeding population. It appears likely these dogs had two copies of the CDDY gene but were within standard at the time. Today, the standard calls for “slightly off square” proportions with the length of the leg equal to the depth of body, which would eliminate a long and low chondrodystrophic dog from show and breeding. We still see dogs of certain lines pop up with these proportions today, however. The breed Illustrated Standard discusses proportions in more detail and includes photos of short-legged Kooikers.
  • When tested with Embark, my own dog came back as carrying ONE copy of the CDDY mutation. This heterozygosity implies that Kooikers fall into the category where the mutation segregates (or, both the normal and mutated allele are present in the population). He does not exhibit the chondrodystrophic phenotype, and is proportioned rectangularly with legs the same length as his depth of body, as called for by the standard.
  • One of the UC Davis researchers involved has informally speculated that Kooikers and Tollers are likely to exhibit similar phenotypes (body types) in response to variation in the CDDY gene. Tollers, or Nova Scotia Duck Tolling Retrievers, have been extensively studied for their “skeletal dysplasia” (now known as chondrodystrophy) and may serve as a model for Kooikers. In Tollers, the rectangular body shape is what is seen commonly in the show ring, but longer and shorter (chondrodystrophic) leg proportions also exist in the population. It’s been shown that the shorter-legged dogs have one or two copies of this CDDY mutation. If Tollers and Kooikers show similar variation in leg length correlated with their CDDY copies, it is reasonable to conclude – as the UC Davis researcher speculated – that the short-legged Kooikers seen may also follow this genetic pattern.

To Test Or Not To Test?

The UC Davis Veterinary Genetics Laboratory recommends CDDY genetic testing “for those breeds in which CDDY has been documented to segregate, in other words, both the CDDY allele and the normal allele are present in the breed” – which appears to describe Kooikers. Their recommendations depend more specifically on knowing allele frequency in the breed, which without testing we do not currently know.

Breed parent clubs set health testing requirements. While every breed can be tested for every health issue and mutation, the approach is typically conservative – to increase compliance and reduce cost, breed clubs tend to only require tests for the most medically impactful, prevalent, and/or lethal disorders. A health issue that effects a dog’s quality of life is prioritized over an issue that may just leave it unsuccessful in the show ring. In the past, chondrodystrophy was considered just an oddity of physical proportions that occasionally cropped up, but now with new research and association with crippling IVDD, genetic testing for CDDY deserves a second look. For Kooikers the issue is even more constrained due to the breed’s dangerously narrow gene pool – dogs with one or two CDDY mutations may not be able to be excluded from the gene pool, especially if the allele frequency in the population is high. It is a permanent balancing act between the long-term health of the breed’s gene pool and the shorter-term health of individual dogs – a balancing act that each owner and breeder must independently contemplate. However, I would also call upon the international parent clubs to establish standardized guidance about testing and recording of dogs with IVDD, any CDDY mutations, and/or chondrodystrophic phenotypes.

Soon, the University of Utrecht will be able to do these SNP genetic tests on blood submitted for routine health testing for von Willebrand’s and ENM. The program is not yet complete – the submission form will allow the owner to opt in to all three tests instead of the two (but it is not yet updated to include this option) and thus we will not need to submit Kooiker DNA to a third-party genetics company like Embark or Wisdom Panel. The team at the University IS aware of the CDDY mutation possibilities in the breed. It remains to be seen whether this mutation will be tested for during routine health testing for dogs in the future, and if so how it would be communicated to owners and/or included in the Fit2Breed program or Register. The timing of these discoveries and advances means there may be an opportunity for parent clubs to require and record testing for CDDY mutations in Kooikerhondjes.

If you have data, anecdotes, photos or genetic panels that relate to the speculative points I’ve made in this article – I’d love to hear from you! I’m happy to revise this article with more information, so please feel free to email me.