Health

The harm arises in the mating rather than in the individual, and what the hearing evidence does and does not establish

The material on this page is drawn from the Global Cat Health Survey 2026, which is where its sources and its declarations of interest are set out in full.

The harm arises in the mating rather than in the individual

A cat with one copy of one variant is, on present evidence, unaffected. What is recorded below is what happens when a kitten inherits two. The two rows of the table are the two ways that can occur.

The last row repeats the configuration of the published case, which is what makes it worth recording rather than filing as an anecdote. In Abitbol et al. 2024 the dam was latent: she carried DBE^ALT, showed no blue iris, and the mating produced two white deaf kittens. In the 2026 report the dam is again a cat showing no blue eye, recorded latent by the compiler, standing six generations below Nikita'l Seymour by unbroken parent links, bred to a blue-eyed sire of a second line. Two daughters, both reported deaf. A single copy of DBE^ALT does not do this: nine genotyped heterozygous carriers of it were BAER tested at Glasgow and all nine heard normally in both ears. The reported harm in the survey is confined to animals standing below two determinants.

And that carries an inference about the Igor determinant. A compound heterozygote is two different alleles at one locus, and the published case is the PAX3 compound: the two white deaf kittens carried DBE^CEL and DBE^ALT, both PAX3 variants, one on each chromosome. A cross to a second line producing the same outcome is evidence that the second determinant is also at PAX3, because a variant at another locus would give a double heterozygote at two loci and not a compound at one. All six identified determinants are PAX3 variants, so the prior is already high and this cross is consistent with it. The step that settles it is a PAX3 screen on one Igor-line cat, a targeted read of a single gene rather than the whole-genome discovery project the survey has been asking for, and one a commercial variant panel does not perform: a panel reports the variants already named and returns clear on a seventh. The white these animals carry remains on the record as an alternative the pedigree cannot exclude, and the PAX3 read is what distinguishes them.

Reading the table has two consequences for a breeder. Every row in the lower section involves two different variants, which is the combination a breeder working carefully within a single line will not produce, and which arises when cats from two separate lines are brought together. And the empty cells matter as much as the filled ones: for three of the six variants nobody has recorded what two copies does, so a mating that would produce one cannot be assessed at all.

The mating to be avoided is therefore variant to variant, whether the two variants are the same one twice or two different ones. As the lines mix across breeds, that mating becomes easier to make unintentionally.

The Topaz, a breed founded on two variants at once

The clearest illustration of that principle is a breed that no longer exists. The Topaz was founded deliberately on two different dominant blue eye variants. Its two founding studs came from the same Kazakh breeding programme: Roxi Darlin, carrying DBE^CEL (Roxi), and Nikita'l Seymour, carrying DBE^ALT. The published description records that the breed was created using both, thereby combining two DBE lineages. The Topaz has since been discontinued. The full history of those two cats, and of the breeds created from them, is the subject of case study two of the survey.

A breed founded on two variants will produce compound heterozygotes in successive generations by design rather than by accident. A compound heterozygote is a cat with one copy each of two different variants, and it is the state in which harm is documented. Two kittens genotyped as compound heterozygous for DBE^CEL and DBE^ALT were white and deaf. A further kitten presumed to carry DBE^CEL and DBE^RE died at birth with limb contractures and an abnormal head.

The compound heterozygote is evidenced, not inferred. Abitbol et al., Animals 2024;14:1845 reports a five-kitten litter born to two DBE parents with DNA held for both parents and four kittens: a latent dam of the Seymour line heterozygous for DBE^ALT, a sire heterozygous for DBE^CEL, one kitten heterozygous for each allele, and “the two white and deaf kittens were compound heterozygous for both DBE^CEL and DBE^ALT alleles”. The paper's own conclusion is that such cats “are viable but are at high risk of deafness” and that “breeders are warned that mating two DBE cats from the Roxi and Seymour origins may be deleterious”.

The dam was latent. She carried DBE^ALT and did not show the blue iris, so a rule triggered by eye colour would not have reached her, and the mating that produced two deaf kittens would have been permitted. And the DBE^ALT variant was itself identified from the whole-genome sequence of one of those two deaf kittens.

Six black-eyed kittens: the clearest reported instance of the compound state

In later generations of the Topaz, kittens were born whose irides were so dark they read as black, and on close inspection were a very dark olive green. Six were born, from particular parent combinations. All six died of infections that were successfully treated in their normal littermates. The account concludes that they carried a defect of the immune system alongside the eye colour, and records that the lines producing the trait were sterilised.

The homozygote explanation was considered and set aside on stated grounds. None of the six carried the white distribution characteristic of homozygotes, none was deaf, and no other indication of homozygosity was present. The cause given is a mix of the two genes, DBE^ALT and DBE^CEL.

This is the strongest reported instance of compound heterozygote harm in the survey, and it is stronger than the deaf kittens on three counts. It carries a visible marker, the black iris, so the state can be recognised without a test. It carries a distinct clinical outcome, failure to survive infections that littermates survived, which is not deafness and not neonatal death. And the competing explanation was examined and excluded with reasons given rather than left unaddressed.

What it is not. It is a breeder account and not a laboratory result. None of the six was genotyped, the lines are sterilised, and the breeding records were lost with the computer that held them. Six animals is a small number and infection has many causes.

Two things nonetheless follow that do not depend on accepting the cause. The observation was made before any of the variants had been named, so it cannot have been shaped by the literature. And the same account asks the question the survey asks, in its own words, of the high-white cats: which of the two genes are they homozygous for, “could they be homozygous for both genes?” A breeder working without a test had reached the compound heterozygote question from the phenotypes alone.

Source: messybeast.com, Blue-Eyed Breeds: Topaz (Ukraine), the Seymour DBE^ALT and Roxi DBE^CEL genes, 2016 to 2026, supplied to the survey on 15 August 2026. A breeder account throughout; no genotype appears in it.

No published source states why the Topaz was discontinued. What can be stated is that the mechanism was present in the breed's foundation and that the documented harm arises in precisely that pairing.

The record is missing. The cats are not

The Topaz genealogy continues, through the British Shorthair and Longhair, the German Longhair, the Maine Coon, the Persian, the Siberian, the Sphynx and Elf, the Ragamuffin, the Highlander, the Munchkin and Minuet and the Lykoi. The survey evidences the cats leaving Ukraine between 2014 and 2018, well before the invasion. The loss of the records in 2022, and of the cats that were still in Ukraine, did not end the line.

The records were lost to the invasion of Ukraine. The breeder fled to Poland with her cats and dogs; the computer holding the breeding records was stolen; and the Polish authorities required most of the cats, of which there were a large number, to be sterilised and homed as pets or sold. The source records that she was unwilling to see the cats taken up by other breeders who would have the work for nothing. The original Topaz is therefore either lost or suspended and many of the records with it, and cats described as Topaz still appear for sale online. Where the cats appear after that point, they appear in whatever register would take them: chart C6 annotates both Lazuli and Kaena, the two cats the Céleste was founded on, “Registered as European / XSH”, and Roxi, Nikita'l Seymour, Nikita'l Pheer-Ca and Nikita'l Markiss stand as XXH, XSH, XSH and XLH. Those codes are the successor records, not the original ones.

The survey identifies 174 records, 34 catteries and 11 breeds in which the Topaz lineage continues to progress, the most recent cat born in 2024. Nine were born in 2023 or later. The line is not historical.

Why no one has recorded it. The cats went out under the prefixes of the catteries that bought them and under whatever breed codes those catteries could get them admitted under, so the descent is spread across surviving registers in fragments that do not name the breed they came from. The only way to follow the line is to piece it together by name and by pedigree, one animal at a time, from charts, from individually held certificates and from later registrations elsewhere, which is what this register does and why the count can be given at all.

If the association holds, the Topaz has evidential value disproportionate to its size, because it represents a cross that could not ethically be designed. It indicates that the risk in these variants lies not in carrying one but in holding two within the same population. The rule that follows is neither a ban on blue-eyed cats nor a ban on carriers, but a ban on combining two variants, whether the same variant twice or two different ones.

That rule cannot be enforced with a single-variant test. To know whether a proposed mating puts two variants together, a breeder has to know which variants both cats carry, and a test that looks for one variant reports only on that one. A cat clear for DBE^RE may still carry DBE^ALT: the mating that produced the two deaf kittens would have passed a DBE^RE test on both parents. The seven products on sale cover between one and five variants each, so they are not interchangeable, and a rule that does not say which variants must be covered has not specified the test it is asking for.

A DNA test cannot establish whether a cat is deaf

One of the seven products is sold under the name Deafness in cats with DBE. It is a DNA test. It reports whether the cat carries the DBE^RE variant. It does not report whether the cat can hear, and no DNA test can, because hearing is measured by a BAER recording at a veterinary practice and not by a cheek swab in the post.

The name assumes the very link the test is being sold on, and that link has since been qualified in the peer-reviewed literature. It rests on a series of eight affected cats reported when the variant was first described, in Rudd Garcés et al., G3 2024. A later study, Abitbol et al., Animal Genetics 2025, tested ten more cats carrying the same variant, from different lines, and found that all ten heard normally in both ears. The product name reflects the first paper and not the second.

A registry writing a requirement has to say which of the two it wants, because “a DBE test” and “a hearing test” are different things with different costs, different providers and different answers. A rule that asks for the wrong one will be satisfied by a result that does not address the risk it was written for.

Why two blue-eyed cats of the same breed can carry different variants

Two breeders may each advertise a blue-eyed British Shorthair, using the same breed name and describing the same eye colour, while one cat carries DBE^CEL (Roxi) and the other DBE^ALT. Nothing a buyer can see, and nothing on either advertisement or either pedigree, distinguishes the two.

This happens because breeding cats move between catteries. A variant enters a breed through a single outcross and kittens from that line go to other breeders; a second variant enters the same breed later through a different outcross made by a different breeder. Within a few generations the breed holds both variants, in different catteries and often in different countries, described in identical terms. The same confusion now occurs one level down. A single named line may contain more than one variant, and the breeder-created names built on the trait, among them Azul, British Ice, Heavenly Eyes, Pantherina and Pharaoh, are names for an appearance rather than for a variant, so two cats sold under one of those names need not carry the same thing either.

The first consequence is evidential, and it governs how the breed table in the survey should be read. PAX3 DBE by breed, on the Laboratories page, records the variants that have been found somewhere in each breed. It does not state that the breed is uniform. The British Shorthair row carrying DBE^ALT does not mean that blue-eyed British Shorthairs are DBE^ALT cats; it means at least one was. This also explains how competing breed lists may both be accurate: if DBE^CEL (Roxi) and DBE^ALT are both present in the British Shorthair in different catteries, a list naming either is correct and a list naming only one is incomplete.

The second consequence, and the technical heart of this directory

How these variants are governed turns on the distinction below.

1. Two copies of one variant is a risk a careful breeder can already avoid. A breeder working within a single line knows which variant runs in that line. If it carries DBE^ALT, mating two carriers risks a homozygote, a known and avoidable pairing. This is the risk existing rules are written for.
2. One copy each of two different variants is a separate risk, and it is the one documented to cause harm. The two kittens born deaf carried DBE^CEL (Roxi) and DBE^ALT, one copy of each. The kitten that died at birth is presumed to have carried DBE^CEL (Roxi) and DBE^RE, one copy of each. No homozygote was involved in either.
3. The mating that produces the harm is between two cats of the same breed carrying two different variants, each with a single copy. A cat with one copy of one variant is, on present evidence, unaffected, so both parents are healthy animals.
4. Whether that mating is avoidable depends entirely on whether the cats show the trait, and it divides into two cases.
   Blue eye to blue eye. Both cats visibly carry something. The risk is apparent to the breeder, the pairing is already prohibited by every registry rule written for this trait, and a breeder who makes it anyway is not acting carefully.
   Blue eye to latent carrier, or latent carrier to latent carrier. A latent carrier has the variant and shows no blue eye, sometimes with no white marking either. The breeder cannot know, the mating does not present as a blue-eyed pairing at all, and no degree of care would prevent it. This is the case in which nobody has been careless and the harm still occurs.
5. In the latent case, nothing in current practice detects the pairing. Knowledge of one's own line does not, because the two variants sit in two different lines and each breeder knows only their own. Eye colour does not, because the point of a latent carrier is that there is no blue eye to see. The pedigree does not, because almost no pedigree records which variant a cat carries. A single-variant test does not. Only genotyping both parents, across the full panel, reaches it.

The only instrument that closes that gap is a genotype recorded against the individual animal and transferred with it when it is sold, which is why that requirement appears twice in the protocol below.

Why a requirement written on appearance cannot work

Such a requirement fails in both directions, and both failures are documented rather than hypothetical.

It misses carriers. A latent cat carries a variant and never displays a blue eye, in some cases with no white marking either. Latent cats are recorded by genotype in Abitbol et al., Animals 2024;14:1845, Table 2, two in the Seymour lineage, three in the Nanotigr and one in the Roxi, and in Abitbol, Couronné et al., Animal Genetics 2024;55:670-675, Table 1, one Céleste. One latent queen was the dam of the two deaf compound heterozygotes. Under a requirement triggered by eye colour she would have bred without restriction.

It restricts cats that carry nothing. Blue eyes arise independently through the white spotting allele and through the colourpoint allele. The working rule used by many breeders, that a blue eye in a coloured coat indicates DBE while a blue eye in a white cat indicates white spotting, is incorrect: a documented mating between two cats carrying no DBE variant, one of them carrying white spotting, produced a blue-eyed offspring with parentage confirmed by DNA.

A ban on blue or odd eyes therefore excludes cats that present no risk while leaving the carriers that do. It regulates the appearance of the problem rather than the problem.

It also removes the evidence that would settle the question. Registration is the only independent certification of parentage a pedigree cat has, and a claim about descent that rests on a breeder's own record does not meet the standard applied at the higher tiers of peer review. Closing registration to a trait therefore ends the certified record of the line at that generation, while the cats themselves continue to be bred. The mechanism, its two consequences and the registration class that would resolve it are set out in the survey under the heading what a registry certifies, and what a ban removes.

The deafness evidence for DBE^RE, and what it does and does not establish

The deafness evidence is set out in full below because the misstatement of it is now more widely circulated than the finding itself, including by a commercial testing company and by the general purpose chatbots most breeders will consult first.

The DBE^ALT hearing data, reported here for the first time

Deafness is the reason dominant blue eyes is treated as a welfare question rather than as a colour. Hearing is established by a BAER test, a brainstem auditory evoked response recording, which measures the response of the hearing nerve to a stimulus in each ear separately. It yields a result in a kitten or in an animal that will not co-operate with other forms of hearing assessment, and it is the only method that detects unilateral deafness.

For DBE^ALT no such testing had been carried out. The published papers list the DBE^ALT lines with hearing recorded as assumed normal, and state the reason: no BAER recording existed for any of those animals. The literature therefore carried an assumption in place of a measurement, for the variant most widely distributed in the British Shorthair.

Fourteen British Shorthair and British Longhair cats of a DBE^ALT line, nine carriers and five wild type relatives, were BAER tested at the Neurology and Neurosurgery service of the Small Animal Hospital at the University of Glasgow on 17 and 24 September 2024, on a TruTrace EMG system, each ear recorded separately at 80 and 100 dB. All fourteen heard normally in both ears. The carriers spanned silver and golden backgrounds and ages from two months to three years, and one of the nine was a latent cat whose carrier status was established only by DNA testing. To the author's knowledge this is the first BAER series in DBE^ALT British cats.

All fourteen animals were genotyped, the nine carriers heterozygous and the five relatives wild type, so the result applies to the heterozygous state only, which is the state in which the breed is ordinarily maintained. No homozygote occurred in the cohort, and only three DBE^ALT homozygotes have been described, two Altai and one Nanotigr, all white, with deafness in some of them, in Abitbol et al., Animals 2024;14:1845, Table 2. The cohort is small and drawn from a single breeding programme, so the result is preliminary.

A latent carrier bred to a carrier, and what the litter showed

The prevention control at the end of this page rests on one proposition: that a cat can carry a PAX3 variant, show no blue eye at all, and pass the variant on. Every registry rule the survey coded triggers on the colour of the eye, so none of them can see such an animal. The proposition is established by genotype in Abitbol et al., Animals 2024;14:1845, Table 2 and Abitbol, Couronné et al., Animal Genetics 2024;55:670-675, Table 1. A mating in a British Shorthair programme demonstrates it directly.

The published position first. Latent carriers are a genotyped fact and not an inference. In the DBE^ALT lines, three Nanotigr cats and two of the Seymour lineage were found heterozygous for the insertion while showing no blue eye. One Celestial cat is likewise heterozygous for DBE^CEL (Roxi) without the phenotype, and in 2025 two latent cats were identified in the DBE^RE Maine Coon line, where none had been reported before. Latency appears in three separately arisen variants, so it is a property of the trait rather than a peculiarity of one line.

The mating. A latent British Shorthair queen, a black smoke carrying no blue eye, was bred in May 2024 to a carrier male of the same line. Of four kittens, two carry the trait and two do not. A cat that no eye-colour rule could have identified produced blue-eyed offspring.

The design matters: it is the control operating voluntarily. On present evidence the harm arises when two variants meet in one animal, so a carrier-to-carrier mating is the pairing the proposed control exists to prevent. Pairing a latent carrier with a carrier answers the transmission question without making that pairing. The queen's own certified pedigree carries no eye code, and neither does the sire's, though his sire is coded 61 and his great-granddam 69: a registry reading either document would see nothing at all.

Two limits apply. This is one litter of four in one programme, so it demonstrates that transmission from a latent carrier occurs and establishes no rate. And the queen's carrier status was known only because she had been DNA tested; had she not been, the litter would have appeared as blue-eyed kittens from an ordinary-eyed mother, which is how such litters are ordinarily recorded and why the register holds so many untested possible carriers.

The DBE^ALT series is cited as a released summary statement: Abitbol M, Cloquell A, Kaczmarska A, Holmes K, Macaulay K. BAER hearing screening in British Shorthair and British Longhair cats carrying DBE^ALT: Summary Statement. Belle Ayr Cats, 3 June 2025. It is not peer reviewed. Its method and its cohort, cat by cat, are reproduced in the sources. The ten Maine Coon results from the same testing sessions are published: they are the BAER data reported as Table 2 of Abitbol et al. 2025, on which the author is a named co-author.

The mechanistic passage above is the assessment of Prof Marie Abitbol, VetAgro Sup, who described four of the six variants. The three sentences in quotation marks are her wording; what follows them, including the genealogy effect, is a summary of her position and not a quotation. Rudd Garcés et al., G3 2024, 10.1093/g3journal/jkae131, Results, clinical investigations. Abitbol et al., Animal Genetics 2025, 10.1111/age.70020, Table 2 and pages 3 to 4. The 2025 BAER cohort is the author's own; see the declaration of interest in the accompanying paper.

A proposed PAX3 prevention control, for every breed

This is one of 6 recommendations about where to spend a test. The other five are test the cats whose status cannot be seen, on Findings, trace the descendants of the unmapped lines, on Spread across breeds, screen PAX3 in one cat of an unmapped line, on Findings, test the white cats, on Latent lines and test one cat in each cattery holding two variants in one breed group, on Variant trace. A further three recommendations, about how a registry writes the trait down, are on Recommendations.

Registries have attempted prevention control, and the intention is correct. What is absent is a protocol matched to the behaviour of the variants.

Sources for this section: Abitbol M. CORIN and DBE variants in cats: what we know and what remains to be explored. Presentation to the World Cat Federation, 2026, VetAgro Sup, Campus vétérinaire de Lyon. Abitbol et al., Animal Genetics 2024, 10.1111/age.13433. Rudd Garcés et al., G3 2024, 10.1093/g3journal/jkae131. Abitbol et al., Animals 2024, 10.3390/ani14131845. Abitbol et al., Animal Genetics 2025, 10.1111/age.70020. Declaration of interest: the author of this directory is a co-author of the 2025 paper cited above.