Shih Tzu CDDY Test: What UC Davis Research Says About Disc Surgery Risk

SamI’m about to buy a Shih Tzu puppy. Should I even worry about its back?

Elena MarshIt’s worth asking before the deposit. A 2019 UC Davis study genotyped 110 Shih Tzus and found the disc-related variant in some dogs but not in others.

SamThe breeder said both parents are health tested. Doesn’t that cover it?

Elena MarshThat depends on which tests. The Orthopedic Foundation for Animals publishes the Shih Tzu health-testing program, and it pays to read exactly what it includes.

SamAnd if a test comes back positive, will the puppy end up needing surgery?

Elena MarshNo gene result can promise that. The UC Davis genetics lab says multiple factors decide whether a particular disc herniates, so a result is about odds, not destiny.

Bottom line: In the Shih Tzu, the disc-disease variant CDDY (an FGF4 retrogene on chromosome 12) is present in some dogs and absent in others, so a DNA test can actually tell two Shih Tzus apart. In a 2019 UC Davis study, Shih Tzus with at least one copy had about 10 times the risk of ending up in decompressive disc surgery compared with Shih Tzus without the variant (relative risk 10.3, 95% CI 1.8–62.1) — a real signal, but a very wide range because the sample was small. One copy already counts: for disc disease CDDY acts as a dominant trait, so a report reading “Carrier” or “N/CDDY” means at risk, not “healthy carrier.” Neither the American Shih Tzu Club nor the OFA CHIC program requires this test, so buyers have to ask for the parents’ results. A DNA result is not a diagnosis and cannot predict whether a given dog will herniate a disc; back pain, wobbling or weakness needs a veterinarian, who makes the diagnosis and treatment decisions.

The Shih Tzu ranked No. 21 in the American Kennel Club’s 2025 popularity list, down from No. 20 in 2024, and the AKC gives the breed a life expectancy of 10 to 18 years — so a puppy chosen this year may live with its genotype for well over a decade. This article works from the primary research, chiefly Batcher and colleagues’ 2019 paper in Genes from the University of California, Davis, and turns it into questions a buyer or breeder can act on.

Why the Shih Tzu numbers you find online disagree

Search for Shih Tzu CDDY figures and you will find 0.56, 0.25 and 0.161 quoted as if they measured the same thing. They do not, and mixing them up is the easiest mistake to make with this research.

0.56 comes from Table 1 of the 2019 paper, which describes only dogs that had disc surgery. The UC Davis Veterinary Medical Teaching Hospital (VMTH) keeps a DNA repository; the authors took the dogs surgically treated for intervertebral disc disease (IVDD) and reported how often the chromosome 12 insertion appeared among them. Among operated Shih Tzus the allele frequency was 0.56 — a frequency inside a group selected because their discs failed, inflated by design. It says nothing about how common the variant is in the breed.

0.25 comes from the paper’s Supplementary Table S1, the population sample. These 110 Shih Tzus had no IVDD diagnosis — the paper takes its breed frequencies from such dogs — and came partly from the UC Davis hospital’s DNA bank and partly from a University of Bern biobank that holds no health information; breed was owner-reported. Their genotypes: 64 dogs with no copies, 36 with one copy and 10 with two copies, an allele frequency of 0.25.

0.161 comes from a separate dataset. The UC Davis Veterinary Genetics Laboratory (VGL) publishes CDDY frequencies among dogs its paying clients submitted: 0.161 (95% CI 0.1047–0.2387) for Shih Tzus, in the “30–99 dogs tested” band (as checked in September 2026). Clients who pay for a test are self-selected, so this is not a census either.

Table 1 also shows that 7.61% of the 92 Shih Tzus in the hospital repository had undergone decompressive surgery. That is sometimes passed around as “the share of Shih Tzus that get IVDD.” It is not; it is a rate inside one referral hospital’s biobank, counted from samples collected at random with respect to IVDD before the laboratory began actively recruiting disc cases in 2016. The fair summary: two sources put the breed’s CDDY allele frequency around 0.16 to 0.25 — present, but far from universal.

SamSo where does “over half of Shih Tzus carry it” come from?

Elena MarshMost likely the 0.56 in Table 1, which counts only dogs already operated on at UC Davis. The 2019 population figure for ordinary Shih Tzus is 0.25.

A breed still split on CDDY — and why that changes what a test is for

Dogs carry two different FGF4 retrogenes. The chromosome 18 insertion (CDPA) shortens legs and, per its curated OMIA entry, is not disease-related. The chromosome 12 insertion (CDDY), identified by Brown and colleagues in PNAS in 2017, shortens legs modestly and also makes the center of the discs degenerate and calcify early in life, setting up Hansen type I disc extrusion. The 2019 supplement lists Shih Tzus at 1.0 for CDPA, but only five Shih Tzus were typed for it — read that as “probably common,” not a measured frequency.

The interesting part is CDDY. Where it is almost fixed, testing a pet tells you little: nearly every dog is positive and there is no one to choose instead. The Shih Tzu is different. In the 2019 population sample, by our arithmetic on the published counts, 46 of 110 Shih Tzus carried at least one copy and 64 carried none. When a breed is split like this — “segregating,” in genetics — the test becomes a tool for telling individual dogs apart.

It is also why the Shih Tzu is one of the few breeds with a breed-specific risk estimate at all. The authors calculated relative risks only for breeds that segregated the variant (allele frequency below 0.5 and above 0.05) and had enough surgical cases; in Dachshunds and French Bulldogs, they wrote, the high allele frequency “precluded the calculation of a relative risk.” Their conclusion speaks directly to breeds like the Shih Tzu: “In breeds with lower allele frequencies of 12-FGF4RG, selection against the allele should reduce the incidence of disc disease.”

The tenfold estimate, read the way the authors meant it

Table 6 of the 2019 paper gives the Shih Tzu a relative risk of 10.3 (95% CI 1.8–62.1; p = 0.005). In plain terms: in that US referral-hospital dataset, Shih Tzus with one or two copies of CDDY had about 10 times the risk of ending up in decompressive surgery at UC Davis compared with Shih Tzus without the variant. Each qualifier matters:

• The outcome is surgery, not “getting IVDD.” The abstract calls it the “relative risk for disc surgery.” Episodes managed without surgery, or treated elsewhere, were not measured.
• The comparison is within the breed. The reference group is Shih Tzus without CDDY. The figure says nothing about Shih Tzus versus Dachshunds or any other breed.
• Controls were old and disc-free. They were dogs at least 10 years old with no IVDD diagnosis at their last hospital visit, from samples collected before 1 February 2016 without regard to IVDD.
• One and two copies were pooled, treating the variant as dominant.
• The sample was small. The Shih Tzu row covers 52 dogs (cases plus controls); the number of cases is not reported. An interval from 1.8 to 62.1 means the evidence that carriers fare worse is solid, while the size of the effect is not.

The authors’ summary covers all segregating and mixed breeds together: “The presence of 12-FGF4RG increases the risk for disc herniation 5.5-15.1-fold over the background risk in segregating and mixed breeds.” Each figure is a within-group comparison with heavily overlapping intervals, so ranking breeds by point estimate would read more into the data than it holds.

SamTen times sounds terrifying. Do most carrier Shih Tzus end up in surgery?

Elena MarshNo. A relative risk compares groups; it isn’t one dog’s probability. The 2019 paper gives no absolute surgery rate for carrier Shih Tzus, and its 1.8–62.1 interval is wide.

Decoding the report: N/CDDY, A/N and the misleading word “Carrier”

Labs write the same three genotypes differently: N/N, N/CDDY and CDDY/CDDY; or 0, 1 or 2 copies; or, at the Orthopedic Foundation for Animals (OFA), Normal (N/N), Carrier (A/N) and Affected (A/A). The OFA label is where many owners go wrong.

Usually “carrier” means a dog with one copy of a recessive variant that stays healthy itself. CDDY does not work that way for the discs. Brown and colleagues wrote that the insertion is “semidominant with respect to height” but “dominant for altered IVDs”, and the 2019 paper concluded that it “acts in a dominant manner to decrease the age of onset and increase the overall risk of disc disease.” The OFA page itself says a Carrier (A/N) dog “is at risk for developing the disease.” A one-copy Shih Tzu belongs in the higher-risk group the relative risk describes.

Copy number still matters for some things. Among 423 operated dogs with radiographs, calcified discs were seen in 84.8% (190 of 224) with two copies, 63.8% (51 of 80) with one and 18.5% (22 of 119) with none; adjusted odds ratios versus no copies were 6.02 (95% CI 2.75–13.18) for one copy and 14.82 (6.46–34.04) for two. Age at surgery behaved differently: one copy was associated with surgery 26.6 months earlier than no copies and two copies with 31.2 months earlier, and the 4.6-month gap between one and two copies was not significant (p = 0.266). All of these dogs had already had surgery, and age at first surgery also reflects owners’ finances and access to referral care.

SamThe breeder says the mother is “only a carrier,” so the puppies are fine. Wrong?

Elena MarshFor CDDY, yes. OFA’s own test page says an A/N dog is at risk, because one copy is enough for the disc changes described in the 2017 PNAS paper.

What a Shih Tzu result cannot predict

A genotype is inherited risk information, not a diagnosis, and it cannot tell you whether a particular Shih Tzu will herniate a disc. The UC Davis VGL: “Multiple factors are thought to influence whether a particular disc herniates in an individual dog.” The Cornell Riney Canine Health Center adds that the variant “does not explain all cases,” citing other unknown genetic variants, lifestyle and environment. In the 2019 data, 46 of 378 dogs with calcified discs at surgery had zero copies. A clear result removes one known risk; it does not rule out disc disease.

Clinical data confirm that Shih Tzus do present with disc disease, without telling us their genotypes. A 2008 series of 297 surgically confirmed cases at a Japanese university hospital (1991–2004) counted Shih Tzus as 15 of 132 neck cases (11.4%) and 8 of 165 thoracolumbar cases (4.8%), noting the breed “tended to suffer from both C-IVDH and TL-IVDH at multiple sites” — shares of one hospital’s cases, not a breed risk. The UK VetCompass study by Dale and colleagues (2024) notes earlier reports of higher intervertebral disc disorder prevalence in Shih Tzus; of 89 Shih Tzu deaths in 2013–2016, 5 (5.6%, 95% CI 2.4–12.5) were grouped as a “spinal cord” finding, a category not specific to IVDD.

Whatever the genotype, know the signs: back or neck pain, reluctance to jump, crying when picked up, an arched back, wobbling, knuckling hind paws, weakness, paralysis or inability to urinate. The 2022 ACVIM consensus statement treats loss of deep pain perception as the prognostic dividing line: paraplegic dogs without deep pain recovered in 21% of 48 medically managed and 61% of 502 surgically treated cases. The panel sets no fixed deadline in hours — it lists roughly as many studies finding no effect of surgical timing as finding one — but sudden weakness should be handled as an emergency. In the US, start with your veterinarian or a 24-hour emergency hospital; VECCS facility certification (Level I highest, though VECCS does not judge quality of medicine) and the ACVIM directory at vetspecialists.com, searchable for a neurologist by ZIP code, help you find options in advance. Bring the DNA report to referral; the diagnosis comes from the veterinarian’s examination and imaging.

A CHIC number says nothing about CDDY

Many buyers ask whether the parents are “health tested” and stop at yes. For Shih Tzus, that yes includes CDDY only if the breeder chose to add it. The OFA’s CHIC requirements for the Shih Tzu are an ACVO eye exam and a patellar luxation evaluation, with hip and cardiac evaluations optional, plus microchip or tattoo identification. No DNA test of any kind is required, so a parent with a CHIC number may still have unknown CDDY status.

The parent club is explicit. The American Shih Tzu Club Health Statement, approved on 19 April 2024, says: “The American Shih Tzu Club does not require any specific health testing.” The conditions it asks breeders to track — renal dysplasia, portal systemic shunt, patellar luxation, hip dysplasia, genetic eye problems and cardiac issues — omit disc disease. The AKC Toy Group health-testing page lists the Shih Tzu with “No recommended health tests,” so AKC Bred with H.E.A.R.T. and Breeder of Merit pledges, which follow parent-club recommendations, do not imply a CDDY test either.

The same statement gives buyers an opening: breeders “should furnish a complete health record on each puppy sold and provide the buyer with health clearances on the parents if they are available.” Reasonable questions to put in writing before a deposit:

• Were both parents tested for CDDY, by which laboratory, and may I see the reports or OFA entries?
• Were the results N/N, N/CDDY (A/N) or CDDY/CDDY (A/A)?
• If a parent was not tested, will you test the puppy before it goes home? (VGL accepts puppies from three weeks of age.)
• Has any close relative had neck or back pain, disc surgery or other neurologic problems?

A breeder who has not run the test is within the club’s rules. The research simply adds that in this breed the answer can differ from one litter to the next.

Breeding against CDDY without breaking the breed

For breeders, a segregating breed is the favorable case. The 2019 authors wrote that “breeding priorities should be for dogs with fewer copies of 12-FGF4RG, so that the allele frequency can be reduced.” Bannasch and colleagues (2022), who call CDDY “FGF4L2,” went further: “Reducing or eliminating FGF4L2 from breeds due to its associated significant health concerns and its modest effect on height should be possible, particularly in breeds that segregate FGF4L2.” As quoted in OMIA, they note that breeding dogs with FGF4L1 (CDPA) and without FGF4L2 “would likely lead to a reduction in the FGF4L2-related risk of intervertebral disc herniation while maintaining the reduction in leg length resulting from FGF4L1.” Since Shih Tzus appear to carry CDPA widely, selecting against CDDY should not, on current evidence, require changing the breed’s silhouette.

The inheritance logic is simple. Two N/N parents cannot pass on a CDDY copy. A pairing with an N/CDDY parent can produce both clear and positive puppies, which is why testing the litter is useful. A CDDY/CDDY parent passes one copy to every puppy.

There is a counterweight. Cornell warns that in breeds where the variant is very common, dogs with one or two copies “should not be immediately removed from the breeding population,” because a shrinking gene pool brings inbreeding-related problems, and urges breeders to weigh the whole dog: family history of back, neck or neurologic problems, the breed standard, required health testing, temperament and general health. With CDDY at roughly a quarter of alleles in the 2019 Shih Tzu sample, a gradual approach — favoring clear dogs when other qualities are equal, pairing an excellent positive dog with a clear mate — honors both pieces of advice better than an overnight purge.

SamIf I breed my clear female to a one-copy stud, have I done something wrong?

Elena MarshNot necessarily. Cornell advises weighing the whole dog, and the 2019 paper favors fewer copies over time. Testing the puppies lets you place them with full disclosure.

Ordering the test in the US: labs, prices and turnaround

Owners can order a CDDY test directly, with no veterinarian required. Published prices (check before ordering):

Two cautions. Embark’s condition page states that “one or two copies of this variant creates a 46 fold increase in risk for an IVDD event and a 5-15 fold increase in risk for disc surgery.” The 46-fold figure is Embark’s own wording; the Shih Tzu-specific estimate from the primary study is the 10.3 above. Embark also notes that where the variant is extremely common it may not be the strongest predictor — a caveat that weighs less in a split breed. Second, laboratories frame the variant differently: UC Davis treats it as causal and breeding-relevant, while Orivet calls it uncertain. For breeders, the VGL-plus-OFA route puts the result in a public record future buyers can check.

Puppy lemon laws and insurance clauses that touch a CDDY result

Several states give buyers remedies when a puppy has a hereditary condition, and none yet says how a DNA-only result (a CDDY-positive puppy with no symptoms) should be treated. Treat the following as questions to raise with a seller, not legal conclusions.

• California. Under the Polanco-Lockyer Pet Breeder Warranty Act, a breeder who sold or gave away three or more litters or 20 or more dogs in the previous 12 months (§ 122045) must sign a statement that the dog has no known congenital or hereditary condition affecting its health “at the time of the sale or that is likely to adversely affect the health of the dog in the future,” or disclose it with a veterinarian’s sign-off (Health and Safety Code § 122050). If a veterinarian finds the dog unfit within one year because of a congenital or hereditary condition, the buyer may keep the dog and recover reasonable veterinary fees up to 150% of the purchase price plus sales tax (§ 122100). Whether a known CDDY genotype is a “known hereditary condition” is not settled by the text — another reason to ask for the parents’ results in writing.
• Florida. Section 828.29 gives one year for a veterinarian to certify a congenital or hereditary disorder. The 2026 text covers dealers selling more than three litters or 30 dogs or cats a year (whichever is greater), including breeders selling directly. Written disclosure of a specific hereditary disorder at sale, with the buyer’s signed acknowledgment of a veterinarian’s determination, removes the refund right for that disorder.
• New York and New Jersey. New York General Business Law § 753 allows 180 calendar days for a veterinarian to certify a congenital malformation affecting health. New Jersey’s Pet Purchase Protection Act gives 180 days for congenital or hereditary conditions in pet-shop sales, with veterinary reimbursement capped at twice the purchase price.

Insurance timing matters more for a CDDY-positive Shih Tzu than for most dogs. Embrace Pet Insurance excludes intervertebral disk disease that occurs or shows symptoms during the first 180 days of a dog’s policy, treating it as pre-existing for the life of the policy. AKC Pet Insurance defines “orthopedic” to include intervertebral disc degeneration, says hereditary and congenital coverage “may be excluded,” and treats hereditary or orthopedic illness arising in its 14-day illness waiting period as pre-existing (per its California disclosure; terms vary by state). If you plan to insure, do it while the puppy is young and symptom-free, and check the declarations page for a hereditary exclusion. For scale, NAPHIA reports an average US accident-and-illness premium of $836 per dog in 2025.

Frequently asked questions

Q. My Shih Tzu’s report says “Carrier (A/N).” Is she just a carrier, as with other diseases?
No. For disc disease CDDY behaves as a dominant trait, so one copy places a dog in the higher-risk group; OFA’s own page says an A/N dog “is at risk for developing the disease.” It does not mean she will develop it. Discuss the result with your veterinarian.

Q. Is it true that more than half of Shih Tzus carry CDDY?
Not according to population data. The 0.56 figure is the allele frequency among Shih Tzus already operated on at UC Davis. The 2019 population sample gives 0.25 (110 dogs; 46 had at least one copy), and the UC Davis VGL’s client data give 0.161.

Q. Does a positive result mean my dog will need surgery?
No. A DNA test is not a diagnosis and cannot predict whether an individual dog will herniate a disc. The relative risk of 10.3 (95% CI 1.8–62.1) compares groups of Shih Tzus in one hospital dataset. Diagnosis and treatment decisions belong to your veterinarian.

Q. My puppy’s parents have CHIC numbers. Were they tested for CDDY?
Not necessarily. The Shih Tzu CHIC program requires an ACVO eye exam and a patellar luxation evaluation and no DNA test, and the American Shih Tzu Club requires no specific health testing. Ask the breeder for CDDY results directly.

Q. If my dog tests clear, can I stop worrying about disc disease?
No. A clear result removes one known risk factor, but 46 of 378 dogs with calcified discs at surgery in the 2019 study had no copies. Watch for back pain, weakness or wobbling regardless of genotype and see a veterinarian promptly.

Q. How fast does a Shih Tzu with sudden hind-leg weakness need to be seen?
Treat it as an emergency and contact your veterinarian or an emergency hospital the same day. The ACVIM consensus sets no fixed number of hours for surgery, but dogs that lose deep pain perception have the poorest outcomes. The veterinarian decides on treatment.

References

• Batcher K, Dickinson P, Giuffrida M, Sturges B, Vernau K, Knipe M, Rasouliha SH, Droegemueller C, Leeb T, Maciejczyk K, Jenkins CA, Mellersh C, Bannasch D. Phenotypic Effects of FGF4 Retrogenes on Intervertebral Disc Disease in Dogs. Genes. 2019;10(6):435. doi:10.3390/genes10060435
• Brown EA, Dickinson PJ, Mansour T, Sturges BK, Aguilar M, Young AE, Korff C, Lind J, Ettinger CL, Varon S, Pollard R, Brown CT, Raudsepp T, Bannasch DL. FGF4 retrogene on CFA12 is responsible for chondrodystrophy and intervertebral disc disease in dogs. Proc Natl Acad Sci U S A. 2017;114(43):11476-11481. doi:10.1073/pnas.1709082114
• Bannasch D, Batcher K, Leuthard F, Bannasch M, Hug P, Marcellin-Little DJ, Dickinson PJ, Droegemueller M, Droegemueller C, Leeb T. The Effects of FGF4 Retrogenes on Canine Morphology. Genes. 2022;13(2):325. doi:10.3390/genes13020325
• Olby NJ, Moore SA, Brisson B, Fenn J, Flegel T, Kortz G, Lewis M, Tipold A. ACVIM consensus statement on diagnosis and management of acute canine thoracolumbar intervertebral disc extrusion. J Vet Intern Med. 2022;36(5):1570-1596. doi:10.1111/jvim.16480
• Itoh H, Hara Y, Yoshimi N, Harada Y, Nezu Y, Yogo T, Ochi H, Hasegawa D, Orima H, Tagawa M. A Retrospective Study of Intervertebral Disc Herniation in Dogs in Japan: 297 Cases. J Vet Med Sci. 2008;70(7):701-706. doi:10.1292/jvms.70.701
• Dale F, Brodbelt DC, West G, Church DB, Lee YH, O’Neill DG. Demography, common disorders and mortality of Shih Tzu dogs under primary veterinary care in the UK. Canine Med Genet. 2024;11(1):2. doi:10.1186/s40575-023-00135-y
• OMIA:000157-9615: Intervertebral disc disease, predisposition to in Canis lupus familiaris (dog). Online Mendelian Inheritance in Animals (OMIA), Sydney School of Veterinary Science, University of Sydney.
• OMIA:002542-9615: Chondrodysplasia, FGF4 retrogene-related in Canis lupus familiaris (dog). Online Mendelian Inheritance in Animals (OMIA), Sydney School of Veterinary Science, University of Sydney.
• Veterinary Genetics Laboratory, University of California, Davis. “Chondrodystrophy (CDDY and IVDD) and Chondrodysplasia (CDPA)”.
• Collins B, Dockweiler J. “Chondrodystrophy and intervertebral disc disease (CDDY/IVDD)”. Cornell Richard P. Riney Canine Health Center, Cornell University College of Veterinary Medicine. Updated Aug. 2024.
• Orthopedic Foundation for Animals (OFA). “Chondrodystrophy with Intervertebral Disc Disease (CDDY/IVDD)” DNA test page.
• Orthopedic Foundation for Animals. CHIC Program Requirements: Shih Tzu.
• American Shih Tzu Club. Health Statement (approved 19 April 2024).
• American Kennel Club. Toy Group Health Testing Requirements.
• American Kennel Club. The Most Popular Dog Breeds of 2025.
• American Kennel Club. Shih Tzu breed page.
• Embark. Chondrodystrophy and Intervertebral Disc Disease, CDDY/IVDD, Type I IVDD.
• Wisdom Panel. Dog DNA tests.
• Orivet. Chondrodystrophy with Intervertebral Disc Disease Risk Factor (CDDY with IVDD).
• California Health and Safety Code § 122045.
• California Health and Safety Code § 122050.
• California Health and Safety Code § 122100.
• Florida Statutes § 828.29 (2026).
• New York General Business Law § 753.
• New Jersey Pet Purchase Protection Act (N.J.S.A. 56:8-92 et seq.), Animal Legal & Historical Center reproduction.
• Embrace Pet Insurance. What is the waiting period for orthopedic conditions?
• AKC Pet Insurance. California insurer disclosure.
• North American Pet Health Insurance Association. State of the Industry 2026 report highlights.
• Veterinary Emergency and Critical Care Society. Facility Certification.
• American College of Veterinary Internal Medicine. Find a Specialist.

About the author

Elena Marsh

Editor & writer (not a veterinarian)

A writer with a molecular-biology background and a lifelong dog and cat owner. Not a veterinarian — she translates peer-reviewed genetics research and primary data into plain language, always as information rather than diagnosis.