Nova Scotia Duck Tolling Retriever and Collie Eye Anomaly (CEA): What the NHEJ1 Gene Test Really Shows

SamMy Toller breeder mentioned a CEA DNA test before I even pick a puppy. Should I be worried?
Elena MarshWorth learning about — NHEJ1-related CEA does show up in Tollers, though the story here turns out to be more complicated than in Collies.
SamComplicated how? Isn’t a DNA test just… a test?
Elena MarshNot quite — a 2018 UC Davis study found the standard NHEJ1 test didn’t match eye findings in several Tollers, so genetics alone doesn’t tell the whole story in this breed.
SamSo how do I actually protect my dog’s eyes, then?
Elena MarshA combination — DNA testing, an early eye exam by a specialist, and knowing what the parent clubs in Canada, the US, and the UK actually require of breeders.
SamAnd is CEA the only hereditary thing I should be checking for?
Elena MarshNo — Tollers carry a handful of other inherited conditions worth knowing about too. We’ll walk through all of it, plus testing costs and insurance.

Conclusion: In the Nova Scotia Duck Tolling Retriever, Collie Eye Anomaly (CEA) traces to the same NHEJ1 gene deletion identified across herding and gundog breeds in 2007. But a 2018 UC Davis study found that of seven Tollers with optic nerve head coloboma, only three were homozygous for that deletion — meaning the DNA test alone cannot fully predict or explain the condition in this breed. A DNA test is still useful for breeding decisions, but a veterinary ophthalmologist’s exam remains essential.

The Nova Scotia Duck Tolling Retriever — usually just called the “Toller” — was developed in the Little River Harbour area of Yarmouth County, Nova Scotia, by Acadian communities working from retriever, spaniel, and setter stock, and was first registered as a purebred with the Canadian Kennel Club in 1945. Its name comes from “tolling,” a distinctive hunting method in which the dog plays at the water’s edge to lure curious ducks and geese within range before the hunter flushes them. The breed became Nova Scotia’s official provincial dog in 1995 and was recognized by the American Kennel Club’s Sporting Group in 2003. Tollers are intelligent, energetic, and known for a distinctive high-pitched vocalization sometimes called the “Toller scream.”

The breed’s global population is also relatively small and closely related: a 2010 study of nearly the entire worldwide registration history of the breed found that more than half of the genetic diversity in the reference population traced back to just two founding ancestors, with an average inbreeding coefficient of 0.26 (Mäki 2010). In a breed this genetically concentrated, tracking hereditary eye disease carefully matters, and Collie Eye Anomaly (CEA) — caused by a deletion near the NHEJ1 gene — is one of the conditions most directly documented in Tollers going back to the mutation’s original 2007 discovery. This article walks through what that research actually shows, the important caveat specific to this breed, how to get a dog tested in the US and UK, what breed clubs expect of breeders, and what an owner can realistically do today.

The NHEJ1 gene and how CEA/CH affects the eye

NHEJ1 (nonhomologous end-joining factor 1) sits on dog chromosome 37. The variant linked to CEA is not a change inside a coding exon but a 7,799-base-pair deletion in intron 4 of the gene, inherited in an autosomal recessive pattern — a dog needs two copies to be genetically affected (OMIA:000218-9615). Because the deletion falls in a non-coding region, researchers believe it disrupts normal gene regulation during eye development rather than eliminating the NHEJ1 protein’s function outright.

The defining lesion is choroidal hypoplasia (CH) — an underdeveloped choroid, the vascular layer beneath the retina that nourishes it — usually seen as a pale, thin patch on a veterinary ophthalmologist’s fundus exam. In more affected dogs, the anomaly can extend to coloboma (a pit or gap in the optic disc, choroid, or sclera) and, in the most severe cases, retinal detachment or intraocular hemorrhage that can threaten vision.

Discovery history: a multi-breed study, and a Toller-specific question mark

The causative deletion was identified by Heidi G. Parker and colleagues in a 2007 study in Genome Research, “Breed relationships facilitate fine-mapping studies: a 7.8-kb deletion cosegregates with Collie eye anomaly across multiple dog breeds.” Using pedigree relationships among Collies, Shetland Sheepdogs, Border Collies, and Australian Shepherds to fine-map the mutation, the study also tested affected dogs in other breeds and found the same deletion homozygous in affected Boykin Spaniels and in affected Nova Scotia Duck Tolling Retrievers — the original evidence base for NHEJ1’s role in this breed.

A decade later, that picture got more complicated. Brown, Thomasy, and colleagues at the University of California, Davis published “Genetic analysis of optic nerve head coloboma in the Nova Scotia Duck Tolling Retriever identifies discordance with the NHEJ1 intronic deletion” in Veterinary Ophthalmology (2018). Of seven Tollers examined with optic nerve head coloboma, only three were homozygous for the NHEJ1 deletion — meaning the deletion did not explain the coloboma in the other four dogs. The authors concluded that relying on the NHEJ1 test alone is not sufficient to predict coloboma in this breed, and recommended pairing genetic testing with a puppy eye exam when making breeding decisions. This is the single most important caveat in this article: the NHEJ1 DNA test cannot fully predict or explain CEA/coloboma in the Toller the way it does in some other breeds. Consistent with this, the Nova Scotia Duck Tolling Retriever Club of Canada (NSDTRCC) notes that, unlike Collies — who tend to show CEA alone or CEA combined with coloboma — Tollers can present with CEA alone, CEA plus coloboma, or coloboma without CEA at all, and that researchers at the University of Pennsylvania have an ongoing study aimed at identifying additional causal variants.

SamSo if my Toller tests clear (N/N), its eyes are automatically fine?
Elena MarshNot necessarily — Brown and Thomasy’s 2018 study is exactly why a clear or affected DNA result in a Toller isn’t the last word; an ophthalmologist’s exam still matters.

Symptoms, the severity spectrum, and the “go normal” phenomenon

CEA/CH spans a spectrum. Mild choroidal hypoplasia, the most common presentation, typically causes no noticeable visual impairment and is usually non-progressive. In the middle of the spectrum, some puppies show retinal folds that often resolve naturally, or a coloboma whose impact depends on its size and location. At the severe end, retinal detachment — sometimes with intraocular bleeding — can cause partial or complete blindness, though this is uncommon.

Because pigment develops in the retina over the first weeks and months of life, mild choroidal hypoplasia that would be visible in a very young puppy can later become obscured — the so-called “go normal” phenomenon. The dog’s underlying genetics and original eye findings don’t change; only what a later exam can detect does. For this reason, an ophthalmologic exam around 6 to 8 weeks of age is considered the most reliable window for a first diagnosis, with a follow-up exam in adulthood (six months or older) recommended by some breed resources to check for signs that may not have been apparent earlier.

Beyond CEA: other hereditary conditions Toller owners should know about

CEA is not the only inherited condition documented in the breed, and breed clubs generally look at the whole picture rather than CEA in isolation. Other conditions reported in Tollers include progressive rod-cone degeneration (a form of progressive retinal atrophy, PRA-prcd), autoimmune thyroiditis, Addison’s disease, a breed-specific cleft palate variant (CP1), and — notably documented in a Norwegian population — steroid-responsive aseptic meningitis, which showed an estimated prevalence around 2.5% in dogs born 1994–2003 in that cohort (Anfinsen et al. 2008). Older, industry-sourced estimates place PRA-prcd affected rates around 7% with a considerably higher carrier rate, autoimmune thyroiditis around one in six dogs in some surveyed populations, and Addison’s disease around 1% affected with a meaningfully higher carrier frequency than in dogs generally — but these figures vary by source and survey period, so treat them as general context rather than precise current statistics, and ask a breeder or veterinarian for their most current health-testing panel.

How to get tested: US and UK routes and pricing

In the United States, the Orthopedic Foundation for Animals (OFA) offers a CEA DNA test via a cheek swab processed by the University of Missouri College of Veterinary Medicine, reporting results as Normal (N/N), Carrier (A/N), or Affected (A/A). OFA does not offer a bundled breed-specific panel discount for the Toller — every DNA test, CEA included, is ordered and priced individually (OFA’s volume discounts apply only to litter, kennel, or multi-registry submissions, not to a curated set of breed-relevant tests). As of this writing, OFA/University of Missouri DNA tests, including the standalone CEA test, are generally priced around $65 per test, though it’s worth confirming current pricing on the official site before ordering. The UC Davis Veterinary Genetics Laboratory (VGL) also offers a standalone CEA test with the Toller listed among its covered breeds; VGL does not publish a standalone price for this test on its public page, so request current pricing directly. PawPrint Genetics (now part of Neogen) offers a Toller-specific panel covering CEA, degenerative myelopathy, and PRA-prcd together; check their official site for current panel pricing. Consumer DNA kits are another option: Embark’s Breed + Health kit, which screens CEA/choroidal hypoplasia among 270-plus conditions, was priced around $199 at full price as of September 2026 (frequently discounted in sales). Wisdom Panel also screens for CEA in its health panels and reports the mutation as “found in 1 in 38 dogs” in its testing population — the company does not specify whether that figure reflects carriers, affected dogs, or a mix of both, nor whether it is an all-breed average or Toller-specific, so treat it as a rough population-level signal rather than a precise carrier rate for this breed.

Canadian owners are not left without options, even though no Canada-specific lab is tied to the parent club for this test: both OFA (via the University of Missouri) and UC Davis VGL accept international submissions, including from Canada. OFA/Missouri’s cheek-swab kits can be mailed by standard post; UC Davis VGL asks international shippers to use a courier service (FedEx or UPS, not DHL), include the lab’s institutional EIN on the package, and confirm current import-permit requirements before shipping. Budget the listed US-dollar price plus ordinary cross-border shipping costs, and allow a few extra days of transit time compared with a US-based submission.

In the United Kingdom, clinical screening runs through the BVA/KC/ISDS Eye Scheme, a decades-old joint program between the British Veterinary Association, the Kennel Club, and the International Sheep Dog Society, under which an owner books directly with a BVA-appointed eye panellist — there are 42 across the UK, and the scheme is open to any dog, pedigree or crossbreed. CEA is a congenital condition on the scheme’s list and can be checked in puppies as young as 5 to 12 weeks. For DNA testing, the UK lab CAGT (Canine & Animal Genetic Testing) offers a CEA/choroidal hypoplasia test listing the Toller among its covered breeds, priced at £49.50 (inc. VAT) with a one-to-two week turnaround.

SamThat’s a lot of options — which one should my breeder actually use?
Elena MarshWhichever accredited lab they prefer for the DNA side, but pair it with an eye exam — OFA and CAGT both report the same three genotype categories, so results are comparable across labs.

Breeding considerations and parent-club guidance

Because CEA is autosomal recessive, pairing two carriers (N/CEA × N/CEA) risks producing affected (CEA/CEA) puppies, while pairing a carrier with a clear dog is not expected to produce affected offspring, though roughly half the litter may themselves be carriers. In Canada — the breed’s country of origin — the Nova Scotia Duck Tolling Retriever Club of Canada’s Code of Ethics requires breeding dogs to have hip clearances through an accredited body (OFA, OVC, or PennHIP) and a veterinary ophthalmologist’s eye clearance before breeding, ideally renewed annually and no less often than every 18 months; where pedigree does not already establish status, it also calls for PRA-prcd and CEA DNA results from an approved lab. In the United States, NSDTRC-USA participates in the AKC’s Canine Health Information Center (CHIC) program, where CEA appears as an available DNA test — not a required one, since CHIC’s required categories for this breed are a hip evaluation, an ophthalmologist’s eye exam, and a PRA-prcd DNA test. An eye exam for puppies around 6–8 weeks of age reflects general clinical practice for detecting mild choroidal hypoplasia before pigmentation masks it, rather than a specific age published by NSDTRC-USA, CHIC, or any single paper. Published CEA carrier estimates for the breed range widely and are not traceable to one dated primary source: secondary sources cite NSDTRC-USA figures anywhere from a roughly 5% carrier rate and 0.5% affected rate up to a 10–15% carrier estimate, but the club’s current health-overview page does not itself publish these percentages, so treat any specific figure with caution until it can be checked against a dated primary source. At the upper end of the spread, a small 2022 Italian population study that screened 20 NSDTRs for the NHEJ1 deletion found a striking 45% carrier frequency (Marelli et al. 2022) — a useful data point, but from a sample far too small to generalize to the breed worldwide.

Given the Toller-specific NHEJ1 discordance described above, breed clubs’ practical message is consistent: DNA test results support breeding decisions and pedigree tracking, but they are not a substitute for the ophthalmologist’s exam when the goal is actually confirming a dog’s eye health.

It is worth being precise about how much of this is actually law versus club guidance, because the two markets differ sharply. In England, anyone breeding three or more litters in a 12-month period (unless they can show none of the puppies were sold) must hold a local-authority licence under the Animal Welfare (Licensing of Activities Involving Animals) (England) Regulations 2018; the statutory guidance for that licence states that no dog may be kept for breeding if it can reasonably be expected, on the basis of its genotype, phenotype, or state of health, that breeding from it could harm its own welfare or that of its offspring, and a higher-standard licence additionally requires breeding stock to be tested through health-screening schemes recognized for the breed before mating. Separately, “Lucy’s Law” — in force since 6 April 2020 — bans the commercial third-party resale of puppies under six months old, so a buyer must now deal directly with the breeder (or a rehoming centre) rather than a pet shop or dealer, which in practice makes it easier for a buyer to ask that breeder directly for NHEJ1/CEA results and eye-exam history. Against that statutory backdrop, the Kennel Club’s own breed-specific health-testing schedule for the Toller — which lists both the CEA/CH DNA test and the BVA/KC/ISDS Eye Scheme exam under its “Best Practice” category (its “Good Practice” tier for this breed instead covers the DM and prcd-PRA DNA tests and prioritising genetic diversity) — is a voluntary accreditation recommendation, not a legal requirement; a breeder can be fully licensed and lawful under the 2018 Regulations without ever completing it, provided nothing about the specific dog’s known health makes breeding from it reasonably foreseeable to cause harm.

In the United States, there is no comparable federal requirement at all. USDA APHIS licenses breeders under the Animal Welfare Act only when they keep more than four “breeding females” and sell dogs sight-unseen rather than face-to-face; a breeder who lets every buyer physically see the puppy before taking possession is exempt regardless of how many litters they produce, and most hobby and small show breeders — including the great majority of Toller breeders — fall outside USDA licensing entirely. Even for breeders who are federally licensed, the Animal Welfare Act’s standards govern housing, veterinary care, and handling, not hereditary-disease or DNA testing, which the federal government does not regulate in any form. Whether a given litter’s parents are CEA-tested comes down entirely to state-level law (which varies by state and rarely mandates genetic testing specifically) and to voluntary parent-club guidance, such as the NSDTRC-USA/CHIC recommendations described above — not to any binding federal rule. In practice, this means a US buyer carries more of the verification burden personally, since no government licence in the US functions as a proxy for genetic-testing compliance the way the UK’s statutory breeding-licence conditions at least gesture toward.

SamIf both parents test clear, is a litter guaranteed to be fine?
Elena MarshNo — NHEJ1-clear only rules out that variant. In 2018, two of seven coloboma-affected Tollers were clear of the deletion entirely.

Pet insurance implications

In the UK, pet insurance policies broadly fall into two structures that matter a great deal for a lifelong condition like CEA: “lifetime cover,” which renews a condition’s claim allowance each policy year (a model insurers such as Agria specialize in), versus “time-limited” or “maximum benefit” policies, which cap either a payout amount or a time window per condition, after which further CEA-related treatment may no longer be covered. Because eye-related monitoring or treatment can recur over a dog’s life, confirming which type of cover a given UK policy actually is matters more here than the DNA result itself. In the United States, insurers such as Trupanion, Healthy Paws, and Embrace generally do cover hereditary and congenital conditions like CEA under standard accident-and-illness plans, provided the dog was enrolled — and the condition not yet diagnosed or symptomatic — before the policy began; a condition already noted in vet records prior to enrollment is typically treated as pre-existing and excluded in both markets. Because CEA can be present from birth, insuring a Toller puppy early — ideally before or shortly after DNA testing — and disclosing any known result accurately tends to leave owners better positioned than waiting until symptoms appear. Exact terms vary by insurer and policy, so the lifetime-vs-time-limited distinction (UK) and the pre-existing-condition clause (US) should always be confirmed directly with the provider.

One legal point is worth clarifying directly, since it is commonly misunderstood: the US Genetic Information Nondiscrimination Act (GINA) protects people from genetic discrimination in health insurance and employment — it applies to human genetic information only and has no bearing on a dog’s DNA test result or on pet insurance underwriting in any way. Neither the US nor the UK currently has a law specifically restricting how a pet insurer may use a known genetic test result when underwriting or reviewing a policy. That regulatory gap is exactly why the disclosure question above is a practical one, not just an ethical one: nothing in law prevents an insurer from treating a known CEA/CEA result as a pre-existing or excludable condition, and nothing in law obliges an owner to volunteer an untested dog’s status before enrollment — but withholding a materially relevant known result can itself support a non-disclosure-based claim denial later, which is the practical case for testing and insuring early rather than waiting.

What Nova Scotia Duck Tolling Retriever owners can do today

• Ask breeders for both parents’ NHEJ1/CEA DNA results (N/N, N/CEA, or CEA/CEA) before committing to a puppy.
• Schedule a veterinary ophthalmologic exam early, ideally around 6 to 8 weeks of age, and consider a follow-up exam in adulthood given the “go normal” masking effect.
• Remember that a clear or affected NHEJ1 result does not fully settle the question in this breed — Brown and Thomasy (2018) found the deletion explained only 3 of 7 coloboma cases studied — so treat the DNA test and the eye exam as complementary, not interchangeable.
• Ask about the breed’s other documented hereditary conditions (PRA-prcd, autoimmune thyroiditis, Addison’s disease) when reviewing a breeder’s health-testing panel.
• Favor breeders who follow NSDTRCC (Canada) or NSDTRC-USA/CHIC health-clearance guidance, and confirm current DNA test pricing directly with OFA, UC Davis VGL, PawPrint Genetics, or CAGT before ordering.
• Treat this article as general background only — always defer diagnosis, interpretation of a specific result, and treatment decisions to a licensed veterinarian or veterinary ophthalmologist.

Frequently Asked Questions

Q. What is Collie Eye Anomaly (CEA) and how is it linked to the NHEJ1 gene in Tollers?
CEA, also called choroidal hypoplasia, is an inherited eye condition linked to a 7,799-bp deletion in intron 4 of the NHEJ1 gene. Parker et al. (2007) found this deletion homozygous in affected Nova Scotia Duck Tolling Retrievers as well as in Collies, Shetland Sheepdogs, and several other breeds.

Q. Does a clear or affected NHEJ1 DNA result guarantee my Toller’s eyes are healthy or diseased?
No. A 2018 UC Davis study (Brown, Thomasy et al.) found that of seven Tollers with optic nerve head coloboma, only three were homozygous for the NHEJ1 deletion — meaning the DNA test alone cannot fully predict or explain coloboma in this breed. A veterinary ophthalmologist’s exam is still needed alongside the DNA result.

Q. How common is the CEA carrier status in Nova Scotia Duck Tolling Retrievers?
Estimates vary widely and are not traceable to one dated primary source: secondary sources cite NSDTRC-USA figures ranging from roughly 5% carriers and 0.5% affected up to 10–15% carriers, though the club’s current health-overview page does not itself publish these percentages. A small 2022 Italian study that screened 20 NSDTRs found a 45% carrier frequency for the NHEJ1 deletion (Marelli et al. 2022) — likely an upper-bound outlier given the tiny sample size. Check the parent club’s current published information and treat any single percentage with caution.

Q. Where can I get my Toller tested, and what does it cost?
In the US, OFA (via the University of Missouri) offers a CEA DNA test around $65, and UC Davis VGL and PawPrint Genetics also offer CEA testing; consumer kits like Embark’s Breed + Health test (around $199 at full price) screen for CEA as well. In the UK, CAGT offers a CEA DNA test for £49.50 (inc. VAT), and the BVA/KC/ISDS Eye Scheme provides the clinical eye-exam pathway. Prices change, so confirm current figures with each provider.

References

• Parker HG, Kukekova AV, Akey DT, et al. (2007). “Breed relationships facilitate fine-mapping studies: a 7.8-kb deletion cosegregates with Collie eye anomaly across multiple dog breeds.” Genome Research 17(11):1562-1571.
• Brown EA, Thomasy SM, et al. (2018). “Genetic analysis of optic nerve head coloboma in the Nova Scotia Duck Tolling Retriever identifies discordance with the NHEJ1 intronic deletion (collie eye anomaly mutation).” Veterinary Ophthalmology 21(2):144-150.
• OMIA:000218-9615 — Choroidal hypoplasia, NHEJ1-related, in Canis lupus familiaris (dog).
• Anfinsen KP, Berendt M, Liste FJ, et al. (2008). “A retrospective epidemiological study of clinical signs and familial predisposition associated with aseptic meningitis in the Norwegian population of Nova Scotia duck tolling retrievers born 1994-2003.” Canadian Journal of Veterinary Research 72(4):350-355.
• Mäki K (2010). “Population structure and genetic diversity of worldwide Nova Scotia Duck Tolling Retriever and Lancashire Heeler dog populations.” Journal of Animal Breeding and Genetics 127(4):318-326.
• Marelli SP, Rizzi R, Paganelli A, Bagardi M, Minozzi G, Brambilla PG, Polli M (2022). “Genotypic and allelic frequency of a mutation in the NHEJ1 gene associated with collie eye anomaly in dogs in Italy.” Veterinary Record Open 9:e26.
• Orthopedic Foundation for Animals (OFA). “Collie Eye Anomaly (CEA).”
• UC Davis Veterinary Genetics Laboratory. “Collie Eye Anomaly (CEA) Test.”
• CAGT (Canine & Animal Genetic Testing, UK). “Collie Eye Anomaly / Choroidal Hypoplasia.”
• Embark Veterinary. “Collie Eye Anomaly (Choroidal Hypoplasia / CEA).”
• Wisdom Panel. “Collie Eye Anomaly (CEA).”
• Nova Scotia Duck Tolling Retriever Club of Canada (toller.ca). “Collie Eye Anomaly (CEA) and Coloboma Study.”
• Nova Scotia Duck Tolling Retriever Club of Canada. Bylaws, Code of Ethics and Constitution.
• Nova Scotia Duck Tolling Retriever Club, USA. “Health Overview.”
• UK Government (Defra). “Dog breeding licensing: statutory guidance for local authorities,” under the Animal Welfare (Licensing of Activities Involving Animals) (England) Regulations 2018.
• The Animal Welfare (Licensing of Activities Involving Animals) (England) Regulations 2018 (SI 2018/486).
• UK Government (Defra). “Gove delivers Lucy’s Law to protect puppies and kittens” (in force 6 April 2020).
• The Kennel Club (UK). Nova Scotia Duck Tolling Retriever breed page — health testing and screening recommendations.
• American Kennel Club (AKC) Government Relations. “Understanding USDA Dog Breeder Licensing.”
• National Human Genome Research Institute (NHGRI). “Genetic Information Nondiscrimination Act (GINA)” — scope limited to human health insurance and employment.

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.