Most inherited diseases we write about are stories of damage limitation: you test, you avoid producing affected dogs, and you manage the ones already living with the condition. Imerslund-Gräsbeck syndrome is the rare exception with a genuinely happy ending. An affected Border Collie puppy can go from failing to thrive, anaemic, and neurologically wobbly to completely normal within a few weeks — provided someone recognises the problem and reaches for the right, almost absurdly cheap, treatment.
What Goes Wrong: A Broken Door for Vitamin B12
Vitamin B12 (cobalamin) is essential. It is a cofactor for enzymes that build DNA, maintain the nervous system, and keep red blood cell production running. Dogs cannot make it; they absorb it from food, and that absorption happens at one specific place: the lining of the ileum, the final section of the small intestine.
To cross that lining, B12 must be carried in by a receptor complex. A key component of that complex is cubilin, a protein encoded by the CUBN gene. In affected Border Collies, a mutation in CUBN produces a faulty cubilin, and the receptor cannot do its job. Dietary B12 simply passes through the gut unabsorbed, no matter how much the diet contains.
The body’s B12 reserves, inherited from the dam and built up early, are modest. So affected puppies usually appear normal at first, then begin to falter at a few months of age as those stores run dry — a slow-motion deficiency in the middle of plenty.
The Signs: Subtle, Then Serious
The clinical picture of cobalamin malabsorption is easy to miss because the signs are non-specific and creep in gradually. The classic triad is:
- Failure to thrive. The puppy is small for its age, lethargic, a poor eater, and not keeping up with littermates. Weight gain stalls.
- Anaemia. B12 is needed to make red blood cells, so deficiency causes a particular type of anaemia (often with abnormally large, immature cells — megaloblastic changes). Pale gums and exercise intolerance follow.
- Neurological signs. Cobalamin is vital for nerve function. Affected dogs can show weakness, tremors, a wobbly gait, or behavioural dullness.
A frequent extra clue is proteinuria — protein in the urine — because cubilin also operates in the kidney. A young Border Collie with unexplained poor growth, anaemia, and protein in the urine should put this syndrome high on the list.
Diagnosis is confirmed by a blood test showing low serum cobalamin, often alongside elevated methylmalonic acid (a metabolite that piles up when B12 is scarce). Crucially, none of these signs are unique to this disease, which is exactly why so many affected puppies are misdiagnosed or, worse, euthanised as “failing to thrive.”
The Hopeful Twist: A Disease You Can Reverse
Here is what makes Imerslund-Gräsbeck syndrome remarkable. The gut cannot absorb B12 — so you simply bypass the gut. Regular injections of cobalamin, given under the skin, deliver the vitamin straight into the bloodstream where the body can use it.
The response is fast and dramatic. Appetite, energy, and growth typically rebound within days to a few weeks; the anaemia resolves; the neurological signs reverse. The catch is that the underlying defect never goes away, so treatment is lifelong — injections at intervals (often monthly once stabilised) for the rest of the dog’s life. But B12 is inexpensive, the injections are easy for an owner to learn, and a treated dog can expect a completely normal lifespan and quality of life.
That stands in sharp contrast to most inherited diseases of herding breeds. A puppy with a fatal lysosomal storage disorder, or one carrying a progressive, incurable condition like the SOD1 mutation behind degenerative myelopathy, faces a path with no good destination. Cobalamin malabsorption is the opposite: catch it, inject it, and the dog is essentially cured in practical terms.
Inheritance and Why Testing Matters
Imerslund-Gräsbeck syndrome is inherited as an autosomal recessive trait. A dog needs two faulty copies of CUBN (one from each parent) to be affected. Dogs with a single copy are healthy carriers — they show no signs and absorb B12 normally, but they can pass the mutation on.
Because carriers are invisible, two of them can be bred together for years without incident, until a mating finally produces affected puppies. This is the classic recessive trap, and it is exactly why a DNA test is worth its modest cost. A genetic test reads the CUBN status directly and returns a clear clear / carrier / affected result. If you are unsure how to interpret those categories, our guide to canine DNA testing explains what each result means for a breeding plan.
The breeding rule is the standard recessive one, and it is reassuring: a carrier is not a dog to remove from the gene pool. Border collie populations cannot afford to discard every carrier of every recessive without collapsing their genetic diversity. Instead, breed a carrier only to a tested-clear mate. That mating can never produce an affected puppy — at worst it produces more carriers, which you then identify and manage in the next generation. You keep the dog’s other qualities and the breed’s diversity while ensuring no affected puppy is ever born.
The Practical Takeaway
For breeders, the message is simple: know the CUBN status of your dogs before you plan a litter, and never double up two carriers. For owners and vets, the message is even more important — a young Border Collie that is small, anaemic, and neurologically off, especially with protein in the urine, deserves a cobalamin blood test before anyone gives up on it. Few diseases in canine genetics offer such a large reward for so little effort. A handful of cheap injections is the difference between a lost puppy and a normal, healthy working dog.