Why rare breeds don't breed the carriers out

The result comes back as a single word.

Your vet pulls twenty or thirty hairs from the mane, roots attached, seals them into a labelled bag, and seven to ten working days later an email arrives from the laboratory. Clear. Or carrier. There is no scale, no grade, no breed mean to measure it against. One word, and the sense that something has just been settled about your animal.

The instinct that follows is immediate, and in a numerous breed it is roughly correct: a carrier should not be bred from. Take it out of the pool. Breed the fault away.

In the Fell and the Dales, that instinct would have been a disaster, and the two societies knew it. What they did instead is one of the more interesting pieces of breeding policy in this country, and it is worth understanding — whether you are buying a pony, planning a mating, or simply trying to work out what a piece of paper in a folder actually means.

The disease

Foal Immunodeficiency Syndrome was identified first in Fell ponies and later in Dales. It is recessive, and it is fatal.

An affected foal is born looking entirely normal. At two or three weeks it begins to fade — dull coat, poor appetite, scouring, anaemia, pneumonia. The Dales Pony Society's own description is blunt: the condition is invariably fatal if usual treatments are employed. There is no version of this where careful nursing wins.

Because the mutation is recessive, an affected foal carries two copies, one from each parent. Which means that behind every affected foal stand two carriers — outwardly healthy ponies, sound and unremarkable, each with a single copy and no way of knowing. A carrier-to-carrier mating produces an affected foal one time in four. The other three times it produces a live foal, half of which are carriers themselves, and nothing whatever is learned.

That is the shape of a recessive. It hides, it accumulates quietly through the generations, and it presents its bill in the foaling box.

The number that changed the arithmetic

A carrier test became available in February 2010, out of work led by the Animal Health Trust at Newmarket with the University of Liverpool, where the underlying research on the condition had been done. For the first time it was possible to ask a healthy pony a question it could not otherwise be made to answer.

Then the results came in.

The Dales Pony Society is careful about its figure — it has been suggested at 12 to 15 per cent, and the Society says plainly that this has not been proven. Estimates published for the Fell have been put higher. But even at the low end the implication was the same, and it was not a comfortable one: a meaningful fraction of a very small breeding population was carrying a fatal recessive.

Now apply the obvious remedy. Remove every carrier from the breeding population of a breed that registers a few hundred animals a year. You would eliminate FIS. You would also, in a single generation, discard a large slice of what remains of a native breed's genetic diversity — and you would be discarding those animals on the basis of one gene, with no regard for everything else they happened to carry. You would have solved the disease you could name and worsened the one you cannot: the slow narrowing that is the real long-term threat to every small population. That argument is the whole of why heritage breeds are worth building for.

Culling the carriers would have been a way of losing the breed in order to save it.

What they did instead

The strategy both societies settled on is almost embarrassingly simple, and it works. Test everything, and never mate a carrier to a carrier.

A carrier bred to a clear pony cannot produce an affected foal. Not less likely — cannot. Every foal of that mating is either clear or a carrier, and every one of them is healthy. The carrier keeps its place in the gene pool, the bloodline continues, and the disease has nowhere to go.

The effect was immediate. Professor Stuart Carter of the University of Liverpool analysed the first three years of testing and found the number of affected foals had fallen dramatically; in 2012, one foal tested positive. A fatal disease was switched off, essentially, by arithmetic and record-keeping.

Here is the part most people miss. Carter was explicit that the strategy would not reduce the number of carriers in the population — allowing carriers to breed with clears leaves the carrier fraction roughly where it was found. That is not a failure of the policy. That is the policy. The carriers were never the problem. The pairing was.

Which also means the testing never stops. Every new animal entering the breeding population has to be asked the question again, in perpetuity, because the gene is still there and always will be. The two societies did not eradicate FIS. They chose to live alongside it, permanently and carefully, and judged that the better bargain. Sixteen years on, they were right.

The two societies do not do this identically

Worth knowing if you are buying or planning a mating, because the paperwork differs.

The Fell Pony Society builds the test into stallion licensing. Colts are eligible from two years old, the vet takes the DNA and FIS samples at the inspection, and the licence is issued once the testing confirmation comes back. The fee schedule carries two columns — one including the DNA and FIS kits, one for a colt already tested — which tells you how routine it has become. The Society will also refund the FIS test fee for a stallion once his first progeny are registered, provided the test was bought through them.

The Dales Pony Society goes a step further. Since 2012, a colt must test clear to be licensed at all. Mares are treated differently, and the Society is direct about it: should you avoid breeding from a carrier pony? No — provided the stallion is clear. They will not license a carrier colt, but a clear colt or a clear filly will carry your bloodline forward.

Two societies, the same science, slightly different judgements about where the line sits. Neither is being careless, and the difference is a reasonable thing to ask a secretary about.

What this means if you are buying

Little of this is difficult, and all of it is worth doing.

Ask for the certificate, and read whose name is on it. A clear result belongs to a named animal, not to a stud, a prefix or a bloodline. "The stallion's line is clear" is not a test result.

If a stallion's certificate cannot be produced, assume he is a carrier. That is the Dales Pony Society's own advice, and it is the right default. It is not an accusation about anybody. It is simply how you avoid the one mating that matters.

Certificates can be faked. In 2024, for the first time since testing began in 2010, the Fell Pony Society reported seeing an FIS certificate that had been tampered with, and asked owners with any doubt to send a copy to the office to be checked. If you are about to plan a mating on the strength of a piece of paper, an email to a society secretary is a very cheap insurance premium. It is also one more instance of the point made in how to use a breed society when you're buying: the institution that cannot sell you anything is the one worth asking.

Get the result recorded in the passport. The Fell Pony Society encourages this and notes it cannot be done automatically. A certificate in a drawer is a certificate that goes missing when the pony changes hands. It is precisely what our free health record was built to hold, and it costs nothing to use.

A carrier is not a lesser animal. If you are buying a pony to ride, a carrier result has no bearing on anything whatsoever; the pony will never be ill from it. If you are buying to breed, it is a constraint on your choice of stallion, not a defect in your mare.

The general point

I have used FIS because it is the clearest example I know of, but the thinking travels.

A genetic test is a very narrow instrument. It reports the status of one animal at one place in the genome, and it reports it with a certainty that a hip score or a set of radiographs cannot offer — which is the distinction I tried to draw in how to read a health-testing certificate. What it does not do is deliver a verdict on the animal, and it does not tell you what to do next. That is a breeding decision, and in a small population it is a decision about the whole population and not only about your foal.

The temptation, when you are handed a certain answer, is to act on it hard. The Fell and the Dales were handed a certain answer about a fatal disease and chose, deliberately, to act on it gently — to keep the carriers, control the pairings, and accept that the work would never be finished.

The foals stopped dying and the gene pool stayed intact. It is not the tidiest outcome that was available. It is the right one.

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Correction, 31 July 2026. This piece originally said the FIS carrier test came "out of work done at the University of Liverpool". The mutation was identified and the test developed in work led by the Animal Health Trust at Newmarket, with the University of Liverpool — where the foundational research on the condition was carried out — among the collaborators. The text has been amended.

— Rene