A ridge too far?

Illustration: Kevin Brockbank

The Rhodesian Ridgeback is an undeniably handsome beast: fit, functional and unexaggerated.  They have real presence and to meet one is to get some sense of why they were so valued in their native Africa - not just as a tough multi-purpose dog that could hunt, herd and protect, but one brave enough to track lion. Of course the cats they encounter these days are rarely bigger than next door’s moggie – and am sure they keep a very respectful distance.

As most people will know, the breed is marked by the ridge of hair that grows in an opposite direction along the dog’s spine, usually topped by a couple of swirls of hair, at the withers-end of the spine, called “crowns”.

The ridge originates from the breeds’ native African ancestors, some of whom had ridges, bred with European dogs to create the breed.  Some of these crosses had ridges, too, and the ridge became the hallmark of the dogs’ tough forbears. The belief also grew that the dogs with a ridge were better hunters – eventually leading to the ridge becoming the defining feature of the breed.

In fact, the genes that have been identified as being involved in the development of the ridge are known to govern tissue development, not temperament, so the presence of the ridge is unlikely to link to any functional advantage. But the story endures and the ridge is hugely cherished by Ridgeback breeders.

And there would be no harm in that except for two things – as we highlighted in Pedigree Dogs Exposed three years ago.

The first is that some ridgebacks are born without a ridge and, traditionally, these pups were often put to sleep (although this happens more rarely now). The second is that the ridge on the Rhodesian Ridgeback  (and the lesser-known Thai Ridgeback) has long been associated with a health problem – a condition called dermoid sinus (DS)  - and pups born with one or more DS are also sometimes put to sleep.

Dermoid sinus is a defect that is caused by the incomplete separation of the covering of the neural tube (from which the spine develops) and the skin during the development of the embryo.  It usually manifests in a pinprick-sized hole in the skin along the dog’s spine that leads to a narrow tube (a sinus).  The sinus can be of varying lengths – sometimes very shallow, occasionally deep enough to connect to the spinal cord – and they are troublesome because they can become infected. This infection can be life-threatening (particularly if the sinus is a deep one).

The most reliable data, from Swedish litter records, suggest that about five per cent of Ridgebacks are born without a ridge and about 10 per cent with dermoid sinus.  The most recent breed survey in the US found the reverse (ridgeless – 10 per cent; DS – five per cent)  and this survey also found that being born ridgeless was the leading cause of death in the breed (7.25 per cent) – with dermoid sinus the second (2.5 per cent).

But the US figures are 10 years old now (from 2001) and Ridgeback breeders insist that very few dogs are euthanised for either reason today. “I do not know a single person who puts down dermoids or ridgeless dogs any more,” says US breeder Ann Chamberlain.  “Back in the days, we put down the dermoid pups because otherwise their quality of life was compromised.  Now that the operation to remove the dermoid is so commonplace, and surgical techniques have improved so much, there is not one reason in the world to put the dog down. “

In the UK, rules brought in since Pedigree Dogs Exposed expressly forbid the culling of dogs for cosmetic reasons (in any UK breed), and some DS pups are now successfully operated on  – but unfortunately this is not true elsewhere. The Rhodesian Ridgeback Club of Ireland still requires that DS pups are “culled at birth”; so do the Swedish and Australian breed clubs. In fact, the RR Club of Victoria still advocates culling ridgeless (although does offer neutering as an alternative). 

Breeding for the ridge, then, still results in some puppies being killed.

Four years ago, Swedish scientists announced that they had worked out the genetics of both the ridge and dermoid sinus. Their findings, published in one of the most prestigious science journals in the world (Nature Genetics), caused a firestorm in Rhodesian Ridgebacks.

The scientists reported that:

• the ridge is a dominant trait – in other words, that a Ridgeback only had to carry one copy of the ridge mutation to have a ridge.

• the ridge mutation predisposes the dogs to dermoid sinus.

• the highest risk of dermoid sinus is in dogs that carry two copies of the ridge mutation.

• ridgeless dogs do not carry the ridge mutation and are therefore not at risk of dermoid sinus.

Lastly,  the Swedish researchers stated that that the incidence of dermoid sinus could be drastically reduced, maybe even eliminated, if breeders were willing to breed from Ridgebacks born without a ridge. To many Ridgeback breeders, this was tantamount to sacrilege as they’ve spent decades trying to get rid of ridgeless dogs.

The findings, then, raised a big ethical question: was adherence to the breed standard at simply too great a cost to the dogs – and should breeders dump the ridge as the hallmark of the breed?

Highlighting this in Pedigree Dogs Exposed caused a huge fuss and led to Rhodesian Ridgeback breeders receiving abuse from the public – abuse that they felt was very unfair.

Some of the abuse was unfair. We made a mistake in the UK version of Pedigree Dogs Exposed in referring to the ridge as “a mild form of spina bifida”.  In fact, it is dermoid sinus that some sources within the breed describe as being similar to a mild form of spina bifida, not the ridge. So although we were right in our central tenet – that breeding for the ridge costs some dogs their lives  – our mistake made the problem appear greater than it is.  For this, I would like to apologise to Rhodesian Ridgeback breeders.  

However, the fact remains that breeders have not acted on science that many people outside of the breed believe is sound enough to warrant a reconsideration of the ridge as such a defining feature of the breed.

 “There has never been any attempt to hide our known adverse condition of dermoid sinus in the Rhodesian Ridgeback , quite the reverse. We understand the problem and information is widely disseminated to the best of our ability," insisted  UK breeder Ann Woodrow in a recent article in a dog magazine. Ann featured in Pedigree Dogs Exposed, controversially putting the case for euthanizing dogs born without a ridge (essentially the fear that they would fall into the hands of "the fighting people"). “

Information in respect of how to detect and deal with dermoid sinus is, indeed, widely available - but information on using ridgeless to breed away from the condition is not. The Swedish research findings have never been published on the Rhodesian Ridgeback Club of Great Britain’swebsite - even though the Club part-funded the research - and Club guidelines still insist that ridgeless dogs should never be bred from.

I asked the Club if this was under review; also why they had not published the Swedish findings on their website and whether they were funding any current research.  Back came this brief statement:

“As the scientific research into dermoid sinus in the Rhodesian Ridgeback is still ongoing, there are no further details presently available.”

It essentially boils down to this: Ridgeback breeders believe that that the science – if not flawed - should be considered interim and that until further evidence is forthcoming, they should not change the way they breed their dogs.

 “Any one study's results, especially with such a small sample size, needs to be replicated to verify the original results,” says US Ridgeback breeder Jan Koler-Matznick. “As you know, many study results are amended or discounted with further investigations. So, this one study provided a basis to do a larger study on the inheritance of the DS, not definitive facts.”

It is of course true that all science is always subject to review. So is Jan right? “Nature Genetics only publishes research that meets the very highest standards of expert geneticist,” says editor Myles Axton. "The paper published in Nature Genetics in 2007 identifies a gene duplication in Rhodesian Ridgeback dogs that is responsible both for the ridge trait and for the predisposition to dermoid sinus… [It} provides enough evidence to give sound advice to the dog breeder on how both to produce the ridge trait and how to avoid the dermoid sinus defect.”

Even if it s true, though, Jan argues that the relatively low incidence of dermoid sinus is a mitigating factor. “Only around five per of dogs are born with dermoid sinus and most can be fixed with no resulting health consequence. That is a lot better than say 30% of some breeds having hip or elbow dysplasia and living on drugs or in pain most of their lives. Given the choices available, I don't see how Ridgebcks do not come out good for health issues in comparison to most breeds.”

The breeders have support from one of the authors of the Swedish research, Nikki Salmon Hillbertz. Nikki, who has Ridgebacks herself, is using a £13,250 grant from the Kennel Club to further research the link between the ridge and dermoid sinus. Questions that still need answering include: why doesn’t every Ridgeback with a double-dose of the ridge mutation develop dermoid sinus? And how come some dogs with only one copy of the mutation (although considered low-risk) have been born with a DS (as indeed, the Swedish research found)

The answer might simply be one of “expression” or “penetrance” – genes often have a “volume” button that varies their influence. Or it might be that there are other genes involved. The latter possibility, particularly, offers hope that a new genetic marker could be found to identify which dogs are most at risk of producing DS. Also useful would be a DNA test that can identify dogs that carry two copies of the ridge mutation in order to avoid breeding dogs with the highest risk of DS.

Another reason for caution, say breeders, is that since the research was published, there have been reports of Ridgebacks without a ridge being born with a dermoid sinus.  On the face of it, this is unexpected.  The Swedish scientists looked at 600 litters registered in Sweden and Germany between 1989 and 2003 and, of the 5,376 puppies recorded, not a single ridgeless dog was found with a dermoid sinus.

But there is an explanation: dermoid sinus does occur occasionally in other, non-ridged breeds. Cases have been reported in American and English Cockers, Boxers, Chows, Golden Retrievers, Shih Tzus, Siberian Huskies and Yorkshire Terriers.  In fact, DS sometimes occurs in humans, too. 

It is perfectly possible, then, for the occasional ridgeless ridgeback to have a dermoid sinus as an unlucky one-off ie. it is not thought to be inherited in any other breed than the Rhodesian and Thai Ridgeback.

But the stalemate endures - with most Ridgeback breeders arguing that the science is not definitive enough to change current breeding practice – and others disagreeing.

I asked geneticist and Boxer breeder Bruce Cattenach for his view. He says he is persuaded by the science. "In my view, despite the historical connection to ridging, which is presented as characterising the breed,  RR breeders would be best advised to breed away from the ridge in future.   I think tying the RR to the ridge diminishes this impressive breed.”

But at the last Rhodesian Ridgeback World Conference in 2008, just before Pedigree Dogs Exposed but after the key Nature Genetics paper was published, breeders voted overwhelmingly to keep the ridge as the “escutcheon” of the breed.

Shortly after, the senior authors of the Nature Genetics paper issued a statement* [see below] confirming their findings and again making the point they believed that dermoid sinus could be “virtually eliminated” if breeders were to accept and breed from ridgeless Ridgebacks. Like the original research findings, this has never been published on the UK Club’s website – nor on any other international Club websites that I could find, despite having been distributed to all Rhodesian Ridgeback Clubs worldwide – a strong indication, I believe, of how most breeders feel about the science

But I did find one breeder prepared to challenge the status quo - Ann Chamberlain, an American biologist and teacher who has been breeding Ridgebacks since 1965. Ann recently mated her ridgeless bitch to a dog thought to carry two copies of the ridge mutation. As the science predicts, she got a litter of nine puppies, all with a ridge and none with dermoid sinus (although this was not the reason for the mating).

“I bred my bitch because she is the most conformationally-correct dog I have ever owned and, otherwise, I would have lost the line,” says Ann, who will be donating their DNA to the further research effort. “I hope breeders will realise that under certain circumstances it may be beneficial to use a ridgeless dog or one with an imperfect ridge”.

Ann is not convinced by the science (she says she has only ever produced one dermoid sinus in 20 litters anyway) but she does feel that the breed should accept their ridgeless dogs and less than perfectly-ridged dogs – and says there are a handful of other breeders who feel this way, too.

“Ridgeless pups and those with other than perfect ridges should no longer be summarily discarded from the gene pool,” says Ann, who is currently conducting a world survey on Ridgebacks..  “We are discarding, potentially, the healthiest and most conformationally correct dogs for what is truly a ridiculous reason.  If the ridge had anything to do with function I would not advocate using anything but perfect ridges.  However, this is not the case and therefore, at times, I truly believe a breeder may actually need to use a ridgeless or a dog with a ridge anomaly.”

“Closed stud books are not healthy for pure-bred dogs, especially a breed with very few founders.  Throwing out a third of your gene pool for the ridge is just plain stupid, no matter how you look at it. Since Rhodesian Ridgebacks had so few founders, it is genetically impossible to "outcross", as so many seem to think they can.  They simply do not understand that importing a dog from Australia to mate with an American dog is still shuffling the same genes, and who knows what has been lost along the way.” 

The Ridgeback row will, I fear, continue for many years – and I do understand why it is difficult for Ridgeback breeders to lose their attachment to the ridge.  But there is hope in breeders like Ann Chamberlain  - and in the fact that ridgeless dogs are now in greater demand as a pet (undoubtedly helped by the fact that breeders sell them for less, meaning you get an awful lot of dog for your money).

“You have no idea how many people ask for a ridgeless pup,” says Ann. “Now that the breed is so popular it is easy to place ridgeless and often easier than placing a ‘show’ prospect.“

This is good to hear and it does make it a little less of an issue than it used to be.  I also accept that the breed as a whole enjoys better overall health than many other breeds where a higher incidence of problems causes much less controversy.

But, bottom line, the breed standard still costs some Ridgebacks their lives. That  - in my view – is a problem.  But what do you think?

If you are a Ridgeback breeder, Ann Chamberlain would greatly appreciate help with a current breed survey. The survey form is online here . Or contact Ann Chamberlain:  email: mazoerr@gmail.com. Please feel free to crosspost this information.

THE STATEMENT FROM THE SWEDISH SCIENTISTS

To whom it may concern

A statement concerning the genetic basis for the hair ridge and the congenital malformation dermoid sinus in Rhodesian Ridgeback dogs

It has come to our attention that the recent World Congress of the Rhodesian Ridgeback Association has left some uncertainty concerning the genetic basis for the hair ridge and the dermoid sinus in Rhodesian Ridgeback dogs. We would therefore like to clarify this subject and its implications for breeding.

1. The inheritance of the Ridge

All our data and all data we are aware of indicate that the Ridge is inherited as a fully dominant trait. The dominant allele causing the Ridge is denoted R whereas the recessive wild type-allele is denoted r. This means that a ridged dog may either be homozygous (R/R) or heterozygous (R/r) for the Ridge allele whereas all ridgeless dogs should be homozygous r/r for the normal allele. The result presented in our Nature Genetics paper from 2007 (Hillbertz et al. Nature Genetics 39:1318-1320) provided conclusive evidence that the mutation causing the ridge in Rhodesian Ridgeback as well as in Thai Ridgeback dogs is a 133 kb duplication on dog chromosome 18. [A duplication means that each Ridge chromosome has two copies of this 133 kb fragment whereas a normal chromosome has only a single copy. It is this doubling or duplication of the chromosome region that constitutes the Ridge mutation.] So far all tested dogs with the characteristic dorsal hair ridge have been heterozygous or homozygous for this mutation whereas all ridgeless dogs we have tested lacked the duplication. The duplication contains four complete genes (FGF3, FGF4, FGF19 and ORAOV1) and we assume that it is the higher than normal expression of one or more of these genes, attributable to their greater number, that leads to the development of the hair ridge.

2. The genetic basis for dermoid sinus (DS)

The mode of inheritance of DS is not as clear as the inheritance of the Ridge but our data clearly showed that the Ridge mutation (i.e. the duplication described above) is the major risk factor for the DS malformation in Rhodesian Ridgeback dogs. Most DS dogs in our study were homozygous R/R (10 of 12) for the Ridge mutation but two were classified as heterozygous R/r. However, DS or DS-like malformations also occur in humans so it is possible that this type of malformation may occur in dogs in the absence of the Ridge mutation. But its frequency in unridged Ridgebacks should be as rare as it is in non-ridged breeds.

3. DNA tests

In our Nature Genetics paper we described a simple DNA test that can be used to identify the presence of the Ridge mutation (the duplication). Any DNA laboratory skilled in the art of DNA testing can perform this test and there is no patent protecting its use. It is therefore easy to distinguish a ridged dog (R/R or R/r) from a ridgeless dog (r/r) by the DNA test but we have not yet established a diagnostic test that on a routine basis can distinguish animals that are heterozygous carriers (R/r) from homozygous ridged (R/R) with 100% certainty. The establishment of this test will require some further research. 

4. Recommendations for breeding

The most straightforward way of reducing the incidence of DS in Rhodesian Ridgeback dogs is to reduce the frequency of homozygotes for the Ridge mutation. This can be accomplished by allowing the use of ridgeless dogs for breeding. While we are aware that dogs with a DS are not usually kept for breeding, matings between homozygous (R/R) ridged dogs (presumably without DS) and ridgeless dogs (r/r) would give progeny all of which would be heterozygous ridged (R/r) and therefore show ridging, and the incidence of DS would be low. In matings between a heterozygous ridged dog (R/r) and a ridgeless dog (r/r), 50% are expected to be heterozygous ridged (R/r) and 50% are expected to be ridgeless (r/r). In these matings no homozygous ridged progeny, which are the major problem as regards the incidence of dermoid sinus, would occur. It should therefore be possible to retain the ridge while keeping the incidence of DS to its absolute minimum. It would be useful to develop a diagnostic test to distinguish carriers (R/r) from homozygotes (R/R) because this would allow breeders to avoid matings such as R/R x R/r that will produce homozygous ridged (R/R) progeny.

            It will of course be decided by the individual breeders and to Ridgeback breeding organizations whether they prefer to keep the Ridge and minimize the incidence of DS using the approach described above or whether they would like to completely eliminate the problem with DS by allowing the Ridge mutation to disappear from the population over time. If one decides to eliminate the Ridge mutation it should not be done too quickly (during a few generations) since that will lead to increased inbreeding in the breed as too many potential breeding animals are eliminated.

Leif Andersson¹, Kerstin Lindblad-Toh^1,2 and Göran Andersson³

¹Department of Medical Biochemistry and Microbiology, Uppsala University, Box 597, SE-751 24 Uppsala, Sweden.

²Broad Institute of Harvard and MIT, 7 Cambridge Center, Cambridge, Massachusetts 02142, USA.

³Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences (SLU), Biomedical centre, Box 597, SE-751 24 Uppsala, Sweden.

This article is adapted from the version that appears in the September 2011 issue of Dogs Today magazine.  Dogs Today is now available internationally for iPad and iPhone for the bargain price of 59p for the app, which includes one edition free.