CANINE GENETICS

Does your dog rub his eyes?  He has a painful eye problem, possibly Primary Lens Luxation or canine glaucoma.  Take him to the veterinarian but read this first!

Primary Lens Luxation In The Dog

Editor's Note: Primary Lens Luxation (PLL) is not new but DNA tests have given breeders the genetic component to control and reduce this eye problem which affects many purebreds and mixed breeds.

Lens luxation is the name used to describe a situation in the eye wherein the crystalline lens is not found in its normal position but is lying either in the anterior chamber in front of the iris or behind the iris not within the hyaloid fossa. Luxation occurs due to weakening of the ligaments holding the lens. (PLL - Lens luxation in the dog is not limited to purebred dogs or any one breed even though it is a hereditary eye disease.  The left illustration shows a normal canine lens whereas the graphic on the right depicts a luxated-shifted lens).

Usually lens luxation is associated with secondary glaucoma. The overall condition is painful and requires an operation. The lens is not as important to the dog as is the lens to man and accordingly the operation still leaves the dog with some vision.

The condition was first reported by Nicolas in 1925 and has been reviewed in several books on canine genetics (e.g. Hutt, 1979; Robinson, 1982; Willis, 1989). It does appear to be fairly late in onset; usually after two years of age and often later with cases being observed as late as 7 years. It also appears to be bilateral in that although one lens will luxate eventually the other will follow suit though an interval of weeks or months may occur between the two happenings.

Breeds Affected

Lens luxation is essentially a "terrier" defect. It has been observed principally in Wire Fox Terriers and Sealyhams but it has also been seen in the Smooth Fox Terrier and in the Jack Russell. It is also well established in the Miniature Bull Terrier and has been seen in Scottish Terriers as well as in the Tibetan Terrier which, despite its name, is not a terrier.

Other non-terrier breeds such as the German Shepherd, Border Collie, Greyhound, Welsh Corgi and Miniature Poodle have had cases reported but not at the frequency PLL affects the terrier breeds.

It is known that trauma (accident) can cause lens luxation if damage to the suspensory ligament is occasioned and some of the non-terrier cases might be made in this way but there is ample evidence that the condition is likely to be genetic in most instances and one is wise to assume inherited cases unless trauma is assured by veterinary diagnosis.

Mode of Inheritance

The most extensive report on the genetics of lens luxation was by Willis et al (1979) looking at the Tibetan Terrier using British and Swedish data. The evidence was in support of a simple autosomal recessive in that breed and it is not improbable that the same mode of inheritance occurs in other breeds. In the Tibetan Terrier the condition traced back to dogs of unknown or incomplete pedigree registered in the days when dogs "looking like" the breed could be registered as Class II registrations. Almost certainly, the cases in the Tibetan Terrier traced back to dogs that looked like but were not Tibetan Terriers.

A simple autosomal recessive means that the condition can occur in either sex and that it is generally transmitted by what appear to be normal animals though the late onset of the condition can result in "affected" dogs producing the defect because they were used prior to their "affected" state being known.

With a simple recessive we have three kinds of dog. If we use N as indicating the normal gene (allele) and n the abnormal or lens luxation gene then we have:

1. NN normal dog showing no problems and producing none.

2. Nn apparently normal dog never showing the problem but producing it in some offspring or capable of doing so.

3. nn affected animal.

There are six different kinds of mating that could take place ignoring the sex of parents. These are shown below with expected proportions of progeny.

Three of the matings (3,5 and 6) involve using affected stock. These are not likely to occur often but in view of late onset of lens luxation can take place at least in early life. All three types of mating would result in the n (abnormal) allele being transmitted to the next generation and in two cases (5,6) could give rise to affected stock. Mating 6 (affected to affected) would be expected to give 100% affected progeny while mating 1 gives rise to no affected progeny at all. Mating 2 is a difficult one since it gives rise to affected stock but ensures that the n allele is passed on to half the progeny though the owners may never realize this because they may never know that they were using one Nn parent.

The most common mating giving the problem is mating 4 when two apparently normal dogs are mated and give rise to affected progeny. This mating of two carriers is the commonest way of discovering recessive genes. However, although the mating gives rise to 75% "normal" and 25% affected, it really gives rise to 25% normal, 50% carriers and 25% affected. In any one litter no affecteds may occur especially in a breed like the Miniature Bull Terrier where small litters are likely to occur. The fact that no affecteds occur makes studies difficult but also makes it difficult lo exonerate animals.

One can conclude. quite categorically that if lens luxation occurs in a litter then BOTH parents must, at the very least, carry the affected (n) allele. Their parents must therefore be either Nn or they could be nn in which the condition has not yet manifested itself. On the other hand, the fact that an animal has not produced the condition does not automatically mean that, that animal is free of the allele. An animal, especially a bitch, may be Nn but be mated always to NN mates and thus never give nn progeny or it might be mated to an Nn mate but, by chance, not give rise to other than Nn or NN cases.

If an affected case occurs in a litter then what are the risks from the other littermates, which appear normal? A litter mate to an affected (nn) pup has a 2 to 1 chance of being a carrier (Nn) and a 1 in 3 chance of being normal (NN) but there are no easy ways one can tell which. Clearly one is better avoiding the use of littermates to affected pups although the late onset makes it difficult to find out before littermates are used.

Advice to follow

Data are being collected on cases in the breed but we require more. Any breeder who has had a case diagnosed should send me a 5 generation pedigree of the case animal stating the sex of the affected animal(s) and the age at onset. Also we require to know how many pups of each sex existed in the litter. This will enable clarification of the mode of inheritance. Is it like the Tibetan Terrier or not?

Breeders should not breed from affected (nn) cases as soon as these are identified, They should avoid breeding again from parents of affected stock since these parents are likely to be at least Nn. However if one can identify NN animals with certainly then some Nn to NN mating might occur if the gene pool warrants it and dogs are outstanding in certain attributes. This will, however, transmit the n allele to the next generation so is best avoided except in exceptional circumstances with exceptional dogs. As yet we cannot be certain of source dogs so it would be premature to indicate lines to avoid. The problem is one that will not go away without positive effort from breeders and it requires collaboration if the incidence is to be identified and reduced. If we are dealing with a simple autosomal recessive then it has to be understood that it cannot, with present genetic understanding, be totally eliminated from the breed. What one can hope to do is reduce the incidence of the problem but that requires the whole-hearted support of ALL breeders towards the common good.

Inherited defects can occur in anyone's kennels. If you have not yet had lens luxation then you should thank your lucky stars but you should give every help to those who have had the problem. Genetic problems are not reduced except by all of us pulling together on the same problem.

From The Miniature Bull Terrier Club of England Handbook ? (England) ~ Edited and submitted by Mrs. V. Allenden

References:

Hutt, F.B. 1979. Genetics for Dog Breeders. W.H. Freeman, San Francisco.

Nicolas, E. 1925. Veterinary & Comparative Ophthalmology. H. & W. Brown, London.

Robinson, R. 1982. Genetics for Dog Breeders. Pergamon, Oxford.

Willis, M.B. 1989. Genetics of the Dog. H.F. & G. Witherby, London.

Willis, M.B., Curtis, R., Barnett, K.C. & Temple, W .M. 1979. Genetic aspects of lens luxation in the Tibetan Terrier. Vet. Rec. 104: 409-412.

Dr. M.B. Willis Faculty of Agriculture, The University, Newcastle-upon-Tyne, NE1 7RU

Miniature Bull Terrier Lens Luxation Research ~ Cases of Lens Luxation in the Miniature Bull Terrier ~ How To Correct Any Genetic Fault

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