The Canine eye and Lens Luxation - PLL PRIMARY LENS LUXATION
There are many genetic diseases that have / can develop in any particular breed or species by natural aging, genetically inherited or caused by injury or trauma to the eye. The major eye disease that affect canine breeds can include genetically inherited blindness, forms of canine progressive retinal atrophy (often abbreviated to PRA or PRD), suddenly acquired retinal degeneration (abbreviated to SARDS), primarily lens luxation (Abbreviated to PLL), glaucoma and cataracts. There are others.
Because our main interest and the focus of this article is the Volpino Italiano, then, for the purpose of this discussion, we will centre our attention on the main diseases affecting the Volpino. We will focus our attention on progressive retinal atrophy (PRA) primary lens luxation (PLL), and cataracts. However,
it does appear that the Volpino Italiano is troubled with its share of most of the known eye problems The main effect of the genetic version of PLL is a genetic weakness in the zonular cords and breakage or disintegration of the zonular
cords at a genetically predetermined time. The disease usually (80%) manifests itself between 3 and 7 years old.
Primary lens luxation is, quite simply, the movement, or luxation of the lens. The lens structure of the eye is meant to focus light on the retina and that allows us (and most
animals) to see an image clearly. The lens is held in place by zonal ligaments (cords) and is not allowed to move from its position between the iris and the vitreous humour in the eye. However, if some of the Zonal Ligaments (cords) disintegrate or break it will allow the lens to ‘loosen’ and change position (sub-luxation). This can occur in the early stages of PLL.
At some point the lens may become completely detached (luxated) and is now free to ‘be pushed’ into the anterior chamber (this is anterior lens luxation). Primary lens luxation has then occurred! The dislocated lens is now free to move about in the anterior chamber, damaging the Iris and pupil opening and blocking the drainage of the aqueous fluid - raising the pressure in the eye!! This is Glaucoma. With this type of Glaucoma the pressure rises rapidly and blindness quickly follows. This can be agonizingly painful.
As the animal ages the composition of the vitreous humour degrades from a clear, thick gelatinous type of substance into a more liquid form. If lens luxation occurs later in life it is possible for the lens to move or fall into the vitreous cavity (this is posterior lens luxation). This scenario may not produce the dramatic damage and pain that moving into the anterior chamber would. PLL in this case may not be detected for some time.
Glaucoma can cause lens luxation as a secondary infliction. However, if PLL is caused by glaucoma then that animal would have been blind far before the onset of PLL. One of the resulting injuries of glaucoma is that it stretches the sclera (globe of the eye) and thus stretches to Zonal ligaments to the point where they tear and break. This will not happen until very late in the disease.
It is also possible that PLL can be caused be trauma to the eye or indirectly by trauma to head. In many cases it will be obvious that the trauma has occurred.
PLL is a condition that can cause great pain to the animal and must be rectified as soon as possible. If the eye specialist notices a lens movement before excessive damage is caused he may be able to remove the lenses and allow the eyes to stay intact. Unfortunately if the lens becomes luxated and excessive damage is caused to the eye then the eyes will then need to be removed. This will likely be the condition that the veterinarian eye specialist finds himself in unless the animal’s eyes have been checked regularly. It is often the situation that the owner of the animal will not realize the severity of this condition until the disease and damage to the eye has progressed to this late condition. Unfortunately the expense in helping the animal will be very high and euthanizing the animal will
probably be the route chosen by most owners.
Although the attention caused by PLL has been high past few years it is a condition that has plagued the Volpino population since well before 1995 (An Italian veterinarians report was released in 1995 outlining the condition of PLL in Volpino Italiano dogs at that time). We can assume that most, perhaps all of the large breeders after that must have known of the condition from that point until the late 2010.
Indeed, since the PLL mutation had spread so wide it would not have been possible for most breeders
not to have heard of the condition or of complaints from their customers. However, it is also
understandable that tracking the disease in the past would have been difficult. One way or the other,
the claim that the Volpino Italiano was free of major health concerns was a very untrue statement!
THE GENETICS OF PLL
In late 2009 the mutated gene that causes PLL was discovered by the OFFA at the University of Missouri, USA (1a). Shortly thereafter it was also co-discovered by the AHT in Cambridge, England. It is here that the location of the mutation (described as G1473+1A ADAMTS17 ) was first identified and a DNA test developed to identify the mutation. Although this is mainly a disease which has plagued the Terrier Breeds (and those breeds that share the same ancestry) it somehow developed in the Volpino breed.
Indeed, if you look through the list of PLL affected breeds it seems that the Volpino Italiano ‘is out of place’. It is only recently that the Volpino Italiano is included in most lists of PLL affected (or prone) animals.
It should be noted though that this is not necessarily the only mutation that can cause PLL. Some breeds do suffer from PLL that are separate and caused by a separate mutation unknown at this time but not caused by the G1473+1A ADAMTS17 mutation. At this time that exact mutation has not been discovered.
The PLL mutation is essentially a simple recessive trait and follows the binary rules of Mendelian inheritance.
Normal – The individual has 2 normal (clear or non-mutated) genes inherited from his parents.
This individual cannot develop the genetic version of PLL. (found at G1473+1A ADAMS17). This individual can only pass on normal or clear genes to his offspring.
Affected - This individual has 2 mutated (PLL defective) genes inherited from his parents. This individual will, if he lives long enough develop the genetic variation of PLL. This individual can only pass on a mutated (PLL defective) gene to his offspring.
Carrier – This individual inherited 1 each of the clear and mutated genes from his parents. He will, more than likely, not develop the genetic variation of PLL. He can pass on either a defective or clear gene to his offspring. There is a small chance, above the normal population average that the individual dog may develop the genetic form of Primary Lens Luxation.
These would be the (average) results of inheritance for puppies of breeding parents.
N – (00) NORMAL A – (11) AFFECTED C - (01 or 10) CARRIER
Parents Male Female Possible results (combinations)
Genes N, N, N, N 100% clear All offspring will be clear
N, C, N, C 50% Normal, 50% Carrier
C, C, C, C 100% carrier - All offspring will be Carriers
N, C, N, C 50% Normal, 50% Carrier
N, C, C, A 25% normal 50% Carrier 25% Affected
C, A, C, A 50% Carriers, 50% Affected
C, C, C, C 100% Carrier - All offspring will be Carriers
C, A, C, A 50% Carriers, 50% Affected
A, A, A, A 100% Affected All offspring will be affected
This average should theoretically be maintained over the population average, although the individual results may vary widely.
As noted above, in a true recessive trait both mutated genes must be inherited from individuals parents for the disease to develop. However, there exists a low risk that a carrier will develop PLL probably because of heterozygosity at a different location on ADAMTS17. Although the actual influence is unknown it may be that 2 to 20% of PLL Carriers may develop PLL. For the Volpino population it appears that the risk is closer to 2%. More test data gathered in future incidences of PLL will be needed to solidify the actual risk!
Because of the possibility of producing PLL Affected
animals, PLL Affected animals should never be bred
with PLL carriers, PLL Carriers should not be bred with
another PLL Carrier. This combination will simply
produce too many PLL Affected animals. Indeed; PLL
Affected dogs should never be bred!! Although some
would suggest that breeding the PLL affected dog
with a PLL normal dog would result in PLL Carriers
which would then be bred with PLL Normal/ Clear
animals and so, should produce a few PLL Clear dogs.
That assumed end result is not entirely certain. It is
my suspicion and it does appear so that more PLL Carriers than PLL Clear/Normal will be produced. In
my view though, it is unethical to breed an individual animal that could only produce Carriers.
Breeding a carrier with a normal animal should be considered only if there are certain exceptional traits that the animal exhibits. This is a pairing that should produce more PLL Normal offspring than PLL Carriers and these can be used to continue the blood lines. This is important because of the past excessive in breeding in the Volpino Italiano. It is more that possible that eliminating the Carrier blood lines may cause damage to the entire breed by introducing even more genetic problems.
The breeding of carriers will need to soon come to a stop at some point, and that point in time should be soon. This step should be considered only as a transitory one towards the goal of eliminating PLL. The breeding of Carriers cannot continue indefinitely!!
Unfortunately there is much more to learn about PLL. While it may be possible to breed PLL out of the Volpino Italiano breed it is also possible that a resurgence of the PLL mutation that is not linked to the present genetic testing may also establish itself in the future. This may be a very old, established canine disease that may manifest itself from different sources. Think of it this way, the Volpino Italiano and some other canine breeds may just be prone to PLL (and other eye diseases).
How did Primary Lens Luxation develop in the Volpino Italiano? The actual facts will never be known but here are some facts. PLL is relatively rare in the Spitz breeds in Europe. Remember that the Volpino breed was resurrected by a few individuals recovered from remote farms and bred to stock the present populations.
- Looking at the early breeding records (see genealogy data) it appears that inbreeding was excessive in the early days of the Volpino recovery. It is possible the gene may have mutated here.
- It is possible that another breed or race that had the PLL mutated gene bred with a Volpino. After a few generations the differences in the introduced genes were lost to breeding, but the PLL mutated gene stayed with the breed
-The Volpino has been kept on Italian farms for centuries. It is here that the present day Volpino (as defined in the 1902 Standard) was essentially developed. On most farms breeding may have / would have been left to the Volpino’s themselves. The puppies in some farms would then have been the result of hundreds of generations of inbreeding. It may be here that the gene causing PLL mutated into its present form.
NOTE: In America, NAVC and VCA member breeders, DNA test to be sure their couples are PLL CLEAR since 2010, all puppies born are PLL CLEAR.
Michel (Mike) B. Rubini
Website Terralea Collins