Primary Lens Luxation (PLL)
Primary Lens Luxation (PLL) is a well-recognised, painful and blinding inherited
eye condition that affects many breeds of dogs.
In affected dogs the zonular fibres which support the lens breakdown or
disintegrate, causing the lens to fall into the wrong position within the eye.
If the lens falls into the anterior chamber of the eye glaucoma and loss of
vision can quickly result.
Scientists have identified a mutation that is associated with the development of PLL in several breeds of dog. DNA test we are now available to breeders and results will be sent identifying their dog as belonging to one of three categories:
CLEAR: these dogs have two normal copies of DNA. Research has demonstrated clear dogs will not develop PLL as a result of the mutation tested for, although it cannot exclude the possibility they might develop PLL due to other causes, such as trauma or the effects of other, unidentified mutations.
CARRIER: these dogs have one copy of the mutation and one normal copy of DNA. The majority of carriers do not develop PLL during their lives but a small percentage do.
GENETICALLY AFFECTED: these dogs have two copies of the mutation and will almost certainly develop PLL during their lifetime. Advice states that all genetically affected dogs have their eyes examined by a veterinary ophthalmologist every 6 months, from the age of 18 months, so the clinical signs of PLL are detected as early as possible. Affected dogs often go blind at about 5-6 years old.
Breeding Advice
Responsible breeders will test for PLL and make appropriate breeding choices to minimise or eliminate the occurrence of this disorder.
Carriers of PLL can continue to be used in breeding programs as long as they are only mated with PLL clear animals. This practise should be followed, to allow the PLL mutation to be slowly eliminated from the population.
Congenital Hypothyroidism with Goitre (CHG)
Congenital hypothyroidism with goitre (CHG) is an inherited disease affecting a few lines of Tenterfield Terriers. Affected dogs lack an Enzyme that is important in the production of thyroid hormone which is necessary for the normal development and metabolism of dogs. At 3-8 weeks of age, dogs with CHG are generally noted to have reduced movement and to be small when compared to their littermates. Neonatal affected pups exhibit inactivity, abnormal hair coat, stenotic ear canals, and delayed eye opening. Unfortunately affected pups only survive a matter of weeks as the disease is fatal at the neonatal stage.
A DNA-based
carrier test was developed and currently is used by breeders to eradicate this
disease.
Professor John Fyfe DVM, PhD in the US at Michigan State University was
instrumental in developing DNA tests so we can test for carrier dogs. Mutation
based DNA tests are breed-specific.
Using the test, it is possible to identify dogs that are:
· affected with CHG (affected individuals have two copies of the mutant gene), although the disease is clinically obvious.
· carriers of CHG (have one copy of the mutant gene and one normal gene copy). Carriers do not develop the disease themselves but can pass the gene mutation on to the next generation.
· genetically normal for CHG (have two copies of the normal gene and cannot pass the mutation to subsequent offspring).
Research has shown that this disease is an autosomal recessive gene. What this
means is, to end up with an affected pup you need to obtain a double up of the
defective gene. That is, both parents must be carriers. Consequently, if only
one of a breeding pair is a carrier, they will never produce an affected pup,
but puppies may be carriers.
A mating of carrier to clear will result in approximately 50/50 ratio of carrier & clear puppies.
I should also highlight that a clear to clear mating will never result a carrier pup, let alone an affected pup.
Professor Fyfe has done a lot of work in developing the tests required.
The Tenterfield Terrier National Breed Council also fundraised in order to get samples sent to the states, via a frozen semen container for the professor to work with. Breeders can now send buccal swabs to the US for testing. Knowledge is power, and with selected breeding, breeders can eliminate the disease from their lines if they have any carriers.