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Irish Red and White Setter
Irish (Red) Setter
Gordon Setter
Shineon
Kennels
flourishingfairies@globalzone.co.za
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PPC
Dogs affected with posterior subcapsular
triangular cataract rarely, if ever, become blind. This is the form of
Posterior Polar Cataract most commonly found in Setters.
Affected dogs should not be used for breeding. The
parents should be given a physical eye examination by a registered
opthalmologist to determine which of them is involved in the transmission of
the disease. This is because the disease is thought to be inheritable. There
are numerous instances in which the cataract can only be detected at around 6
years or more. Although it can be argued that PPC does not result in blindness,
the breeding of two PPC-affected dogs has been known to produce blind puppies.
One of the reasons for this is that often PPC is a cataract that is present
secondary to another eye disorder, such as PRA – which is recessively inherited
and does lead to blindness.
It has been noted that some
Gordon Setters affected by Posterior Polar Cataracts (PPC) have also been
affected by Progressive Retinal Atrophy (PRA). Thus, it could be
concluded that the PPC is sometimes a secondary condition, and could be an
indicator that PRA has the potential to develop in certain lines. PPC seems to
be inheritable in two different modes of
transmission.
Dominant
(Puppies receive one gene from each parent at fertilisation that combine and form the double strand found in DNA).
Dominant
gene expression means that only one of the two genes in a combined pair must be
affected in order for the dog to develop the characteristic controlled by that
gene. In a true dominant gene expression, only one parent need carry it in
order for the offspring to be affected. Unfortunately, it is possible to have a
dominant gene with incomplete penetration; i.e., passing on of the characteristic
to offspring will result in the presence of the gene, without that gene showing
expression. It is also possible to be incompletely dominant; i.e., the
phenotype differs between heterozygote (carrying one affected and one
unaffected gene in a combined pair) and homozygote (carrying two affected genes
in a combined pair) with respect to the gene responsible for PPC to express
itself.
One type
of cataract due to a dominant gene is a Posterior Polar Cataract appearing as
an inverted Y or triangle, or irregular circle of opacity, at the pole, and not
associated with the hyaloid artery. These cataracts show little tendency to
progress until senile changes occur in the senior dog. These are usually, but
not always, bilateral and symmetrical. Typically, dogs show defective vision at
1-3 years of age. Congenital cataracts often reveal a posterior capsular
deformity.
Recessive
(Puppies receive one gene from each parent
at fertilisation that combine and form the double strand found in DNA).
The recessive mode of inheritance requires both genes to be present in order for a characteristic to be expressed in an individual; i.e., both parents must carry the gene in order for the offspring to be affected.
The
recessive type cataracts are usually bilateral, symmetrical, and progressive.
This type is observed earlier, sometimes as early as a few months of age, and
progress until total, maturing at 2-3 years of age.
Large
differences in the age of onset and mode of progression of cataracts is found,
especially in different breeds, and some inheritable cataracts don't become
apparent until middle age.
The two
most common modes of recessive inheritance of cataracts are autosomal recessive
and polygenic. For autosomal recessive traits to become expressed, both
parents must each pass an affected gene to the offspring.
Polygenic
traits are those in which many genetic factors
interact to determine the expression or lack of expression of a trait. Dogs
with autosomal recessive traits or genes, that do not express the condition,
are known as carrier animals.
The most
common type of cataracts in dogs apparently results from a simple autosomal
recessive method of inheritance. Autosomal recessive genes usually express
themselves at a younger age than does the dominant type. Where an autosomal
recessive gene was inherited from both parents, PPC may progress until the dog
is completely blind, usually between 2 and 5 years of age. Sometimes both the
recessive and dominant modalities are present in the same dog!
It is
possible for only one puppy in a litter to develop PPC, although both its
parents should be considered carriers, and thus not be bred from. All the
siblings should be tested before breeding, as they may be carriers or affected
and show late onset of symptoms.
A
secondary cataract, for example those occurring as a secondary condition to
progressive retinal atrophy (PRA), may begin as a cortical type, often
posterior with typical appearance of vacuoles (clear spaces) arranged in a
wedge-shaped area from the edge of the lens. These are progressive and mature
to total opacity, thus not allowing examination of the retina for the presence
of retinopathy (retinal disease) as light cannot penetrate the eye.
Breeding
is not recommended for any animal demonstrating partial or complete opacity of
the lens or its capsule unless the examining opthalmologist has also checked
the space for "significance of above cataract is unknown".
The most
practical approach is to assume cataracts to be hereditary. Exceptions are:
in cases
specifically known to be associated with trauma,
other causes
of occular inflammation such as having had cleaning chemicals in the eye,
specific
metabolic diseases like diabetes or pancreatic insufficiency,
persistent
pupillary membrane,
persistent
hyaloid,
nutritional
deficiencies; like a lack of rutin or Vitamin A (too much vitamin A can kill),
or arginine and tryptophan deficiencies
electric
shock,
radiation,
such as exposure to the ultraviolet radiation emitted by sunbeds
toxins, a
commonly used toxin, disophenol – used to treat hookworms, is often implicated
here
The
adenovirus preparation vaccine (CAV-1 or CAV-2) protects against hepatitis and
kennel cough. Clouding of the clear window in the eye may occur one to two
weeks after inoculation, in a minority of cases. This can sometimes be
misdiagnosed as a posterior cataract. The eye does not usually tend to become
clear again once this has occurred. Hepatitis vaccines form part of the DHL
injection. Annual booster shots are given.
It is
possible to use a PPC affected dog to a PPC unaffected one, as no puppies would
probably go blind, as long as both have been tested for PRA and found
clear.
This is
the prudent approach to take where the gene pool for a breed is small, such as
with the Irish Red and White Setter, and for which an effective DNA test for PRA
(rcd-1) exists.
Should
both the proposed Sire and Dam have PPC, it would probably result in some
puppies becoming blind as they mature, should the mating go ahead.
References:
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Ophthalmologic Diseases |
Disorders of the Lens, pp 1120-1121 |
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Abnormalities of the Companion Animals: Analysis of Heritability |
C W Foley, J F Lasley, G D Osweiler, |
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Common Congenital and Heritable Ocular Disorders of the Dog |
Paul H Scherlie, Jr., DVM, and Stephanie L. Smedes, DVM, Cornell
University, Kal Kan Foods, Inc., 1989 |
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Ocular Disorders Proven or Suspected to be Hereditary in Dogs |
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Inherited Eye Diseases in Purebred Dogs |
Lionel F Rubin, DVM, |
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e-mail: flourishingfairies@globalzone.co.za
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