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According to Gelatt's Veterinary Ophthalmology
(1999), PRA is a term for a group of hereditary diseases that are characterized by
a degeneration or dysplasia (abnormal development) of the photoreceptors at the back
of the eye. The changes are progressive and result in a loss of vision. All forms of
PRA can be detected by trained Veterinary Ophthalmologists examining the back of the
eye (once the visible changes are present), and incidences of PRA are included in the
CERF statistics. A dog with PRA will not receive CERF certification. Electroretinograms
(ERG) can detect changes in the function of the retina earlier than CERF examination
can detect visible changes to the eye.
There are two main classifications of the disease
processes involved in PRA:
a primary effect on the photoreceptors (generalized PRA), abnormalities of the pigment epithelium behind the photoreceptors which
causes secondary photoreceptor degeneration (central PRA or, more recently, RPED).
Central PRA is much less widespread in
pure-bred dogs, apparently affecting Retrievers (Labrador and Golden) and Collie
breeds almost exclusively. The progression of the disease is slower than GPRA
and the dog may not become totally blind. This form of PRA has not, to date, been
recorded in Alaskan Malamutes.
Generalized PRA is more widespread among the breeds
and is a form of PRA that has been documented to affect the Alaskan Malamute in North
America and in Europe. In generalized PRA the disease process usually leads to complete
blindness. Generalized PRA is currently divided into two types of diseases:
Early onset (retinal dysplasias), Later onset (prcd - progressive rod-cone degeneration).
The early onset retinal dysplasias result from
abnormal development of the photoreceptors and breeds so affected can start to show
signs as early as 12 weeks of age. For later onset prcd, clinical signs (or even
eye changes) may not be seen until the dog is 4 years of age or older. Most
frequently night blindness is the first clinical sign, but then as the condition
progresses the dog shows decreased and, eventually, total loss of all vision. Owners
may also notice that the dog has dilated pupils and an increase in the shininess at
the back of the eye. Some dogs may also develop cataracts secondary to the PRA, seen
as an increased cloudiness in the eye.
Since the ophthalmologic and clinical signs of
Generalized PRA may not be detected until the dog is 4 years of age, or older, there
have been considerable efforts made to find the gene(s) responsible for this condition,
and with some success, in a number of breeds. Through genetic research it has been
found that there are several different types of prcd with different genes and/or
different mutations responsible in the different breeds. Thus a DNA test for PRA in
one breed may not be helpful for PRA in another, even though the condition looks the
same clinically. Since Malamutes have had a very small number of cases recorded, there
is currently no impetus to develop a DNA test for Malamutes.
Known incidence of PRA in
Malamutes:
North America:
CERF has recorded 2 cases of Generalized PRA in Malamutes (up
to end 1999),
Europe:
Two cases of Generalized PRA have been recorded in Malamutes under
the Swedish Kennel Club's official eye-testing scheme (up to end 2004).
The AMCA health survey conducted in 1996 by Jocelynn
Jacobs-Knoll, DVM, recorded 3 cases of PRA and 4 cases of "Retinal Atrophy". This survey
had respondents from USA, Canada and At Large (International). |
