The visual system is an important sense available to the
canine. The eyeball functions to gather light and transmit
it into electrical impulses which are interpreted by the
brain to form images. The eye is protected by being sealed
in a bony eye socket supported by lubricating tissues,
muscles, and eyelids.
eyeball itself is formed by layers of tissue (see Figures 1
& 2). The white area called the sclera is made of tough
fibers rich in blood vessels which transport oxygen and
nutrients to the eye. The clear portion of the eye is the
cornea and it is made of layers of cells arranged in a
unique fashion so as to be transparent. The cornea lets the
light enter in to the eye. Inside the eyeball are
specialized organs bathed in a liquid called vitreous fluid,
which keeps the eyeball "inflated" and supplies nutrients to
the structure within the eye.
In the interior of the ball is
the colored portion called the iris. As in humans, dogs have
different colored eyes and this is determined by the color
of the iris. In the center of the iris is an opening called
the pupil. This opening can be made larger or smaller as
muscles called ciliary bodies ( which are attached to the
colored iris ) expand or contract. In dim light, the pupil
is made larger so as to let more light enter the eye.
Conversely, in bright light the pupil becomes smaller. This
is important as too much can cause pain and/or damage to the
eye just as in humans.
the pupil lies the lens which is a pea sized organ that is
normally clear. The lens bends, concentrates and focuses the
light so it will land on the rear area of the eyeball called
the retina. The retina contains nerve cells called rods and
cones. The rods are sensitive to light and the cones are
sensitive to color. Unlike humans, the canine possesses very
few cones and that is why domestic dogs are thought to be
color blind. They see only shadow of gray, black, and white.
Dogs do, however, have many rods and other reflective cells
which enable them to see in very dim light. The nerve cells
within the retina transform the light into nerve impulses
which leave the eyeball by the optic nerve and enter the
brain. The brain translates the impulses into images,
Several eye disorders have been identified in the Jack
Russell Terrier. This article touches on only some of the
ones that have come to attention through recurrence. They
vary in severity from those that progress with a few, subtle
signs , like PRA ( Progressive Retinal Atrophy ), to those
that strike like a thunderclap, like PLL ( Primary Lens
Luxation ), when the dog is fine Thursday and blind on
Friday. With the help of the general membership in testing
of their own dogs, more information can be gathered in the
quest to identify and eliminate these problems.
A cataract is defined as a loss of the normal
transparency of the lens of the eye. Any spot on the lens
that is opaque, regardless of size, is considered a
cataract. They may affect one or both eyes. Some are clearly
visible to the naked eye, appearing as white or bluish dots.
Treatment consists of surgical removal of the lens (
cataract extraction ). This operation is usually recommended
for the dog who has so much visual impairment that it has
trouble getting around.
The prudent approach is to assume cataracts to be
hereditary. However, cataracts may result from injuries to
the eye, exposure to great heat, radiation, diabetes, and
old age (senile). Cataracts can also occur from ocular
inflammation, specific metabolic diseases, persistent
pupillary membrane ( PPM ), or nutritional deficiencies.
Breeding of affected animals is not recommended.
Distichiasis is a condition in which small hair
structures abnormally grow on the inner surface of the
eyelids ( see diagram ).
upper and lower lids may be involved. The abnormal hairs
growing on the inner surface of the lids cause irritation to
the cornea. The affected eye will become red, inflamed, and
may develop a discharge. The dog will squint or blink very
often, much like a person when a bug or other foreign matter
enters the eye. In severe cases, the cornea may become
ulcerated and appear bluish in color. Left untreated, severe
corneal ulcerations and infections can develop. The hairs
can cause severe irritation and without treatment will
usually worsen. Blindness can result if infections do
Treatment involves the removal of the hairs through the
use of surgery or electro-epilation. With electro-epilation,
a fine needle is passed into the hair follicle and the
follicle is burned to destroy the hair and its roots. If
surgery is performed, the lid is actually split and the
areas with abnormal hairs are removed. Antibiotic eye drops
will also be used with the surgery to eliminate infections.
This is a serious eye problem. Usually it leads to
partial or total blindness. It is due to an increase of
fluid pressure within
eyeball caused by an interruption of fluid exchange between
the eyeball and the venous circulation. The fluid that
develops the excessive pressure associated with glaucoma is
the aqueous humor, the liquid found in front of the lens. It
is produced by the ciliary body (1), flows past the lens
(2), through the pupil (3), across the inner surface of the
cornea and front side of the iris (4), and then drains from
the eye at the angle ( commonly referred to as the drainage
or iridocorneal angle ) formed where the cornea meets the
iris (5). The cornea and the lens are living tissues but
they have no blood vessels going to them to supply the
needed oxygen and nutrition. These life sustaining materials
are brought to them through the aqueous humor. In glaucoma,
elevations in the pressure of the aqueous humor are most
frequently caused by this fluid not being able to drain
correctly from the eye. When the eye pressure is greater
than the arterial blood pressure, arterial blood cannot
enter the eye to nourish the retina. A sudden build up of
pressure leads to acute blindness and damage to the optic
nerve. A slower build up causes few symptoms, yet leads to
the same results.
Primary (or congenital) glaucoma occurs without prior
disease. It occurs in a dog because it possesses physical or
physiologic traits that predispose it to glaucoma. These are
usually predetermined by genetics. Secondary glaucoma means
that the disease is secondary to, or caused by, another
condition. A common example is a penetrating wound to the
eye. Other causes would include bleeding in the eye,
inflammation within the eye, attachments or scarring between
the iris and lens, and luxation or displacement of the lens.
Lens luxation can cause glaucoma and likewise chronic
glaucoma can cause lens luxation. The early sings of
glaucoma that an owner may watch for or notice are pain, a
dilated pupil, cloudiness within the cornea and /or an
increase in the size of the blood vessels in the white
portion of the eye. The pain may present itself with the dog
rubbing his eye with his paw, against the furniture or
carpet, or your leg. This is a common, and often unnoticed,
early sign. Some dogs will also seem to flutter the lids or
squint with one eye. The pupil of the affected eye will
usually dilate early in the course of the condition. It may
still react to a bright light shining in it, but it will do
so very slowly. Remember that glaucoma, even primary
glaucoma, is usually going to initially affect just one of
the eyes. If the pupil in one eye is larger than the other,
something is definitely wrong.
Just so there is no misunderstanding, if treatment in the
dog is not started to combat glaucoma in a few days or, in
some cases a few hours, vision will probably be lost
completely from the affected eye. The pressure can crush the
cells of the retina and optic nerve, rendering them
nonfunctional. It can break down the structures holding the
lens in place and it can cause damage to the iris and
cornea. After these internal changes have occurred, the
eyeball itself swells in size, tilts off to the side, and
all the surface blood vessels enlarge giving it the
appearance of a large, ugly, bruised radish.
Measurement of the intraocular ( inner eye ) pressure
and interior inspection of the eye is needed for diagnosis.
Chronic glaucoma can be managed for a time with drops and
Breeding of affected animals is not recommended.
Luxated (displaced) lens occurs when the zonula (ligament
fiber) which holds the lens in place deteriorate allowing
the lens to fall out of its normal site behind the pupil.
Subluxation is the partial separation of the lens position
and is often times an indication of eventual total luxation.
When the lens luxates to the posterior (rear) chamber of the
eye, the eye will appear normal, but if it luxates forward
(anterior), the lens will rub against and irritate the
cornea, causing tearing and a bluish cast over the eye.
Anterior lens displacement is the most hazardous form of the
displacement. It has a high probability of causing glaucoma,
and if the lens touches the cornea it will cause damage
leading to cornea edema. This must be attended to
immediately as the lens now restricts the flow of ocular
fluids, creating eye pressure and great discomfort. Before
luxation is apparent, you may notice behavior changes in the
dog; not catching a biscuit tossed to him, bumping into
stationary objects, missing the first step in a staircase.
Treatment varies due to the severity of the disorder.
Surgical removal of the lens will alleviate pain and allow
partial vision. This surgery is expensive and not always
shown to be effective. Sometimes a combination of eye drops
and oral medication is helpful. In severe cases, removal of
the eye is necessary.
Primary lens luxation is assumed to be autosomal
recessive. That is, each parent must at least be a carrier
of the disorder. If a dog is affected, certain facts are
known; both parents are at least carriers and every
offspring of the affected dog is a carrier. Affected dogs
should not be bred and known carriers should be pulled from
the breeding program. This is an insidious disorder as it
appears anywhere between 3-8 years of age, well into the
breeding program. With primary lens luxation, at first only
one eye is affected but it is only a matter of time before
the other eye follows suit. The time can vary from several
weeks to several years, but it will follow. Secondary lens
luxation is associated to trauma to the eye, such as
puncture or injury, and is not hereditary.
Persistent pupillary membranes (PPM) are blood vessel
remnants in the anterior (forward) chamber of the eye which
fail to regress normally in the neonatal period. These
strands may bridge from iris to iris, iris to cornea, iris
to lens, or form a sheet of tissue in the anterior chamber.
The last 3 forms pose the greatest threat to vision and when
severe, vision impairment or blindness may occur. The
membranes appear to be white, gray, or pigmented, and do not
usually involve >25% of the corneal surface. There is no
effective treatment for PPM.
Progressive retinal degeneration ( PRD ) is also known
as progressive retinal atrophy ( PRA ) and refers to retinal
diseases that cause blindness. Some breeds have blindness by
abnormal development of the retina and this is dysplasia.
Other breeds have a slowly progressive degeneration or death
of the retinal tissue and this is degeneration.
As the name PRD implies, a slow death of the retinal
tissue occurs. It is a slowly progressive disease and the
earliest signs may be overlooked. The first indication of a
problem is the loss of night vision. The dog hesitates to go
out at night. He won't jump on or off furniture in a
darkened room. Later he will go up, but not down, stairs. A
loss of day vision will follow.
The veterinary ophthalmologist examines the retina with
an instrument called an indirect ophthalmoscope. Changes in
the retinal blood vessel pattern, the optic nerve head, and
the reflective substance within the dog's eye called the
tapetum can be seen which are classic for PRD. However, in
some breeds PRD characteristics have little or no early
changes. The eyes of these dogs may appear normal until they
are in the later stages of the disease. Progressive retinal
degeneration will progress at different rates in different
breeds. This variation causes difficulty in determining just
how long any particular dog will continue seeing. Cataracts
may occur in some patients with PRD and generally occur
later in the disease. Formation of cataracts may interfere
with the ophthalmologist's direct examination of the retina
and make other tests such as an electroretinogram (ERG)
essential for diagnosis. The ERG is sensitive enough to
diagnose dogs with PRD before they begin to demonstrate
signs of the disease.
In summary, PRD refers to a broad group of inherited
retinal disease which result in the blindness of dogs.
Because of the nature of the disease and sometimes late
onset, repeated examinations may be required to detect
individuals with the condition. Patients affected should not
be used for breeding.
PRA has been shown to be autosomal recessive in the
poodle, Irish setter, Norwegian elkhound, and Samoyed.
Inheritance should be assumed in other breeds. The recessive
nature makes this disease extremely difficult to eliminate
from affected bloodlines. There is no treatment for the
In closing, let me leave you with some facts and
figures from CERF, the Canine Eye Registration Foundation;
from January 1991 to May 1996, a grand total of 364 Jack
Russell Terriers were given eye examinations by board
certified canine ophthalmologists. Of those, 80% were eye
normal and 20% had some form of eye disorder. Of those with
eye disorders, 50% were so severe as to be rejected for
registration by CERF.
Now look at the total membership of the JRTCA and the
number of registered dogs over that same period of time:
probably close to 5000 dogs. Yet only 364 were examined by
CERF. If we, as a club, are ever going to get a grip on the
disorders, of all kinds, that affect our breed, we must
start testing our dogs for everything we can: eyes, hearing,
orthopedics, etc. This can't be stressed enough. It's up to
us, the breeders, to make our breed as healthy and sound as
we possibly can. Only we can do it. And if you profess your
love of the breed, your desire to see it flourish, you must
let your actions speak for you. Do the best you can, test
your dogs, and pull those that fail. Breed only the best. If
we all work together, we can make the Jack Russell Terrier a
strong, healthy, defect free breed.