Persisting
Pupillary Membrane (PPM), so this illness is called by breeders likewise,
is examined in Germany within the Basenji population, which are brought
to breed, since 1998 systematically. Responsible breeders already let
examine their dogs this point in time ago voluntarily. Presently it
can not be estimated that this will be a serious problem for the breed
in Germany, because not all breed-relevant dogs were examined. The result
for 77 Basenjis examined in Finland during 1985-1996 proved 43% of the
dogs as with PPM afflicted. In Norway in the last years 51 Basenjis
were examined on PPM with the result that 22 dogs (= 43%) had PPM. With
16 Basenjis thereby only minimum development could be determined. For
such a Basenji this illness does not have to lead inevitably to complications.
PPMs
are remnants of a fetal structure called the pupillary membrane. This
membrane covers the pupil before an animal is born. It is part of
the blood supply to the developing lens (the structure in the eye
that focuses light on the retina). Normally the pupillary membrane
completely absorbs before birth in foals and calves but is partially
present and continues to disappear in neonatal dogs. Absorption may
not be complete in puppies when the eyes first open and small strands
or a web-like structure may be seen across the pupil. These strands
normally disappear by four to five weeks of age. In some dogs these
strands do not disappear and become PPMs.
PPMs
may be found in several configurations in the anterior chamber (see
figure). They may span across the pupil (iris to iris), from the iris
to the lens, from the iris to the cornea, or they may float free on
one end, only connected to the iris. In general, iris to iris PPMs
cause no problems. They may be single strands or a forked structure.
These PPMs may break and become less prominent as the puppy gets older,
but they usually do not disappear completely. Iris to lens PPMs are
more problematical. These PPMs cause opacities (cataracts) at the
point where they are attached to the lens capsule. The cataracts do
not usually progress and cause only minor visual deficits. Iris to
cornea PPMs cause opacities on the cornea due to their ability to
damage the corneal endothelium (the inner lining of the cornea). These
opacities may be small or may be severe due to the development of
corneal edema (fluid in the cornea). Severely affected puppies (with
numerous strands) may be blind (they may improve as they get older).
The strands may regress but do not disappear.
PPMs
are found in many breeds of dog. In most of these breeds, iris to
iris PPMs are classified by CERF as a "breeder option" problem. This
means that most of the PPMs which have been reported in these breeds
have been small and are probably sporadically occurring and not hereditary
defects. Dogs with these small iris to iris PPMs who have been bred
have not been reported to have puppies with vision problems. This
does not mean that problems will never occur in these breeds. Owners
with dogs diagnosed with PPMs should be aware of the situation and
should probably either not breed affected dogs or should breed the
affected dogs only to unaffected dogs.
In
some breeds, PPMs are known to be hereditary and puppies who have
any type of PPM will not receive a certification number. The Basenji
is the most well known but CERF will also not certify Chow Chows,
Mastiffs, Pembroke Welsh Corgis, or Yorkshire Terriers with PPMs.
Members of these breeds have been shown to produce offspring with
blindness directly associated with their PPMs. In these breeds, the
mechanism of inheritance is not known but breeding any of these dogs
with PPMs is highly discouraged. In Germany the test results of the
veterinarians, who belong to the so called “Dortmunder Kreis (Gesellschaft
für Diagnostik genetisch bedingter Augenerkrankungen bei Tieren e.V.
= society for diagnostics of genetically conditioned eye illnesses
in animals e.V.), are the basis for breed appreciation and breed fitness
check for the pedigree dog breed associations in the VDH consulted.
Abbildung 1
-
One
end of PPM floating in anterior chamber - no clinical significance
- Iris
to lens - results in capsular cataract
- "Y"
shaped iris to iris band - no clinical significance
- Single
iris to iris band - no clinical significance
- Iris
to cornea - results in adherent leukoma of the cornea
