Merle Associated Diseases Merle Ocular Dysgenesis and Pigment Associated Deafness
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
Breeding advice appliesUse the condition’s screening guidance and veterinary or genetics advice before breeding an affected or at-risk animal.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
The merle coat pattern is produced by a genetic variant that alters pigment distribution. Dogs with one merle allele can be healthy, but inheriting two merle alleles substantially increases the risk of developmental abnormalities involving the eyes and ears. These dogs are commonly called double merles. The amount of white coat is not a perfect indicator, because some genetically double-merle dogs can have less obvious external colouring.
Merle ocular dysgenesis is a spectrum of eye abnormalities that can include unusually small eyes, abnormal pupil shape, iris or optic-nerve coloboma, cataract, lens displacement, retinal dysplasia, retinal detachment, persistent embryonic tissues and abnormal development of the reflective tapetum. Severity ranges from minor structural changes with little effect on vision to severe malformation and blindness.
Pigment-associated deafness occurs because melanocytes are required for normal development and function of the inner ear. When pigment cells are absent from critical cochlear structures, hearing can be lost in one or both ears. White pigmentation around the head and ears is associated with increased risk but does not predict the hearing status of an individual puppy accurately.
These problems are developmental rather than acquired later from ordinary coat colour. Affected puppies are born with the underlying abnormal structures, although hearing loss becomes apparent during early development. Eye and hearing problems can occur together or separately.
The most important preventive principle is avoiding matings that can produce puppies with two merle alleles.
Risk is greatest in dogs inheriting two copies of the merle variant, because reduced pigment development can affect structures essential for normal hearing and eye formation as well as coat colour.
The central preventive rule is to avoid producing puppies with two merle alleles. Behavioural response to sound can miss unilateral deafness because the puppy responds through its normal ear.
Screening & Early Detection
Before breeding, merle genotype should be established accurately, particularly when coat pattern is subtle, atypical or potentially cryptic. Genetic testing is especially useful where a dog has an atypical or cryptic pattern that is difficult to identify reliably from appearance alone.
Puppies at risk should have a veterinary ophthalmic examination during early life, commonly around six to eight weeks, to identify structural abnormalities such as microphthalmia, coloboma, cataract or retinal dysplasia. Some subtle defects require specialist examination and should not be assessed only by casual visual inspection.
BAER testing is the definitive method for evaluating hearing in each ear separately. BAER therefore provides much more accurate information for both pet placement and breeding records.
A DNA result showing double-merle status does not determine exactly which abnormalities a puppy will have, so clinical eye and hearing assessment remains necessary. Conversely, a puppy with apparently normal vision and hearing should not be assumed genetically single merle without testing when parentage makes double merle possible.
Screening is most effective when performed before breeding rather than after affected puppies have been produced.
Visual coat assessment alone is not always sufficient because cryptic or atypical merle patterns can be missed. Genetic testing, eye examination and objective hearing testing therefore play complementary roles in responsible breeding and health assessment.
Treatment & Management
Developmental eye and inner-ear abnormalities associated with double-merle genetics cannot be made normal after birth, so treatment is tailored to the functional problems each dog has. Management is therefore directed at the specific problems present in the individual dog.
Dogs with congenital deafness are trained using visual signals, tactile cues and secure environmental management. Outdoor activity should be on lead or within safe fencing because the dog cannot hear traffic or verbal recall. Unilateral deafness may require little day-to-day adaptation, although safety training remains important because the dog cannot reliably localise sound from both sides.
Eye treatment depends on the abnormality. Cataracts may be surgically treatable when the rest of the eye is functional. Painful glaucoma, lens luxation or chronic inflammation requires ophthalmic care. A severely malformed blind eye may occasionally need surgery for comfort even though vision cannot be restored. Dogs with stable, non-painful visual defects generally need environmental adaptation rather than medication.
Partially sighted or blind dogs benefit from a consistent home layout, barriers around hazards and verbal or tactile routines. Many adapt very well when their surroundings are predictable.
Management is tailored to the individual deficits. Deaf dogs benefit from visual training and secure environments, while dogs with malformed or painful eyes may need ophthalmic treatment or surgery for secondary problems. Regular review is important because comfort and functional vision can differ markedly between the two eyes.
Breeds or species affected
American pit bull terrier
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American Staffordshire terrier
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Australian shepherd
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Border Collie
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Cardigan welsh corgi
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Catahoula leopard dog
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Chihuahua
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Cocker spaniel
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Collie
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Collie (rough & smooth)
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Coolie (german collie, koolie)
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Dachshund
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Great Dane
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Old English sheepdog
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Pomeranian
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Shetland sheepdog
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Welsh corgi
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