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Weimaraner hypomyelination is an inherited, autosomal recessive defect that results in delayed myelination of the nerves of the central nervous system. The common name for this disorder is “shaking puppy syndrome” or “shaky puppy”. This term comes from the observation that these puppies show muscle tremors and have jerky movements at a young age. Affected puppies usually improve with age and may look completely normal as adults.
Affected Weimaraner puppies get better as they get older and most survive and live healthy lives. Studies in Weimaraners so far have suggested that the mutation involves a single autosomal gene (ie, a different genetic mutation). The defect is recessive, requiring inheritance of the defect from both parents for manifestation of clinical signs.
The exact cause of the disorder in Weimaraner puppies is not yet known. What is known is that myelin formation is decreased in the central nervous system in affected Weimaraner puppies, but not in peripheral nerves. Myelin is made by cells called oligodendrocytes in the central nervous system, whereas different cells (Schwann cells) make myelin in the peripheral nervous system. It is likely that Weimaraner hypomyelination is caused by abnormal oligodendrocyte development or function. It has been shown that oligodendrocytes are decreased in number in affected puppies, and it has been suggested that this is caused either by oligodendrocyte dysfunction or by hypertrophy of the other cells of the central nervous system, the astrocytes. It has been suggested that “recovery” is caused either by compensation (and extra myelin production) by the oligodendrocytes that do exist, or by eventual development of more oligodendrocytes. Studies of affected Weimaraner puppies are few, but studies of older, “recovered” shakey pups are non-existent. A single study reported in two affected adult Chow Chow dogs that had improved showed an increased number of myelinated nerves, but there were still many hypomyelinated nerve axons present.
Weimaraner puppies with this disorder appear completely normal at birth. They usually begin to show signs at 1-2 weeks of age, although both earlier and later onsets have been reported. Affected puppies begin to show tremors, particularly in the hind legs, around the time that puppies first begin to move around the whelping box. The tremors cause the puppies to bob up and down as they try to move, looking like “low rider” cars. More severely affected puppies may have trouble nursing due to their tremors. From the time signs are first noticed, affected puppies tend to get worse before they get better. Affected pups may need help as they are started on weaning formula, because the shaking tends to cause them to move backwards, away from the food dish. They need to be fed alone, away from competition and shoving by the other puppies. Steadying their rear ends as they eat, with a hand supporting the crotch, is helpful. When urinating and defecating, they also tend to shake backwards into their own messes, requiring frequent cleaning. Affected puppies should be kept on soft surfaces (eg carpet instead of concrete) so that they do not get rub spots or sores on their feet or legs. They have difficulty moving forward on the slippery surface of newspaper; rubber matting gives better traction and can be hosed or mopped.
The good news is that many of these puppies do eventually improve, becoming essentially normal at 4-6 months, and can lead happy healthy lives. Some pups are improved enough by 8 weeks to be able to go to their new homes on time. While others require a little more time. In general, residual signs (if any) apparent at 1 year of age will probably remain. Many of these dogs appear completely normal to the untrained eye. However, if watched closely, they may continue to exhibit a mild tremor in the hind legs, particularly when excited or after hard physical exertion. Affected pups make good pets, but also are quite capable of pursuing athletic endeavors, including obedience, tracking, agility and hunting.
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