"Wasting" or "fading" syndromes are common causes of puppy and kitten mortality. Numerous infectious and toxic, metabolic, or nutritional factors could potentially be responsible for wasting and death in young animals. Evidence has been presented that infectious canine hepatitis virus
infection, beta-hemolytic streptococcus
infection, and feline infectious peritonitis virus
infection are responsible for a significant number of deaths due to
wasting syndrome. However, many cases of
wasting syndrome cannot be attributed to infectious agents or other specific etiologies. The thymus gland warrants special attention when one is evaluating an animal with a
wasting syndrome because it is known that, in some species, neonatal
thymectomy results in wasting and death. Unfortunately, most reports describing fading syndromes in puppies and kittens do not mention the gross or histologic appearance of the thymus gland at postmortem examination. When examining the thymus gland, one must keep in mind that the thymus may be hypoplastic owing to a congenital or genetic defect in its structure and function or it may be atrophic secondary to whatever is causing the fading syndrome. If a thorough history, clinical examination, and/or postmortem examination do not reveal a cause for the fading syndrome, then defective thymus function should be considered as a possible causative or contributing factor to the fading syndrome. In these cases,
therapy designed to replace or improve the defective thymus function should be considered. At least one form of
wasting syndrome in puppies (immunodeficient
dwarfism) has been found to respond to short-term
therapy with a thymus
hormone (
thymosin fraction 5) or with
bovine growth hormone (which is thymotropic) in limited clinical trials. It is possible that other forms of wasting or fading syndromes would also respond to
therapy with thymus
hormone or
growth hormone. Certain
thymus hormones (
thymopoietin pentapeptide,
thymosin alpha 1,
facteur thymique serique, and rabbit thymus
acetone powder) and
bovine growth hormone are commercially available. Before initiating
therapy, one should consider that if the cause of the
wasting syndrome is genetic, then successful treatment may perpetuate a genetic defect. More research (both basic and clinical) is needed to determine the role of thymus gland dysfunction in fading syndromes of puppies and kittens and if
therapy with one or several of the
thymus hormones or with
growth hormone could reverse the symptoms of wasting.