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Tapetal Degeneration
The tapetum is part of the choroid (a thin layer of the eye between the retina and sclera containing many blood vessels). Lack of tapetum is not an abnormality. It occurs especially in dogs with white or dilute hair color such as Dalmatian dogs, white cats, some merle collies, and some small breed dogs. The Chihuahuas in particular have very small tapetal areas.
Tapetal degeneration and ectasia syndrome are genetic conditions of the retina. Beagles have this abnormality which appears to be heritable. The initial lesions are oval areas of tapetal hyperreflectivity. As the condition progresses, a second lesion develops. The two areas eventually join to form a thin hyperreflective band. Degeneration of the entire tapetal retina may occur.
Forms of Tapetal Degeneration
Nutrition-induced Tapetal Degeneration
Zinc, a trace element that influences cell metabolism through a variety of mechanisms, appears to play an integral role in maintaining normal eye function. This element is present in high concentrations in eye tissue, particularly in retina and choroid. Zinc deficiency has been shown in a number of species to result in a variety of eye abnormalities. Zinc is believed to interact with taurine and vitamin A, regulate the light-rhodopsin reaction, and serve as an antioxidant. Both taurine and zinc are localized on the periphery of the tapetal rods and they contribute to the stability of the membrane. Cats fed on taurine-depleted dog food often develop this condition.
Chemical-induced Tapetal Degeneration
Zinc pyrithione (ZPT) is an antibacterial and antifungal agent that is often utilized in antidandruff shampoos with a composition level up to 1% in the formulation. It has some adverse effects to human and animal if ingested. In some experimental studies the administration of ZPT to cats resulted in ocular lesions characterized by tapetal degeneration and atrophy. ZPT also caused ocular lesions in Beagle dogs. The initial lesions consisted of a degeneration of tapetal cells, soon accompanied by intense tapetal inflammation which progressed to inflammation of the retina (retinitis), and bleeding, resulting in retinal detachment and blindness.3
Hereditary Tapetal Degeneration
An autosomal, recessively hereditary tapetal degeneration has been described in Beagles. Normal number of tapetal cells are present at birth. With time, however, progressive degeneration of the tapetal cell layer occurs.
References:
1. Comparative Ophthalmology Notes: Chapter 12 - Retina, Optic Nerve, Vitreous Body [106K] Feb 2002 (www.vetmed.ucdavis.edu)
2. Bruce H. Grahn, DVM, Phyllis G. Paterson, PhD, Katherine T. Gottschall-Pass, PhD and Zhen Zhang, MSc. Zinc and the Eye. Journal of the American College of Nutrition, Vol. 20, No. 2, 106-118 (2001), American College of Nutrition
3. Ying Shih, Jyh-Myng Zen, Annamalai Senthil Kumarb and Pei-Yen Chen. Flow injection analysis of zinc pyrithione in hair care products on a cobalt phthalocyanine modified 'screen-printed carbon electrode. Talanta Volume 62, Issue 5, 19 April 2004, Pages 912-'917
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Canine Eye Diseases and Vision Disorders
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