Originally written in 2005 for the Rat Society of America
Avascular necrosis of the tail, commonly referred to as ‘ringtail,’ is a series of annular constrictions caused by low environmental humidity. Because the condition is so photogenic, ringtail is often described in veterinary textbooks. The condition is also reported in mice and deer mice.
At first, the tail and distal extremities develop dry dermatitis, with erythema and inflammation, which progresses to edema and annular constrictions. Severely affected animals can develop dry gangrene, necrosis, and sloughing of the tail due to the periodic constrictions and annulations. If the tail is sloughed, the stump usually heals without any complications. In most cases, the animal is simply left with a few permanent annular constrictions on the tail. The toes may also be affected.
Low humidity appears to result in an aberrant response of the temperature-regulating vessels in the tail or the neonatal or weanling rat. Ringtail is seen in rats when circular constrictions of the skin covering the tail occur stopping the blood supply and causing skin sloughing. A detailed histological study may serve to shed light on the etiopathogenesis of this disease. Microscopically, the rings are eroded or ulcerated, with acanthotic and hyperkeratotic epidermis with a marked dermatitis.
Ringtail is most commonly seen in young, pre-weaned rats around 7-15 days old. The cause of ringtail is commonly attributed to low humidity of 25% or less, and has been caused experimentally by reducing the humidity. Since it’s due to a drop in humidity and higher environmental temperatures, ringtail is most commonly seen in the winter when heating systems are in use. However, the causes of avascular necrosis of the tail could actually be multifactorial. Genetics, elevated environmental temperatures, low relative humidity, nutritional deficiency (low essential fatty acids) in lactating dams, dehydration, and iatrogenic trauma from blood sample collection have all been implicated in the development of this condition. Other predisposing factos are excessive ventilation, hygroscopic (over-absorbant) bedding, and housing in hanging wire cages.
Treatment is palliative. Recognition of the signs and an understanding of husbandry practices are important in the diagnosis and treatment of the condition. Ringtail can be controlled by maintaining a relative humidity of over 50% (ideally 70%) and by housing in solid-bottom cages with adequate bedding. Hygroscopic bedding should be avoided. The air flow should be reduced to slow air turnover rate. If the condition has been allowed to continue to severe constrictions, treatment also includes amputating the tail proximal to the necrotic annular constriction.
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