URTICARIA— NETTLE RASH 275 



URTICARIA. NETTLE RASH. 



Defmition. —Urticaria is an eruption of the skin appearing 

 in the form of rounded, flattened elevations, which appear 

 and disappear suddenly, due to an infiltration of the papillary 

 bodies and the Malpighian layer of the skin. 



Etiology. — Urticaria is common in horses, cattle and 

 swine. The causes are in part external and in part internal. 

 Urticaria is often secondary to infectious diseases. 



External Causes. — The external causes are irritant sub- 

 stances, such as insect stings, chemical agents (turpentine), 

 nettles, and rubbing and scratching the skin. When the 

 body is covered with sweat and suddenly cooled as by a 

 -shower of rain urticaria often develops in horses. 



Internal Causes. — Urticaria is very often a sjTnptom of 

 digestive disturbances, gastro-intestinal catarrh, constipa- 

 tion, and so-called indigestion, or of general infectious dis- 

 eases. There are probably formed in these cases in the fer- 

 menting ingesta toxins which are absorbed and irritate the 

 peripheral bloodvessels. Certain foods like rye, potato tops, 

 and wheat are causes. The toxins of specific microorganisms 

 also are causative, viz., doiu-ine, swine erysipelas ("diamond 

 disease"), purpura, influenza, and strangles. Urticaria also 

 occurs after injections with tuberculin or mallein. Cows 

 which have gone long unmilked sometimes suffer from it. 

 In certain individuals there is an idiosyncrasy existing favor- 

 ing urticaria when certain foods are eaten. In some instances 

 no cause can be determined. 



Symptoms. — ^Usually without any prodromal symptoms 

 there suddenly appear upon the skin of the neck, shoulder, 

 sides of the chest, and croup, swellings or nodes flattened on 

 top and varying in size from a pea to a clenched fist or by 

 confluence even larger. The hairs over the swellings are dry 

 and stand erect; the skin between is normal. In some 

 instances vesicles appear on the swellings which erupt, causing 

 them to be moist or covered with dry exudate. In other 

 instances the centers of the nodes sink, leaving a wall-like 

 periphery (U. annularis). By confluence grotesque figures 

 may appear. 



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