706 RADIATION BIOLOGY 



livestock diseases in which photosensitivity is a consequence of hver dys- 

 function (Rimington and Quin, 1934; Clare, 1944). Certain precautions, 

 however, are necessary in such an approach. The animals must be photo- 

 sensitive at the time the sample is taken; some attempt should be made 

 to relate the amount present in the blood to the amount needed to pro- 

 duce photosensitivity ; and failure to detect a pigment does not ehminate 

 it from consideration, since photosensitivity produced by injection of 

 phylloerythrin may still occur, presumably through deposition of sensi- 

 tizer in the skin, after the level in the blood has fallen below the detectable 

 limit (Clare, 1944). Furthermore, as Blum has emphasized, the appear- 

 ance of photodynamically active porphyrins in blood and urine of con- 

 genital porphyria and other conditions is inadequate grounds for the 

 popular belief that these porphyrins are responsible for the sensitivity to 

 light and skin lesions as seen in hydroa aestivale. 



A further criterion that can be applied to distinguish between true 

 photodynamic action and other effects of light, such as sunburn, is based 

 on the requirements of oxygen in the former. By this means it has been 

 shown that the dermatitis that affects some persons after treatment with 

 sulfanilamide (Blum, 1941b) and the condition known as "urticaria 

 solare" (Blum ct al., 1935) are not due to photodynamic action. 



PHOTOSENSITIVITY IN ANIMALS 



SYNDROME OF PHOTOSENSITIZATION 



The symptoms and clinical signs observed in photosensitization do not 

 depend upon the nature of the agent or the wave length of the light 

 (provided the sunburn radiation is excluded), but they do vary with the 

 intensity of the photodynamic action. Erythema accompanied by pru- 

 ritus is commonly the first observable effect and may appear within a 

 few minutes of the start of insolation. If the photoreaction is sufficiently 

 strong, edema of the exposed area follows within a few hours, even if the 

 skin has meanwhile been protected from exposure, and exudation of this 

 edema through the skin may occur. Gravitation of the edema may 

 result in swelling of areas that have not been exposed, such as the sub- 

 mandibular space and the legs. In animals such as sheep the ears swell 

 and droop in a characteristic attitude. Necrosis and sloughing of the 

 exposed skin may follow within a few days, the ears of animals being 

 again particularly affected in this way. In cases of older standing in 

 sheep, for example, the tips of the ears are often withered and curled up 

 in a distinctive manner, and wool growth over the exposed areas is 

 inhibited. Formation of suppurating sores in animals is, of course, a 

 consequence of secondary infection of the damaged tissue, but it has 

 occasionally given rise to an erroneous belief among workers with farm 

 animals that the disease may be transmitted by contact. 



