710 RADIATION BIOLOGY 



Finally, to complete the list of primary photosensitivities and in defer- 

 ence to the contributions by experimental animals to our knowledge of 

 the subject, mention must be made of photodynamic action so frequently 

 induced in such animals by injection of fluorescent substances. 



DISEASES OF TYPE II 



The only photosensitivity of this type among diseases of animals is 

 that accompanying congenital porphyria in cattle reported from South 

 Africa by Fourie (1936) and Fourie and Rimington (1938). There has 

 been much discussion, which will be referred to in a later section, as to 

 whether skin lesions seen in some human cases of congenital porphyria 

 are due to photodynamic action by porphyrins, and Blum (1941a), with 

 an impressive array of evidence, concluded that there was no experi- 

 mental basis for this belief. The description given by Fourie of the skin 

 lesions in the congenital porphyria of cattle leaves little doubt that these 

 lesions were due to photosensitization, but it is to be hoped that use will 

 be made of the cattle remaining at the Onderstepoort Veterinary Labo- 

 ratory for action-spectra studies or other experiments on this aspect of 



the disease. 



DISEASES OF TYPE III 



Included in this group are most of the economically important live- 

 stock diseases in which photosensitivity has been reported. The criterion 

 adopted for classification under this type is the occurrence of bilirubinemia 

 (generally indicated by clinical icterus) or liver lesions, since study of the 

 causal relation between hepatic damage and photosensitivity, or proof 

 that phylloer3^thrin is the photodynamic agent, has been attempted in 

 very few cases. In two diseases (Mathews, 1938; Steyn, 1943) it has 

 been suggested that a pigment other than phylloerythrin is responsible, 

 but in neither case is the evidence convincing. Although this possibility 

 is admitted, the assumption that photosensitivity associated with liver 

 dysfunction is due to deranged phylloerythrin excretion is considered 

 justifiable, where evidence to the contrary is lacking, provided that the 

 animal is consuming chlorophyll in its diet. A plea is entered at this 

 stage for the examination of the blood for phylloerythrin and bile pig- 

 ments in all investigations of photosensitivity in farm animals. Since 

 the photodynamic aspects are similar throughout, the following discussion 

 deals with only a few examples of this type selected to illustrate the 

 general features of these diseases. Details such as the nature of liver 

 lesions and the influence of climate and management on these diseases 

 do not fall within the scope of this review. 



Geeldikkop {Yellow Thick Head). This disease of sheep in South Africa 

 was the first of the hepatogenous type to be carefully studied and may be 

 taken as representative of this class. It is characterized by severe photo- 

 sensitivity, icterus, and liver damage in sheep grazing the plant Tribulus 



