Types of Photosensitization

Photosensitization is typically classified according to the source of the photodynamic agent.

Ø  Type I/ Primary Photosensitization,

Ø  Type II/ Congenital erythropoietic porphyria,

Ø  Type III/ Secondary/ Hepatotoxic/photosensitization and

Ø  Type IV/ Idiopathic photosensitization. 

Type I/ Primary photosensitization

It occurs when photodynamic agents are ingested, injected or absorbed and enter the circulation. Examples of primary photosensitizing agents include hypericin (St. John's wort), fagopyrin (buckwheat), Bishop's weed, clovers, alfalfa, furocoumarins, (plants of Umbelliferae and Rutaceae families), aromatic hydrocarbons, tetracyclines, some sulfonamides, phenothiazine anthelmintic that cause photosensitization in livestock and poultry.

Type II/ Congenital erythropoietic porphyria

It occurs due to defect in the normal synthesis of the heme pigment. Enzyme deficiency in heme synthesis leads to over production of various porphyrins in blood.

•      Porphyrin can accumulate in bone and teeth and the condition is called ‘osteohemochromatosis/ Pink teeth’ in livestock.

•      It can also accumulate in lung, kidney, erythrocyte and even it can pass through urine causing amber-brown color (porphyrinuria). 

Type III/ Secondary/ Hepatoxic photosensitization

Secondary or type III photosensitization is the most frequent type of photosensitivity observed in livestock. The photosensitizing agent, phylloerythrin (a porphyrin), accumulates in plasma because of impaired bile excretion.

Phylloerythrin is derived from the breakdown of chlorophyll by microorganisms present in the GI tract. Phylloerythrin, but not chlorophyll, is normally absorbed into the circulation and is effectively excreted by the liver into the bile. Failure to excrete phylloerythrin due to hepatic dysfunction or bile duct lesions increases the amount in the circulation. Thus, when it reaches the skin, it initiates a photosensitivity reaction.

Type IV/ Idiopathic photosensitization

Photosensitivity where the pathogenesis is unknown or the photodynamic agent is not identified is classified as type IV/ idiopathic photosensitization.