Photosensitivity in animals

Photosensitivity reactions, aka photosensitization, are abnormal skin reactions in animals when exposed to direct sunlight. It is unrelated to a sunburn. These reactions are due to the accumulation of photosensitive compounds beneath the skin. These compounds can be the metabolites of drugs or photodynamic substances seen in plants which gain entry to the body after ingestion and get accumulated in the system either unchanged or after metabolism inside the body. In some other cases, the photodynamic substances can be produced in the body itself due to some inborn errors of pigment metabolism, especially those involving the heme synthesis. Photosensitivity reactions are usually seen in herbivorous or omnivorous animals, though such reactions are not uncommon among carnivores.

Effects of photosensitivity
The photosensitivity reactions are characterized by the formation of severe inflammation of the skin with depigmentation and ulceration. In some cases, (Type I), there will be excretion of pigments, which may be fluorescent in nature, through urine or deposition of such pigments in hard tissues such as bones and teeth causing discolouration. The affected animals should be protected from direct sunlight, until the photodynamic compounds have been completely eliminated from the body or appropriate treatments have been given.

Biological mechanism of photosensitivity reactions
The photodynamic compound can be activated by sunlight and they are excited to a higher energy state. They immediately transfer their energy to the surrounding acceptor molecules like enzymes or free radicals. These molecules are activated and they will undertake the skin reactions.

Classification of Photosensitivity reactions
Based on the origin, the photosensitivity reactions have been classified into four types - Type I, Type II, Type III and Type IV.

Type I Photosensitivity
It occurs when the photodynamic compounds are directly accumulated under the skin of the body following ingestion. Such compounds need not undergo metabolism to get activated. The examples of plants producing Type I Photosensitivity are Buckwheat and Klamath Weed. Some drugs like phenothiazine or its derivatives can also lead to photosensitivity reactions, commonly in sheep. The phenothiazine is converted into phenothiazine sulfoxide, which is the photodynamic compound.

Type II Photosensitivity
It occurs due to inborn errors in the metabolism of some biological pigments. Due to absence of some key enzymes of metabolism, the products of intermediary metabolism get accumulated. They are either eliminated through the urine and body fluids or are deposited in some body tissues like bone and teeth. A common condition seen in animals is congenital porphyria due to the accumulation of Uroporphyrin. This compound get deposited in the teeth and bones giving them a pink discolouration. Some times, this compound may get excreted through urine, showing a pinkish fluorescence under ultra violet light.

Type III Photosensitivity
It is the most common type of photosensitivity reaction seen in animals. It is also known as hepatic photosensitivity. Here, the photodynamic substances are normally synthesized in the body, but they fail to get excreted due to damages in the biliary transport mechanism. The photodynamic substance produced in the body is phylloerythrin, a derivative of chlorophyll. Normally, the phylloerythrin is secreted into the intestine by the biliary system and get excreted through the dung. But due to failure or damage to biliary system, phylloerythrin enters the systemic circulation and is accumulated under the skin. This condition is exacerbated in the presence of hepatic parasites.

Type IV Photosensitivity
It is another type of photosensitivity reaction which occurs following the ingestion of some plants like alfalfa. The reason is obscure or idiopathic.

Treatment
The affected animals should be transferred to shade. To overcome shock, corticosteroid therapy and fluid infusions may be tried. Antihistamines are also useful.