Photosensitivity in animals
Encyclopedia
Photosensitivity
is an abnormal skin reaction to direct sunlight exposure. It is unrelated to a sunburn
. These reactions are due to photosensitization, the accumulation of photosensitive compounds beneath the skin. In some cases, the photodynamic substances come from ingested plants or drugs, after being metabolized or not. In other cases, the photodynamic substances may be produced in the body itself due to inborn errors in 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.
of the skin with depigmentation and ulceration. In some cases (Type I), pigments are excreted through urine or deposited in hard tissues such as bones and teeth, causing discolouration. Affected animals should be protected from direct sunlight until the photodynamic compounds have been completely eliminated from the body or the appropriate treatments have been given.
and St John's wort
(Hypericum perforatum). Certain drugs such as phenothiazine
or its derivatives may also lead to photosensitivity reactions, commonly in sheep. Phenothiazine is converted by the body into the photodynamic compound phenothiazine sulfoxide.
under ultraviolet light.
. The reason is obscure or idiopathic.
therapy and fluid infusions may be tried. Antihistamines are also useful.
Photosensitivity
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...
is an abnormal skin reaction to direct sunlight exposure. It is unrelated to a sunburn
Sunburn
A sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet radiation, commonly from the sun's rays. Usual mild symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV...
. These reactions are due to photosensitization, the accumulation of photosensitive compounds beneath the skin. In some cases, the photodynamic substances come from ingested plants or drugs, after being metabolized or not. In other cases, the photodynamic substances may be produced in the body itself due to inborn errors in pigment metabolism, especially those involving the heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
synthesis. Photosensitivity reactions are usually seen in herbivorous or omnivorous animals, though such reactions are not uncommon among carnivores.
Effects of photosensitivity
Photosensitivity reactions are characterized by severe inflammationInflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
of the skin with depigmentation and ulceration. In some cases (Type I), pigments are excreted through urine or deposited in hard tissues such as bones and teeth, causing discolouration. Affected animals should be protected from direct sunlight until the photodynamic compounds have been completely eliminated from the body or the appropriate treatments have been given.
Biological mechanism of photosensitivity reactions
Photodynamic compounds are activated by sunlight, which excites them to a higher energy state. They immediately release the excess energy and transfer it to the surrounding acceptor molecules, such as enzymes or free radicals. These molecules are thus activated and cause the skin reaction.Classification of photosensitivity reactions
Photosensitivity reactions are classified by origin into four types: Type I, Type II, Type III and Type IV.Type I Photosensitivity
Type I Photosensitivity occurs when the photodynamic compounds need not undergo metabolism to be activated and so may accumulate under the skin directly following ingestion. Examples of plants producing Type I Photosensitivity include BuckwheatBuckwheat
Buckwheat refers to a variety of plants in the dicot family Polygonaceae: the Eurasian genus Fagopyrum, the North American genus Eriogonum, and the Northern Hemisphere genus Fallopia. Either of the latter two may be referred to as "wild buckwheat"...
and St John's wort
St John's wort
St John's wort is the plant species Hypericum perforatum, and is also known as Tipton's Weed, Chase-devil, or Klamath weed....
(Hypericum perforatum). Certain drugs such as phenothiazine
Phenothiazine
Phenothiazine is an organic compound that occurs in various antipsychotic and antihistaminic drugs. It has the formula S2NH. This yellow tricyclic compound is soluble in acetic acid, benzene, and ether. The compound is related to the thiazine-class of heterocyclic compounds...
or its derivatives may also lead to photosensitivity reactions, commonly in sheep. Phenothiazine is converted by the body into the photodynamic compound phenothiazine sulfoxide.
Type II Photosensitivity
Type II Photosensitivity is caused by inborn errors in the metabolism of certain biological pigments. In the absence of some key metabolic enzymes, the products of intermediary metabolism accumulate. They are either eliminated through the urine and body fluids or are deposited in some body tissue, such as bone and teeth. A common condition seen in animals is congenital porphyria due to the accumulation of Uroporphyrin, which is deposited in the teeth and bones, giving them a pink discolouration, or excreted through the urine, exhibiting a pinkish fluorescenceFluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
under ultraviolet light.
Type III Photosensitivity
Type III (aka hepatic photosensitivity) is the most common type of photosensitivity reaction seen in animals. In this type, the photodynamic substance is phylloerythrin, a derivative of chlorophyll produced in the body. Normally, phylloerythrin is secreted into the intestine by the biliary system and excreted through the feces. In Type III Photosensitivity, damage to the biliary transport mechanism prevents the excretion of phylloerythrin, allowing the photodynamic substance to enter the circulatory system and accumulate under the skin. This condition is exacerbated by the presence of hepatic parasites.Type IV Photosensitivity
Type IV Photosensitivity occurs following the ingestion of certain plants, such as alfalfaAlfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
. The reason is obscure or idiopathic.
Treatment
Affected animals should be transferred to shade. To overcome shock, corticosteroidCorticosteroid
Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...
therapy and fluid infusions may be tried. Antihistamines are also useful.