Photosensitization

In the presence of photodynamic agents, skin (especially areas exposed to light and lacking substantial protective hair, fur, or pigmentation) becomes more susceptible to ultraviolet light. Sunburn and photodermatitis are not the same as photosensitization since all of these disorders cause pathologic skin changes without the involvement of a photodynamic agent.

When photons react with a photodynamic agent in photosensitization, unstable, high-energy molecules are formed. These high-energy molecules react with skin substrate molecules, releasing free radicals that increase membrane permeability in outer cells and lysosomes. Damage to the outer cell membranes causes cellular potassium to leak out and cytoplasmic extrusion to occur. Damage to the lysosomal membrane allows lytic enzymes to enter the cell. Skin ulceration, necrosis, and edema are also possible outcomes. The length of time between exposure to a photodynamic agent and the onset of clinical symptoms is determined by the type of agent, its dosage, and the amount of sunlight exposure.

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The source of the photodynamic agent is usually used to classify photosensitization. The three forms of photosensitivity are main (type I) photosensitivity, aberrant endogenous pigment synthesis (type II) photosensitivity, and hepatogenous (secondary, type III) photosensitivity. Idiopathic (type IV) photosensitivity has been identified as a fourth group.

Photosensitizing agents may come in a variety of forms, including those that are fungal or bacterial in origin. Plant-derived compounds, on the other hand, are the most common causes of photosensitivity in animals. Photosensitization may affect any animal, but cattle, sheep, goats, and horses are the most commonly affected.

Photosensitivity has been recorded in 25 percent to 90 percent of patients receiving demethylchlortetracycline, 20 percent of patients receiving doxycycline, 7% of patients receiving methacycline, and just a few patients receiving minocycline in people belonging to Tetracycline photosensitivity group.

Primary Photosensitization and Secondary Photosensitization

When a photodynamic agent is ingested, injected, or absorbed through the skin, it causes primary photosensitization. After the animal is exposed to ultraviolet light, the agent enters the systemic circulation in its native form, causing skin cell membrane damage. 

Example of Photosensitizer - Hypericin is an example of a primary photosensitizing agent.

In horses, primary photosensitization from other plants like buckwheat (Fagopyrum toxicosis), which contains many toxins similar to hypericin in St. John's wort, is uncommon. Photosensitization (furocoumarin toxicosis) caused by spring parsley, Bishop's plant, and Dutchman's breeches are more common in sheep, cattle, and pasture-raised swine.

Photosensitivity in Sheep

Sporadic photosensitization in sheep occurs in conjunction with the grazing of a variety of plant species, including grasses, cereals, and legumes, but it is uncommon. Below is a specimen of how photosensitivity in sheep affects their body. 

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Mechanism of Tetracycline Phototoxicity

Seven clinically used tetracyclines were studied to see what factors were relevant in the phototoxicity process (TC). The TCs' clinical phototoxicity, photochemical degradation rates, and in vitro phototoxicity were all qualitatively related but not quantitatively. In vitro, phototoxicity was partly oxygen-dependent, and singlet oxygen could be involved. The observed differences between the in vivo action spectrum and the absorption spectrum of demethylchlortetracycline may be due to the contribution of photoproducts to the phototoxic process. The absorption of UVA radiation by TC contributes to at least two key processes, according to a mechanistic model for in vivo phototoxicity.

(i) development of one or more photo products that photosensitizer through absorption of visible radiation; (ii) photosensitization of biologic molecules by the drug to induce phototoxicity.

Talaporfin has been studied to treat Port-Wine Stain and Benign Prostatic Hyperplasia.

Clinical Signs of Photosensitization

Photosensitivity can develop within a few days of sheep moving to biserrula pastures, but it can take weeks to develop. The following are the first signs that an animal could be photosensitized:

• Restlessness

• Head shaking

• Rubbing

• seeking out of shade.

Swelling of the head, eyelids, mulesed area (tail), backline, and muzzles are early signs of photosensitization. These are all the areas of the body that are the most visible. If left untreated, these areas can become reddened and inflamed, with the overlying skin dying and peeling away, revealing raw tissue underneath. Lambs may lose the tips or both of their ears in extreme cases, there may be a general split in the fur, and sheep may appear lame due to inflammation of the coronets.

Methylene Blue Photosensitizer

Methylene blue (MB) is a hydrophilic phenothiazine derivative also known as methylthioninium chloride. It's a photosensitizer that absorbs light at 660 nanometers. This limit is well within the emission range of most low-level laser therapy diode lasers. These lasers are often available at hospitals that treat patients with head and neck cancer, or they can be purchased for a low price. MB is barely activated by ambient light. As a result, health effects from environmental light exposure are unlikely. Furthermore, MB is a low-cost photosensitizer. Because of its effectiveness against a wide spectrum of pathogens, including bacteria, fungi, and viruses, MB is used in antimicrobial photodynamic therapy (APDT) and as a potent PDT medication for local treatment of periodontal diseases.

Photosensitization in Sheep

Swelling of the ears, eyelids, mulesed area (tail), backline, and muzzles are early signs of photosensitization. Lambs may lose the tips or all of their ears in severe cases, there may be a general break in the wool, and sheep may appear lame due to inflammation of the coronets.