Photosensitization may be associated with liver disease or damage caused by ingestion of poisonous substances. It should be noted that photosensitization may resemble sunburn, but it is a distinct condition.

Photosensitization is a condition in which a cat’s skin is extremely sensitive to sunlight. The disorder occurs when skin molecules react with a photodynamic agent, or a molecule activated by light that causes damage to the skin. Photosensitization is characterized by a marked sensitivity to light and redness and swelling of the skin. There are four types of photosensitization in cats, categorized by the nature or origin of the reaction.

Symptoms of photosensitization may appear similar to those of sunburn, and can cause discomfort for your cat. Additional symptoms that are associated with other conditions such as liver damage may also be present. Seek immediate veterinary attention as soon as you notice any of the following symptoms:

Type I, or primary, photosensitivity occurs when a photodynamic agent is ingested or otherwise absorbed by the skin. Ingesting certain plants, such as St. John’s wort and buckwheat, or harmful chemicals may cause type I photosensitivity in cats. 

Type II photosensitivity, characterized by abnormal pigment creation, is a congenital defect that is more common in cats. 

Type III, or secondary, photosensitivity in cats may occur as a result of liver damage or poisoning. 

Type IV photosensitivity is more rare, and is indicated when the photodynamic agent is unknown.

There may be several causes of photosensitization in cats depending on the type, and range from birth defects to poisoning. Photosensitization that develops due to a congenital defect may be attributed to malformed or defective enzymes involved in the production of heme, a substance found in hemoglobin that gives blood its red color.

The vet will make a tentative diagnosis based on a thorough physical examination and presentation of symptoms. Be sure to inform your vet of any birth defects or liver problems that you know of, as well as instances when your cat may have been exposed to poisonous plants or chemicals.

The vet will make a definitive diagnosis through a number of tests. Your vet may take a biopsy of the liver in addition to analyzing liver enzymes to identify liver damage or disease. Your vet may also perform standard diagnostic testing, including a blood chemical profile as well as fecal and urine analysis. Additional tests may be utilized if concurrent disease is present or suspected.

Treating photosensitization will depend on the type diagnosed and the severity of the underlying condition. The primary objectives of treatment are to soothe your cat’s discomfort and resolve the underlying cause. Your vet will advise you on a treatment plan based on your cat’s specific needs.

Certain types of photosensitization do not require any additional treatment apart from preventative measures. Steroid injections may assist in resolving some cases of photosensitivity, particularly in cases that are detected and treated early. Topical treatments and other wound management medications may be prescribed in order to heal lesions or scabs, especially those accompanied by discharge.

Recovery and prognosis depend on the underlying cause, but are  generally good to excellent following treatment. Always follow your vet’s post-treatment instructions carefully. Always administer any prescribed medications exactly as directed for the full duration of the treatment period. Never apply sunscreens or topical treatments made for human use to your cat unless explicitly instructed to do so by your vet.

You will need to employ preventative measures to avoid exacerbating the condition. Prolonged exposure to sunlight may lead to excess stress, serious disease, death of skin tissue, or even death of the animal in severe cases. Ensure your cat has access to shade from sunlit areas at all times. Monitor and limit your cat’s outdoor activity. If you must let your cat outside, it is generally recommended to allow them outside only at night.

Skin damage attributed to photosensitization attracts flies, which may lead to severe illness and/or maggot infestation. You should always monitor your cat’s outdoor activity, preventing them from accessing areas in which flies are present; this includes trash dump sites and animal carcasses. Always maintain a sanitary environment.

Your vet may schedule follow-up appointments as needed to monitor the underlying condition if it is severe. If you have any questions, or if the condition does not seem to be improving with treatment, contact your vet immediately.