Photosensitization

Question 1

Name 3 drugs responsible for primary photosensitization

Answer 1

Phenothiazines, thiazides, methylene blue, tetracyclines, sulphonamides, furosemide

Question 2

You diagnosed this sheep to have facial eczema

What agent is responsible for this and where does it commonly grow?

Answer 2

Saprophytic fungus: Pithomyces chartarum

Grows commonly on perennial ryegrass

Produces sporidesmin (liver damaging toxin) –> liver unable to metabolize phylloerythrin –> photosensitization

Question 3

Name 3 plants and their photodynamic agents in primary photosensitization

Answer 3

St John’s wort – hypericin

Buckwheat – fagopyrin

Perennial ryegrass - perloline

Question 4

Briefly describe the 4 types of photosensitization

Answer 4

1. Primary photosensitization - ingestion of photodynamic agent
2. Abnormal porphyrin metabolism so photodynamic agents increase
3. Hepatogenous (secondary) photosensitization: liver disease caused by plant toxin –> phylloerythrin accumulation
4. Idiopathic

Question 5

A farm vet reached out to you on a derm case. A young Holstein with stunted growth, and exfoliation of non-pigmented skin. The teeth are discolored red/brown. What is your prelimary diagnosis?

Answer 5

Congenital erythropoietic porphyria

Autosomal recessive

ANEMIA

Deficiency of uroporphyringogen III cosynthetase–> accumulation of uroporphyrin & coproporphyrin

Question 6

T/F

All photosensitization caused by abnormal porphyrin metabolism presents with concurrent anaemia and teeth discoloration

Answer 6

False

Congenital erythropoietic porphyria does often have “pink tooth” and anemia

Bovine protoporphyria does not

Both diseases are autosomal recessive

Question 7

Which is the most common form of photosensitization?

Answer 7

Secondary (hepatogenous) photosensitization

Question 8

What is the breed of sheep with congenital photosensitization?

Answer 8

Southdown and Corriedale sheep may inherit a hepatobiliary incompetence that results in photosensitization.

Question 9

How would you diagnose photosensitization?

Answer 9

• History and PE
• Liver function testing
• Primary photodynamic agents identified with biological assays
• Measure blood and tissue porphyrins

Question 10

Define photosensitization

Answer 10

Photosensitization occurs when skin (especially areas exposed to light and lacking significant protective hair, wool, or pigmentation) becomes more susceptible to ultraviolet light because of the presence of photodynamic agents.

Question 11

What is the photosensitizing agent in hepatogenous photosensitization?

Bonus points: How is this agent derived?

Answer 11

Phylloerythrin (a porphyrin), accumulates in plasma because of impaired hepatobiliary excretion.

Phylloerythrin is derived from the breakdown of chlorophyll by microorganisms present in the GI tract.

Question 12

What is the pathogenesis of photosensitization? And how does it create skin lesions such as ulceration, necrosis and oedema?

Answer 12

In photosensitization, unstable, high-energy molecules are formed when photons react with a photodynamic agent. These high-energy molecules initiate reactions with substrate molecules of the skin, causing the release of free radicals that in turn result in increased permeability of outer cell and lysosomal membranes. Damage to outer cell membranes allows for leakage of cellular potassium and cytoplasmic extrusion. Lysosomal membrane damage releases lytic enzymes into the cell. This can lead to skin ulceration, necrosis, and edema

Question 13

T/F

Important causes of photosensitivity in veterinary medicine are fungal and bacterial in origin

.

Answer 13

False

A wide range of chemicals, including some that are fungal and bacterial in origin, may act as photosensitizing agents. However, most compounds that are important causes of photosensitivity in veterinary medicine are plant-derived.

Question 14

What are the common species affected by photosensitization?

Answer 14

Photosensitization occurs worldwide and can affect any species but is most commonly seen in cattle, sheep, goats, and horses.

Question 15

T/F

Primary photosensitization occurs only when the photodynamic agent is ingested

Answer 15

False

Primary photosensitization occurs when the photodynamic agent is either ingested, injected, or absorbed through the skin.

Question 16

What are the 2 most commonly reported cattle diseases of ‘aberrant pigment metabolism’ photosensitization (some texts will call this type II)?

Answer 16

Bovine congenital erythropoietic porphyria and bovine erythropoietic protoporphyria

Question 17

Name two common sheep diseases where Phylloerythrin is the phototoxic agent

Answer 17

Facial Eczema and congenital photosensitivity of Southdown and Corriedale sheep

Question 18

What are the clinical signs of photosensitization (regardless of cause)?

Answer 18

Animals are photophobic, agitated and uncomfortable

+/- scratch or rub lightly pigmented (ears, eyelids, muzzle)

Erythema develops rapidly and then edema.

Lesions may progress to vesicle and bulla formation, serum exudation, ulceration, scab formation, and skin necrosis.

Finally skin sloughing

Question 19

What is the prognosis for animals with hepatogenous photosensitization and porphyria?

Answer 19

Poor

(prognosis for animals with primary photosensitization is generally good)

Question 20

What is the treatment for animals with photosensitization?

Answer 20

Mostly palliative

Animals should be shaded fully or housed and allowed to graze only during darkness.

Corticosteroids, given parenterally in the early stages, may be helpful.

Secondary skin infections and suppurations should be treated