Photosensitization in Livestock

Updated: July 2002

Photosensitivity is caused directly by photodynamic agents, such as hypericin in St. John's Wart (H. perforatum) which is absorbed and then reacts with light at the skin's surface. Secondary photosensitization is caused by phylloerythrin, a metabolite of chlorophyll, which is normally excreted in the bile. Agents such as alkaloids causing liver damage result in phylloerythrin entering the general circulation and causing skin lesions when it react with ultraviolet light.

Signs

The white or light coloured areas of the animal become sunburned with skin lesions, photophobia, death caused by starvation or infection of skin lesions. The cow will often not let the calf suckle.

Causative Agents

Buckwheat from a green manure crop, etc. Both the seed and forage can cause photosensitization.

Alsike clover poisoning is usually seen in horses and the unknown agent acts on the liver causing failure. Other symptoms include icterus, depression, stupor and "head pushing" from elevated blood ammonia and copper levels and neurological involvement.

Furocoumarins are a photosensitization agent present in Bishops weed and spring parsley in S. California and Utah/Nevada resp. Also present in celery and parsnips.

Hypericin from St. John's Wart (Hypericum perforatum) (Goatweed) is a photosensitizing agent. Found in wet meadows, shorelines and marshy areas; may be more common in high country areas. It is considered unpalatable to livestock.

Kochia scoparia (kochia weed) toxicity includes signs of icterus and photosensitivity with progressive central nervous system involvement, cirrhosis, Gastro-Intestinal (GI) tract inflammation and polioencephalomalacia from a thiaminase or liver toxin causing impaired thiamine utilization. Kochia is usually found in arid and saline soils and is most toxic at seed and in drought periods.

Lupinosis arises from several varieties of lupines which are found in meadows, wooded and submountainous areas. Sheep, late pregnant or recently calved cows are more susceptible.

Mouldy straw induced photosensitization is not uncommon in the southern US and could possibly apply here. Cattle fed mouldy straw show liver damage and even when off straw for some time and placed on lush spring pasture will show photosensitivity (phylloerythrin reaction) .

Pyrrolizidine alkaloids give similar symptoms as alsike clover poisoning.  Tansy ragwort (Senecio family -ragwort/groundsel) is well documented to cause the necessary liver damage.

Ranunculin from buttercups in the fresh state (not in dried) which generally grow in wet soils and marshy areas, cause blistering of the lips and irritation to the mouth and GI tract. This is not a major problem maker.

Sporidesmin in ryegrass pastures is caused by spores of a fungus (Pithomyces chartarum) which grows in the dead litter of ryegrass pastures. Action is through liver damage as spore numbers increase dramatically with warm weather. Facial eczema and lesions of the udder and leg areas predominate.

Lady's thumb and Water Smartweed (Polygonum spp.) found in shallow water and slough and meadow margins can also cause photosensitivity when eaten green.

Another photosensitizer is fall rye when grazed during flowering. Problems seem to be related to the occurrence of pollen, possibly an allergic reaction.

A solution is to remove cattle from sources of the problem. If the problem arises from primary photosensitization, symptoms should disappear within a few days. If liver damage has occurred, the phylloerythrin reaction may continue for some time (mouldy straw above) .

Most cases arise when cattle eat something they don't eat normally as in overgrazed situations. Some plants may grow in abnormal or unexpected areas. Most troublemakers are unpalatable and cause problems in only a few animals. If photosensitization is throughout the herd, it is not usually an abnormal plant consumption problem, but an abnormal plant physiology problem.

Producers are advised to work closely with their veterinarians if photosensitivity reactions occur in their animals. Photosensitization may be an indicator of larger problems related to liver damage and production losses and deaths may be the outcome.

original document by Bob Drysdale MSc., P Ag.

July 2002 update by Bryan Doig, PAg, Livestock Agrologist, SAF