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This cow’s coming apart: Photosensitivity linked to dry conditions

Veterinary Case Study: A dry summer likely encouraged Rosie the milk cow to graze plants she’d normally avoid, and the results were nearly disastrous

Rosie is an Ayrshire-cross milk cow, the kind of cow you could milk anywhere if you offered a handful of oats.

Kirby’s ranch incorporated a section of short grass prairie and bush southeast of Regina. Kirby’s family lived comfortably on what 30 commercial cows, two sows, a milk cow, 20 chickens and a full-time job at the lumberyard provided. Kirby could have been a stand-up comedian because he had more one-liners than most could imagine and an indomitable spirit that attracted people. 

I sensed Kirby’s concern when he called one morning wanting me to stop by next time I was in the area to have a look at Rosie, their Ayrshire-cross milk cow that Kirby had found standing in mud on the edge of a slough. Normally an authority on the ailments of cows and horses, this one went beyond Kirby’s veterinary knowledge. He barely got her to limp home. 

“I really need you to stop by, Doc. Rosie is coming apart.” Kirby’s description of the family cow signaled the need for a special call over and above the type of “when-in-the-area.” Rosie, now two months from freshening, provided milk and cream for four kids. Rosie could be milked if offered a small portion of oats wherever she happened to be grazing. A dry summer had pushed her grazing range deeper into the bush and made plants, normally avoided, more palatable to a hungry cow. Rosie had fallen victim to photosensitization, probably induced from a variety of strange plants. 

Photosensitization occurs when the presence of a chemical makes skin sensitive to sunlight (particularly UV wavelengths). The result: skin damage (primarily of white and unpigmented areas), blistering, ulceration and skin loss. The coronary band, a non-pigmented area at the top of the hoof, can be affected, causing severe lameness and a tendency for affected animals to seek relief from pain in water and mud. 

Photosensitization can cause significant economic loss when it affects groups of animals. Horses are also susceptible. 

There are three sorts of photosensitization: 

1. Direct photosensitization occurs when the chemical comes from a defect in the animal’s metabolism of its red blood cells, or, more commonly, from plants such as St. John’s wort. 

2. Secondary photosensitization occurs in animals with liver damage. This damage interferes with the complete breakdown of chlorophyll, resulting in the accumulation of photosensitive chemicals. 

3. Local photosensitivity can also result as a reaction to the sap of some plants. The most commonly affected sites are those exposed to direct sunlight, including the udder (which is exposed when the animal lies down). 

Clinical signs

  • Non-pigmented skin is affected.
  • Hair loss, reddening, peeling.
  • Ulceration of skin.
  • Crusting, bleeding. 


• A veterinary diagnosis is important to rule out liver failure.
• Animals appear agitated and uncomfortable. They may scratch or rub lightly pigmented, exposed areas of skin (e.g., ears, eyelids, muzzle).
• The skin lesions heal remarkably well, even after extensive necrosis. 


  • Removal to cool, shaded housing.
  • Fly control.
  • Supportive therapy.
  • Treatment of liver failure (if present).
  • Prevention: Do not breed commercial animals showing photosensitization because genetic defects can be involved.
  • Identify and remove possible plant sources of photosensitizing chemicals. A few species include brassica species, cow parsnip, some species of clover, Bermuda grass, anise, cilantro, dill and cowbane.

Rosie’s comfort level under cover quickly improved. Fly control and tender, loving care returned her to near normal by the time she calved. 

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).



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