Vet School Team Solves Dog's Mystery Rash

Max’s itchy skin problem resolved.
By Eleanor K. Sommer, December 2010, Updated February 2022

When we impulsively rescued an eight-week-old ball of yellow fur from the back of a pickup truck headed for the pound, we had no idea how much our lives would change. This Lab/German Shepherd mix, whom we named Max, grew quickly. Before we knew it, we had a 108-pound ballistic missile with a wet tongue and four paws — and, in his second year, a severe allergy to an unknown substance.

We tried steroids, antibiotics, topical applications, herbs and acupuncture, yet the rashes, oozing and itching persisted. The steroids worked in the short term, but made him hungry, thirsty and susceptible to ringworm, and were bad for his bones, joints and kidneys over the long haul.

The rashes typically began with papules — red dots — and pustules, and then spread into a solid flaming mass of irritation in his inguinal (groin) and axillary (armpit) areas — sometimes on his belly as well. I kept telling vets that it looked like poison ivy, and they assured me that dogs do not react to poison ivy.

No one had any idea what Max was allergic to. Because the rashes appeared initially in summer, they were first attributed to our typical Florida weather — hot and humid — exacerbated by insects of all kinds. That theory was disproved when Max’s condition continued into the cooler, drier months.


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We investigated food as a source, and put Max on a special diet of an allergy-free amino acid–based food for three months. Then we reintroduced his regular diet. No change. We gave him raw milk at the suggestion of a farmer friend, who had tried it successfully for her dog’s itchiness. No change. We thought he might be allergic to turkey and chicken and eliminated them from his diet. Nothing seemed to make a difference. The typical culprits — fleas, ticks and mites, both demodex and scabies — were ruled out or did not fit the symptoms to begin with.

He wasn’t better in any particular season, so inhalant allergies were out. Our allopathic vet suggested he was just allergic to “a lot of stuff,” got itchy and then got various infections from romping around. That’s when drugs were prescribed: antibiotics for the secondary infections and steroids to suppress his reaction to the allergenic agents. Our holistic vet offered various herbs and did some acupuncture, but she seemed as perplexed as everyone else about what could be causing this awful skin condition. Although we were frustrated, we could not fault the professionals we consulted. Presented with the facts we had, they did the best they could.

We’re fortunate to have a veterinary college near us, part of the University of Florida (UF) in Gainesville. Although we hesitated at first because of the cost, we finally decided it was time to have him checked out there — Max was still miserable despite the fortune we’d already spent on medications, herbs and topical applications.

The great thing about visiting a university clinic is that a team of students and residents reviews a case before the attending veterinarian weighs in. We liked the idea of having several bright, inquiring minds looking for solutions to this mystery. We were there for half the day, and the intake exam was thorough — the students carefully inspected every inch of Max’s body, then took him away for testing that included skin scrapings for cultures and microscopic examination. Finally, they all re-entered the tiny treatment room where my husband and I were waiting. The attending veterinarian, Rosanna Marsella — a diplomate of the American College of Veterinary Dermatology — came in with them. “Max definitely has a contact allergy,” she told us, as opposed to an atopic allergy. (Atopic means the reaction can be distant from the cause — for example, a rash that is a result of pathogens entering through the respiratory or digestive system.)

So what was he coming in contact with? Dr. Marsella said the rash was consistent in character with that caused by plants in the Commelinaceae family. Her Italian accent, coupled with the challenging pronunciation of the full botanical name (genus and species), made it difficult for me to understand what she was saying. “Does it have a common name?” I asked.

“‘Wandering Jew,’” she said. We knew this plant as an invasive exotic that many people have in their ornamental gardens. But, since we make an effort to maintain native plants, this was not one that we had growing on our 10 acres of land. “Well, we don’t have any of that,” I said, disappointed that we had reached another dead end. My husband, however, made the connection between the botanical family name and the genus of “Wandering Jew.” He asked if she had said Tradescantia fluminensis.

“Yes, that’s it.”

And then it all fell into place. We are surrounded by acres of Tradescantia — not the invasive T. fluminensis, but the beautiful Florida native species, T. ohiensis, commonly called “spiderwort” or “blue jacket.” There was still a disconnect for the medical team because most plants in the Commelinaceae family are ground creepers. But the Tradescantia that grows abundantly on our property gets as high as 36 inches, which accounted for the reaction on Max’s torso.

“Do you live nearby? Can you get some of that Tradescantia and come back with it and Max?” Dr. Marsella asked. Of course, we said. Later that afternoon, the veterinary team made a slurry from the plant and coated shaved areas of healthy skin on Max’s flank and one of his ears with the paste. They added two control spots as well. To be sure he could not paw the test areas on his side, the students made him a spectacular outfit from pink vet wrap decorated with little red hearts.

The next day, we returned to the clinic and were told that the test was absolutely conclusive. Max had developed the same rashlike pustules that we had come to expect on his underside. And once the vet team saw field photos of the plant, they understood why this dog, with his 20-inch leg length, had extensive rashes in his armpits and groin and not just on his feet, where he would have come in contact with the creeping T. fluminensis, T. pallida, and T. zebrina. The rash between Max’s toes was tame compared to those on his underside because, as Dr. Marsella explained, fur prevents direct contact with the skin.

Just as with people who are allergic to poison ivy or oak, the culprit is a chemical, in this case calcium oxalate, a compound with a microscopic crystalline structure. The irritating needlelike formations penetrate the skin, causing an inflammatory response in animals that are sensitive to them. There is no cure for the rash and no vaccine to prevent reaction. The rash almost always leads to secondary infections, which is why most dogs (and cats) that have this allergy are treated with steroids and antibiotics. Fortunately, Dr. Marsella was the lead author of a study published in Veterinary Dermatology (volume 8, issue 2) by the team of veterinarians that pioneered the use of pentoxifylline (Trental®) to mitigate reaction to calcium oxalates when used prior to exposure. The only other treatment is avoidance. Max is now on this medication 24/7, which initially worried me because it is a vasodilator and in the caffeine class of drugs. Recently, though, a friend who is a pharmacist likened Max’s dose to a person drinking a daily cup of Earl Grey tea, and I calmed down a bit. Pentoxifylline lasts about eight hours in a dog’s system, and because it can irritate the stomach, we administer it day, eight hours apart (morning and late afternoon), so it is in his system and “on duty” during the times he is most active outside.

Other options for reducing contact with the plant include booties and/or a body suit. Unfortunately, Max is too large for a body suit to be effective, as it leaves exposed the very areas that need protecting. Another way to mitigate the rash is to rinse or thoroughly wipe down the dog after every potential exposure. By also using shampoos such as Malaseb, alone or mixed with a bit of Selsun Blue, the chance of secondary staph and yeast infections can be reduced.

Studies have shown that dogs build sensitivity over time but may not show a reaction until as late as two years old. And once started, it tends to get worse, according to the research. “Every time the dog comes in contact with the substance, the reaction is likely to worsen,” wrote Gail Kunkle, DVM, in a paper about canine allergies presented at a 2000 Dog Owners and Breeders Symposium at University of Florida College of Veterinary Medicine. “There are steroids which are used in severe cases to relieve the inflammation but these are not good long term solutions,” she wrote. “The most difficult part of managing a contact allergy is that it [requires] a lifetime commitment from the owners, as it is unlikely the dog will ‘outgrow’ the reaction or improve over time. Some owners have resorted to making concrete or rock kennels in their yards or even using Astroturf if the problem is a grass or weed. The common plants that cause this reactivity in the dog are very difficult to eliminate from your yard and, in some cases, clients have literally killed the grass and the weed has thrived.”

We have chosen to eradicate this otherwise wonderful plant on nearly an acre around our house, and we are considering a fence to keep Max from roaming on the rest of the acreage, where we are unable to tame this native menace. Botanical sources indicate there are 350 species in the Commelinaceae family, and all contain calcium oxalates. Some species are so tiny (e.g., Murdannia spp.) that they look like grass until you get on your hands and knees to see their minuscule purple flowers! Other plants, such as taro (Colocasia esculenta), also contain calcium oxalates, which should not be confused with plants that contain oxalic acid, such as spinach or sorrel.

Max is only outside when we are, so we can keep an eye on him. But still, he occasionally experiences a thrilling moment when a deer wanders by and taunts him into a good run, which the deer invariably wins. On his return, after successfully evicting an “intruder” from our property, Max gets a sudsy rinse on his underside, just in case.

Dr. Marsella told us that Commelinaceae allergy is a greatly underdiagnosed condition in dogs. She hopes that more exposure to the issue will encourage veterinary clinicians to consider it as a cause of chronic rashes and infections. We can vouch for the fact that the patch test is simple, painless, and conclusive. We are grateful to the students and faculty at the UF College of Veterinary Medicine for improving Max’s quality of life.

Eleanor K. Sommer is a freelance editor and writer with a background in the traditional use of herbs.