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Lessons abound for dermatologists when animal health and human health intersect



NEW YORK – We share more than affection with our dogs and cats. We also share diseases – about which our four-legged furry friends can teach us plenty.

That was the conclusion of speakers at a session on “cases at the intersection of human and veterinary dermatology,” presented at the summer meeting of the American Academy of Dermatology.

“Human health is intimately connected to animal health,” said Jennifer Gardner, MD, of the division of dermatology, University of Washington, Seattle, and a collaborating member of the school’s Center for One Health Research. The One Health framework looks at factors involved in the human, environmental, and animal sectors from the molecular level to the individual level and even to the planetary level.

Dr. Gardner challenged her audience to think beyond their individual areas of expertise. “How does the work you’re doing with a patient or test tube connect up the line and make an impact to levels higher up?” she asked.

The One Health framework also challenges practitioners to look horizontally, at how work done in the human world connects to what’s going on in the veterinary world – that is, how treatments for dermatologic conditions in dogs may one day affect how dermatologists treat the same or similar disorders in humans.

Learning from the mighty mite

For example, the study of mites that live on the skin of animals could eventually shed light on how dermatologists treat mite-related conditions in humans.

Dirk M. Elston, MD, professor and chair of the department of dermatology at the Medical University of South Carolina, Charleston, noted that Demodex mites occur in humans and in pets.

Dr. Dirk M. Elston, former president of the American Academy of Dermatology
Dr. Dirk M. Elston
In people, they play a role in papular eruptions in immunosuppressed patients, and in rosacea, alopecia, and blepharitis, he said. Patients with pityriasis folliculorum may look like they have rosacea, “but with little spines” – which are Demodex mites dining in. “They are so crowded in there that their backsides are sticking out,” he said. “They’re all there munching on the sebaceous glands.”

In such cases, “sulfur tends to be my most reliable” treatment, he said, noting that it releases a rotten egg smell. “You’re basically gassing the organism.” Dr. Elston said he frequently gets calls from fellow dermatologists whose antimite efforts have failed with ivermectin and permethrin and does not hesitate to give his advice. “I’m like a broken record,” he said. “Sulfur, sulfur, sulfur, sulfur.”

The Demodex mite affects dogs to varying degrees, depending on where they live, said Kathryn Rook, VMD, of the department of dermatology at the University of Pennsylvania School of Veterinary Medicine, Philadelphia. In North America, demodicosis occurs in 0.38%-0.58% of dogs, and in 25% of dogs in Mexico, she said.

Amitraz, the only Food and Drug Administration–approved treatment for canine demodicosis, is available only as a dip. But it has fallen from favor as a result of sometimes serious side effects, which can include sedation, bradycardia, ataxia, vomiting, diarrhea, and hyperglycemia.

Daily administration of oral ivermectin – often for months – also carries a risk of side effects, including dilated pupils, ataxia, sedation, stupor, coma, hypersalivation, vomiting, diarrhea, blindness, tremors, seizures, and respiratory depression.

But the discovery of isoxazoline has “revolutionized” the treatment of demodicosis and other parasitic infestations in dogs, Dr. Rook said, citing quicker resolution of disease and improved quality of life for both the patient and its owner.

Isoxazoline, which Dr. Rook said carries little risk for side effects, is licensed in the United States only as a flea and tick preventive.

Atopic dermatitis

Atopic dermatitis (AD) tends to be similar in people and dogs, according to Charles W. Bradley, DVM, of the University of Pennsylvania School of Veterinary Medicine, Philadelphia. About 10%-30% of children and up to 10% of adults have the disorder, the prevalence of which has more than doubled in recent years, he said.

In dogs, the prevalence is 10%-20%, making it “an extraordinarily common disorder,” he said. Lesions tend to be located on the feet, face, pinnae, ventrum, and axilla/inguinum. Additional sites vary by breed, with Dalmatians tending to get AD on the lips, French Bulldogs on the eyelids, German Shepherds on the elbows, Shar-Peis on the thorax, and Boxers on the ears.

In humans, Staphylococcus aureus is the chief microorganism of concern, said Elizabeth Grice, PhD, of the department of dermatology at the University of Pennsylvania, Philadelphia, who copresented the topic with Dr. Bradley.

Dr. Elizabeth Grice
Concern about drug resistance is “one reason why we want to better understand the entire microbiome and other organisms that are colonizing the skin,” she commented. That means better understanding the relationship among S. aureus, microbial diversity, and disease severity.

“My true love is anything to do with the skin microbiome,” she said. “The more severe the disease, the lower the skin microbiome diversity.”

Though most studies of AD use mice as animal models, dogs would be better, according to Dr. Grice and Dr. Bradley.

That’s because canine AD occurs spontaneously and exhibits immunologic and clinical features similar to those of human AD. They include prevalence, environmental triggers, immunologic profiles, genetic predispositions, lesion distribution, and frequent colonization by Staphylococcus species. In addition, dogs and their owners tend to share the same environment.

A rash of itches

Among dermatology patients – man or beast – itch can outweigh rash as a key focus of concern, according to Brian Kim, MD, of the division of dermatology at Washington University in St. Louis, and codirector for the University’s Center for the Study of Itch. “The problem is my patients don’t complain about their rash; they complain about their itch,” he said. “But we don’t understand the basic question of itch.” In fact, the FDA has not approved any drugs for the treatment of chronic itch, he said.

Dr. Brian Kim
Toward that end, veterinary medicine is moving faster than human medicine, he said, citing work in mice that has succeeded in killing itch.

For dogs, advances have been made with Janus kinase (JAK) inhibitors, which “may function as immunomodulators,” Dr. Kim said. And JAK-1 selective inhibition “may be more effective than broad JAK blockade for itch.”

‘The perfect culture plate’

Lessons can be learned from studying canine AD, which “is immunophysiologically homologous to human AD,” said Daniel O. Morris, DVM, MPH, professor of dermatology, at the University of Pennsylvania School of Veterinary Medicine, Philadelphia. “The main difference: My patients are covered in dense hair coats.” Because of that, systemic treatment is necessary, he said.

Canine AD primarily affects areas where hair is sparse or where the surface microclimate is moist, he said. A dog’s ear canal, which can be 10 times longer than a human’s, harbors plenty of moisture and heat, he said. “It’s the perfect culture plate.”

But, he added, the owners of his patients tend to resist using topical therapies “that could be potentially smeared on the babies and grandma’s diabetic foot ulcer.” So he has long relied on systemic treatments, initially steroids and cyclosporine. But they can have major side effects, and cyclosporine can take 60-90 days before it exerts maximum effect.

A faster-acting compound called oclacitinib has shown promise based on its high affinity for inhibiting JAK-1 enzyme-mediated activation of cytokine expression, including interleukin (IL)-31, he said. “Clinical trials demonstrate an antipruritic efficacy equivalent to both prednisolone and cyclosporine,” he noted. Contraindications include a history of neoplasia, the presence of severe infection, and age under 1 year.

Monoclonal antibody targets IL-31

The latest promising arrival is lokivetmab, a monoclonal antibody that targets canine IL-31, according to Dr. Morris. It acts rapidly (within 1 day for many dogs) and prevents binding of IL-31 to its neuronal receptor for at least a month, thereby interrupting neurotransmission of itch.

But side effects can be serious and common. Equal efficacy with a reduced side effect is the holy grail, he said.

Some doctors are not waiting. “People are throwing these two products at anything that itches,” he said. Unfortunately, they tend to “work miserably” for causes other than AD, he added.

Dr. Gardner, Dr. Elston, Dr. Rook, Dr. Bradley, and Dr. Morris reported no financial conflicts. Dr. Grice’s disclosures include having served as a speaker for GlaxoSmithKline and for L’Oreal France, and having received grants/research funding from Janssen Research & Development. Dr. Kim has served as a consultant to biotechnology and pharmaceutical companies.

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