Allergic dermatitis in horses likely to be more than skin deep – review

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A horse diagnosed with environmental and insect allergies. The itchiness in this patient was intense and led to significant self-trauma.
A horse diagnosed with environmental and insect allergies. The itchiness in this patient was intense and led to significant self-trauma. Photo: https://doi.org/10.3390/vetsci8070124

What first appears as allergic dermatitis in horses is likely to be part of deeper-rooted problems linked to the body’s immune response.  

“It is now recognized that atopic dermatitis is not a single disease but a multifaceted clinical syndrome with different pathways in various subgroups of patients,” Rosanna Marsella writes in the journal Veterinary Sciences.

“Appreciating this complexity is clinically relevant as we develop more targeted treatments which may work well in some patients but not in others.”

Marsella, a professor with the College of Veterinary Medicine at the University of Florida, conducted a review of the current understanding of atopic syndrome in domestic animals, highlighting opportunities for further studies in the future.

She says sustainability and safety are central to long-term management of the condition, with re-educating the immune system and decreasing the need for rescue medications the desired strategy.

Atopic dermatitis is a clinical syndrome that affects both people and animals. Dogs closely mimic the complexity of the human skin disease, and much progress has been made in recent years in the understanding of the role of skin impairment and the identification of new treatments.

Horses and cats also develop atopic syndromes which include both skin-related and respiratory signs, yet studies in these species are lagging, she says.

In people, dermatitis is typically the first manifestation of allergy-related disease and can be followed by respiratory disease later in life as part of what is called the “atopic march”, she writes.

Dealing with horses, Marsella says allergy-related problems can manifest as respiratory or skin-related diseases. The respiratory disease has been recognized to be similar to human asthma.

“Our understanding of atopic dermatitis in horses is very limited,” she says.

“It is commonly accepted that this disease is the result of genetic and environmental factors, and it is frequent to see horses that were raised in colder climates manifest disease only later in life when moved to a warmer climate with more insect and environmental pressure.”

Many allergy-affected horses are sensitized to a range of allergens and have hypersensitivity to both insects and various pollens.

Very little is known about the skin barrier and atopic disease in horses. One study in allergy-affected horses showed abnormalities on electron microscopy when compared to normal horses but it is unclear if this is the result of inflammation, or may suggest some primary skin impairment which could facilitate absorption of the allergen and increased risk for sensitization.

The findings of a recently published study suggested skin impairment was a factor in insect allergic horses.

“It is possible that some of these allergic horses were also atopic. Clearly, more work is needed before any conclusion can be made about the existence of a primary skin impairment and what the pathogenetic relevance could be for the equine disease.”

Atopic dermatitis in horses presents as a relapsing, itchy inflammatory disease that typically affects the face, ears, and hair-free areas. Some horses may have a history of heaves as well. Many atopic horses are also insect allergic.

Controlling all triggers for itchiness is crucial to the success of therapy, she says.

As is in other species, the diagnosis of atopic dermatitis is a clinical diagnosis based on suggestive history, compatible clinical signs, and exclusion of other diseases. More accurate serology testing is aiding in making a diagnosis, she says.

Treatment primarily still involves the use of glucocorticoids and antihistamines, but no controlled studies have been done to evaluate the efficacy of these treatments in a controlled fashion.

“The reports of these treatments are retrospective and uncontrolled studies where owners report on the beneficial effects of these strategies.

“The same limitations hold for reports investigating allergen-specific immunotherapy, which is recommended for atopic horses with a long allergy season.”

The reported success rate of allergen-specific immunotherapy in atopic horses ranges from 64% to 84%, with the bulk of the improvement typically visible after the first year.

“Some horses can be maintained with allergen-specific immunotherapy alone, while others still require other medications, although the amounts of medications necessary to make them comfortable may be decreased.”

Marsella traversed several other potential avenues for easing symptoms such as itching but cautioned that our understanding of atopic dermatitis in horses is still rudimentary.

“Much work needs to be done to understand the role of the skin barrier and immune dysregulation in horses, and how this relates to other species.

“Based on what we know, it is reasonable to hypothesize that skin barrier impairment may exist in horses.

“Identification of specific targets to decrease the use of glucocorticoids would be of immense benefit.”

Interleukin-31, an inflammatory cytokine that can trigger severe itching, appears to be a suitable target for therapy, she says, describing it as a key player in several species.

In summary, Marsella says atopic dermatitis affects animals in a similar fashion to the human disease.

“For animals such as dogs that have embraced lifestyle changes similar to people (e.g., increased exposure to clean, indoor environments and increased consumption of processed foods), these changes have increased the risk of development of allergic disease.

“Whether increased allergies also are developing in horses and cats due to the fact that animals are dewormed more frequently and therefore have less exposure to parasites than they did in the past remains to be established.

“Our approach to atopic dermatitis has changed over time to become more holistic and more focused on restoring rather than suppressing the immune system.

“Sustainability and safety are key for long-term management,” she says.

“Regardless of the species, allergen-specific immunotherapy remains the most desirable long-term option with the intent of re-educating the immune system and decreasing the need of rescue medications.”

Marsella, R. Atopic Dermatitis in Domestic Animals: What Our Current Understanding Is and How This Applies to Clinical Practice. Vet. Sci. 2021, 8, 124. https://doi.org/10.3390/vetsci8070124

The study, published under a Creative Commons License, https://creativecommons.org/licenses/by/4.0/ can be read here

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