Grass Contact Dermatitis in Dogs

In the Literature

Mason K, Ruutu M. Canine dermatitis on contacting grass leaf: a case series. Vet Dermatol. 2023;34(2):115-124. doi:10.1111/vde.13143

The Research …

Contact dermatitis is a type IV hypersensitivity reaction that occurs when chemicals known as haptens come into contact with skin. Protein contact dermatitis (PCD) in humans is thought to involve type I and IV hypersensitivities triggered by insect or animal proteins and has a similar distribution to contact dermatitis.¹ Dogs that develop reactions to contact with grass may experience a similar pathogenesis to PCD.

This retrospective study reports patient history, clinical signs, and diagnostic findings of a previously undescribed pruritic skin disease in 18 dogs in Australia. Primary lesions were confined to contact areas (eg, ventrum, interdigital skin, distal limbs, facial area around the muzzle) and included erythema, macules, and papules with secondary alopecia, hyperpigmentation, and lichenification. Dogs living outside and confined to a grassy area had the most severe signs. Average age at onset of clinical signs was 9.4 months, and average age at presentation was 3 years. Other pruritic dermatoses were ruled out.

PCD was diagnosed based on clinical signs, diagnostic test results, response to therapy, and relapse on challenge. Compatible clinical signs included pruritus, inflammation, and distribution of signs consistent with atopic dermatitis or atopic-like disease, although flexural surfaces were not commonly affected. Concordant test results included cytology negative for infection, histopathology inconsistent with atopic dermatitis, weak or negative allergy tests for common aeroallergens, and immediate positive crude scratch test to local grasses and plants. Resolution of clinical signs after bathing with a medicated shampoo and removal of contact with plant material (not aeroallergens), as well as reappearance of clinical signs within 24 hours after re-exposure to plant material, were also required for confirmation. 

 … The Takeaways

Key pearls to put into practice:

• PCD should be a differential diagnosis for dogs with pruritic skin disease that principally affects contact areas, especially in dogs that appear reluctant to walk on and are irritated by wet grass.

• Other causes of pruritic skin disease, including infection, ectoparasites, and food- and environment-induced atopic dermatitis, should be ruled out. Patients with PCD have weak or no positive responses to allergen testing for aeroallergens and an immediate reaction to local plant allergens on a scratch test (where available).

• Diagnosis of PCD requires demonstration of clinical signs when contact is made with the inciting plant allergen (typically grass) and resolution when contact is withdrawn.

• More studies are needed to confirm diagnosis of PCD in a larger and more geographically diverse population of dogs.