Mind the pine: Suspected environmental allergic contact dermatitis to constituents of pine tar

Wood tars are liquid byproducts obtained from the pyrolysis of wood. Pine tar has been used topically for >2000 years for antiseptic and soothing properties.1 Though specific wood tars differ, a mixture of tree tars (pine, beech, birch, juniper) is generally used in patch testing.2 We present two cases wherein patch test reactions to pine tar and Abitol (a semi-synthetic mixture of different hydroabietyl alcohols produced from colophony) were integral in establishing a diagnosis of allergic contact dermatitis from recreational contact with pine trees. A 25-year-old female was referred to our clinic for patch testing after having several episodes of localised, pruritic and vesicular dermatitis. Her initial episode was the most severe, with edematous papules coalescing into plaques on her buttocks, posterior thighs and low back (Figure 1A). Prior to this eruption, she reported hiking in the woods while on a camping trip and recalled 'sliding' across fallen trees while wearing a bikini. She was patch tested to the North American Contact Dermatitis Group (NACDG) standard series in addition to several supplemental series, including our plants/woods series (SmartPractice, Phoenix, AZ and Chemotechnique Diagnostics, Vellinge, Sweden) using Finn Chambers (SmartPractice). At the final (96-h) reading, several potentially relevant reactions were observed: (3+) to wood tar mix 12% gel (%w/w: beech tar 3%, birch tar 3%, Juniperus oxycedrus extract 3%, pine tar 3%), (2+) to pine tar 3% in petrolatum (pet), and (1+) to Abitol 10% pet. Interestingly, she tested negative to colophonium 20% pet as well as abietic acid 10% pet, oxidised turpentine oil 0.4% pet, fragrance mix 8% pet and other fragrances. A 67-year-old male presented for patch testing after developing erythematous, scaly plaques on the web spaces of his hands each time he returned from camping in Montana. He reportedly used a rope to string up equipment from a pine bough on these trips. A biopsy obtained by the referring clinician demonstrated spongiotic dermatitis with eosinophils. He was tested to the NACDG standard series, our plants/woods series, and other history-driven supplemental series (SmartPractice; Chemotechnique Diagnostics) along with select personal products 'as-is', including the used camping rope. At the final (144-h) reading, he had reactions to several botanicals: (2+) to Compositae mix 5% pet, sesquiterpene lactone mix 0.1% pet, Arnica montana 0.5% pet, and feverfew 0.1% pet, (1+) to chrysanthemum 1% pet, wood tar mix 12% gel, pine tar 3% pet, Abitol 10% pet and the camping rope 'as-is' (Figure 1B). He also did not react to colophonium, abietic acid, turpentine oil or other fragrances. Allergic contact dermatitis (ACD) to wood tars is rare, and reactions have been reported primarily through occupational and airborne exposures.1, 3, 4 Pine tar allergy specifically is not well-described, with prior reports instead utilising pine sawdust mixed with petrolatum or pine needles as-is.5-7 The cases presented herein demonstrate that ACD to pine trees may be clinically relevant through direct environmental exposure. Pine tar may retain constituents of the tree's oleoresinous exudate after synthesis via pyrolysis.1 We hypothesize that this oleoresinous exudate was the likely cause of ACD to pine trees via recreational exposure and underscore the utility of patch testing to pine or other wood tars when indicated. Interestingly, reactions to Abitol (but not colophonium) were positive in both cases. This may be a suitable test substance if pine or wood tar mix is unavailable. Puneet Arora: Conceptualization; investigation; writing – original draft; visualization; writing – review and editing; methodology; data curation; software; project administration; formal analysis. Caroline Brumley: Writing – review and editing; validation; investigation; methodology. Katherine Lee: Conceptualization; investigation; validation; resources; supervision; data curation; methodology. Anne Neeley: Writing – review and editing; conceptualization; investigation; methodology; validation; formal analysis; data curation; supervision; resources. The authors declare no conflicts of interest.