Plant-induced allergic contact dermatitis (ACD) is a common disorder that can arise in numerous situations and places during daily life.1 Diagnosis is often challenging due to our lack of experience in its management.

We present the case of a 51-year-old woman with a long-standing history of rheumatoid arthritis on treatment with leflunomide. She consulted for an outbreak of pruritic lesions that had arisen 2 days earlier on her hands. Two weeks earlier she had developed lesions of similar characteristics that had resolved within a few days, after the application of topical corticosteroids. She stated that for the previous 2 months she had been applying an infusion of a plant called yellow fleabane (also known as sticky fleabane or false yellowhead) (Fig. 1) once a week to her hands to control pain. She prepared the infusion at home. Examination revealed multiple vesiculobullous lesions with a blood-stained content on a background of normal skin on the dorsum, palms, and fingers of both hands; in addition, she had desquamating erythematous plaques on both wrists (Fig. 2). Treatment with oral and topical corticosteroids and discontinuation of the application of the infusion led to complete resolution of the lesions and there have been no recurrences. The unknown irritant capacity of the plant led us to perform semi-open patch testing, applying fragments of the plant (stem, leaf, and flower) to the patient's forearm, covering the samples with porous tape and removing them after an hour. At 48hours, 2+ positive reactions were observed in the area of application of the leaf and stalk, with reactivation of the lesions previously present on the hands; these manifestations started to appear 24hours after performing the test. Control tests were performed on 10 healthy patients using the same technique with dry leaves and stalks, with negative results. Patch testing was also performed with the standard series of the Spanish Contact Dermatitis and Skin Allergy Research Group (GEIDAC), the Martitor plant series, and the infusion of yellow fleabane in water. A 2+ positive reaction to the infusion of the plant was observed at 48 and 96hours. All patch testing was performed in accordance with the criteria of the International Contact Dermatitis Research Group.

Figure 1.

Dry yellow fleabane (Dittrichia viscosa) provided by the patient.

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Figure 2.

Vesiculobullous lesions with a bloodstained content on the dorsum and palms of the hands, with some areas of erosion and desquamating erythematous plaques on the wrists.

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Yellow fleabane—scientific name Dittrichia viscosa, a plant species of the Asteraceae/Compositae family—is an aromatic Mediterranean shrub. This plant is widely used in homeopathy for its medicinal properties as it contains biologically active substances that act as modifiers of the inflammatory response,2 though they also have allergenic potential. These substances are the sesquiterpenic lactones (SL), a group of metabolites found in the oil-resins released from the leaves, flower stalk, and possibly from the polen.3 There are around 1350 identified varieties of SLs and approximately 50% of them have allergenic potential.4 Although the concentration of SLs varies between plant species, small similarities in their molecular structure can give rise to crossreactions.5 Identical SLs from different plant species can be responsible for false crossreactions. This situation, together with the few standardized tests for the study of this pathology, makes it difficult to study plant-induced ACD. Two preparations are available for patch testing. The “SL mix”, which contains 3 forms representative of the 3 main structural groups,6 only enables us to identify 60% of cases of SL sensitization. An alternative preparation for diagnosis is the “Compositae mix”, based on a mixture of multiple plant extracts; this has a considerably higher sensitivity (85%) than can be achieved with SL mix, although specificity is low.7

Ten cases of D. viscosa-induced ACD have been published. As in our case, those reports described positive patch testing to fragments of the plant. Additionally, positive reactions to extracts from other plant and to substances included in the perfumes series were observed in some cases, which supports the high probability of crossreactions.8,9 Negative patch testing to SLs, as in our case, does not exclude the diagnosis because of the test's low sensitivity.

The study of plant-induced ACD is a significant challenge, mainly because the majority of patients do not identify the specific trigger. This is further complicated by the high probability of crossreactions with substances obtained from other plants and the low specificity of standardized tests. The specific substance to which the patient has been exposed, if it can be identified, must be included in the study, avoiding tests with fresh plants or plant extracts due to the high risk of irritation or sensitization.9 We also draw attention to the importance of studying healthy controls using the same techniques, in order to classify the type of reaction as allergic or irritant.

This case highlights the need for dermatologists to recognize this type of reaction and the difficulties that can arise during its investigation.