Contact allergy: another occupational risk to Tetranychus urticae

Tetranychus urticae is a phytophagous, macroscopic mite ubiquitous to warm climates, which causes occupational allergic symptoms in farm workers.

This study was designed to investigate the cause and mechanisms of an otherwise "undetermined" recurrent and occupational dermatitis on the hands of farm workers and to obtain a better diagnostic profile of the urticaria that many farm workers who are allergic to T. urticae experience.

Fifty-six farm workers with occupational cutaneous and/or respiratory symptoms and 10 healthy farm workers underwent in vivo and in vitro diagnostic trials by skin prick testing, serum-specific IgE determination, and three different types of epicutaneous tests.

Four of the 10 patients with recurrent dermatitis had a positive IgE reaction to T. urticae, and five of the 10 had a positive patch test response. The percentage of positive patch test responses between subjects with positive T. urticae IgE and those with negative T. urticae IgE was similar. The diagnostic trial allowed the diagnosis of immune-mediated contact dermatitis in seven of the 10 patients with recurrent occupational dermatitis on the hands. The result of the epicutaneous open test on healthy skin with no history of eczema was positive in almost all patients with IgE to T. urticae and episodes of occupational urticaria.

Urticaria frequently occurring in farm workers allergic to T. urticae is an allergic contact urticaria. T. urticae is also directly responsible for recurrent dermatitis, caused by a different immune-mediated mechanism, in farm workers.