LAUSR

Urtica ferox (ongaonga) is a tree nettle native to New Zealand that has pin-like trichomes (Fig. 1) containing numerous defensive molecules. Exposure to these trichomes results initially in pain, followed by numbness that can last for days. Typically, this effect is localised to the region of exposure; however, systemic effects and even death have been reported after extensive exposure. Recent work has identified two neurotoxic compounds present in U. ferox, yet the clinical characterisation of the postpain sequelae remains incomplete. Following earlier work, here we provide the first prospective clinical characterisation following controlled exposure to a standardised extract of U. ferox. Standardised extracts of U. ferox trichomes were made using an established protocol. The extract was sterile filtered and dried and aliquots of extract were reconstituted in vehicle so that 5 μL of vehicle contained the amount of extract derived from 10 trichomes. The trial protocol was approved by the New Zealand Human Disabilities and Ethics Commission (19/NTA/107) and registered (ACTRN12619000804189). Based on preliminary data, a sample size calculation returned seven participants (sample size for Fisher exact test P1 = 0.9; P2 = 0.1; alpha one-sided = 0.025; power 0.8). Participants were recruited through the local newspapers. Sites for cutaneous treatment applications were drawn as 15mm circles on the forearm with indelible ink and each site numbered (Fig. 1C). Application sites were shaved to remove hair. The extract, negative control (vehicle) and positive control (Emla Cream; 5% lidocaine and prilocaine) were randomly assigned a number for each participant. Participants were blinded to the treatment application. For each treatment, 5 μL of solution was applied, the application area was subsequently scratched in a hashmark pattern using a single-use allergy lancet (Inmunotek PSEP-0032) and covered with a nonabsorbent dressing (Nexcare, Tegaderm, 3M) for 1 h. After the incubation period, the participants removed the