That Primula obconica and Rhus. Toxicodendron cause a very annoying form of dermatitis is well known, but less generally known is the fact that vanilla essence may, in certain individuals, cause equally troublesome symptoms.
A perfectly healthy patient of mine, of an inquiring turn of mind, liking the smell of vanilla, added about one-half ounce of the essence to about five ounces of a hair lotion consisting of quinine, spirits of lavender, and rectified spirit (which he was in the habit of using). About twenty-four hours after its application intense itching of the scalp occurred, which gradually extended over the forehead, behind the ears, and down the neck. It did not at first strike him that vanilla was to blame, and for a day he kept rubbing it in the hope of “cooling” the part, as he expressed it. The result was that his eyes were opened metaphorically to the cause although practically they were almost closed by the swelling which followed. Not content with this experience he performed a control experiment on the front of the forearm, because, as he said, it was difficult to believe that vanilla, which he had frequently swallowed in the form of a flavoring agent with nothing but agreeable results, could irritate the skin to such an extent without causing disastrous results to the more delicate mucous membranes. Twenty-four hours afterwards the same intolerable itching occurred on the forearm with an eruption which, from his description, seems to have been of a close popular nature with no reddening of the skin. This soon faded, only to return every five or six hours, as did the eruption on the scalp and face. This state of affairs continued for ten days in spite of constant washing with soft water and soap and other domestic remedies. He said he was compelled to seek advice, as the itching remained intolerable at night and prevented sleep.
When I saw him there were many excoriations on the scalp and face, and more on the arms, due to the scratching, and a popular eruption over the affected parts. There was still some oedema, but it evidently was much less than it had been, and he seemed to be getting well.
The condition was probably due to some alkaloid in the vanilla, and knowing that mercury iodide precipitates most of the alkaloids, I prescribed a lotion of rectified spirit and mercury iodide, 1 in 2000. This seemed to give relief, and the itching disappeared in two or three days.
The interesting points are: (1) The long time fourteen days – the symptoms lasted, in spite of the washing; (2) the intervals of freedom from irritation – about six hours – when it is remembered that the poison was purely irritant and not due to micro-organisms; (3) the fact, as the patient points out, that the vanilla could be taken by the mouth without producing any irritation when the skin was so susceptible. I am presuming that the irritation was not due to the action of micro organisms owing to the fact that the vanilla had been suspended in strong spirit for over a year; and there is no evidence that the irritation was merely of a mechanical nature. – Wm. Leggett, B.A., M.D., British Medical Journal.