Route Of Exposure

Thymus is a huge genus, containing 300 to 400 species, most of which are aromatic shrubs or perennials. All are native to Asia or Europe, most probably to Southern Europe. Thyme is now cultivated all over the world and has become naturalised in some areas, including the northeastern US.

Thyme is an evergreen perennial shrub with slightly fuzzy gray-green leaves about a 1.5 cm long. The tiny, tubular, lilac flowers are arranged in whorled, compact heads. Thyme grows in a thick, spreading mound 15-30 cm high and up to twice as wide. As with most mints, the stem is square in cross-section, and the leaves are arranged in pairs opposite each other. Leaves are highly aromatic (reaching their peak just before plants flower) and are frequently used fresh or dried as a seasoning.

There is a great variety of cultivars. Aureus, for example, has golden leaves; Orange blossom has leaves that smell like oranges; the leaves of Silver posie have white margins.

Thyme is one of the classic herbs, often used in soups, stews, sauces, and meat and fish dishes. Cajun and Creole dishes are especially likely to contain Thyme. Thyme is also used to make herbal teas.

The main essential oil in Thyme, thymol, is active against salmonella and staphylococcus bacteria. Thyme and Thyme oil are ingredients in fumigants, antiseptics, disinfectants, and mouthwashes. Thymol is one of the active ingredients in Listerine® mouthwash and provides the "medicated" properties of many consumer products. Oil of Thyme also functions as a carminative and counterirritant. It features in a number of homeopathic and folk remedies.

Clinical Relevance

Allergen description

No allergens from this plant have yet been characterised.

Potential Cross Reactivity

An extensive cross-reactivity among the different individual species of the Lamiaceae (Labiatae) family, which comprises plants such as hyssop (Hyssopus officinalis), Basil (Ocimum basilicum), Marjoram (Origanum majorana), Mint (Mentha piperita), Sage (Salvia officinalis) and Lavender (Lavandula officinalis), could be expected.

Clinical Experience

IgE-mediated reactions

Thyme may uncommonly induce symptoms of allergy in sensitised individuals. Although reactions may be uncommon or rare, they may also often be overlooked.

A 45-year-old man reported 3 reactions to food, as a result of Oregano on a single occasion, and twice to Thyme. He developed pruritis and swelling of the lips and tongue, dysphagia, dysphonia, and progressive upper respiratory difficulty as well as intense facial and palpebral oedema. On 2 occasions the reactions were severe, resulting in hypotension, vomiting, and nausea. His first adverse event occurred within minutes of the ingestion of pizza containing Oregano. The 2 adverse reactions to Thyme occurred after eating meat seasoned with Thyme, and Snails seasoned with Thyme. Skin-specific IgE was detected to a number of plants of the Labiatae family when the skin prick technique was used; when the prick-to-prick method was used, specific IgE was detected to members of the family with the exception of Basil and Lavender. Serum-specific IgE was detected to all herbs tested. The authors concluded that plants belonging to the Labiatae family show cross-sensitivity on the basis of clinical history and in vitro and in vivo test results.

Occupational asthma caused by working with a number of aromatic herbs, including Thyme, has been reported. The diagnosis was confirmed by inhalation challenges, and skin- and serum-specific IgE.

Other reactions

Similarly, among 46 farmers studied during the threshing of dried Thyme, 4 developed contact dermatitis after 5 to 30 minutes of exposure to Thyme dust. Thyme-specific IgE was found in 1 person with work-related symptoms, but also in 2 asymptomatic farmers. The authors proposed that an IgE mechanism may be questionable in eczema related to Thyme dust, that the aetiology of Thyme-related skin symptoms was obscure, and that an irritant mechanism seems probable.

A number of reports describe allergic contact dermatitis to Thyme oil or thymol. In a group of 100 patients with ulcus cruris, the incidence of contact allergies was determined with the use of patch tests for 18 products that might have come into contact with the skin. Balsam of Peru (14%), wool-wax alcohols (10%), para-aminobenzoate component (7%), neomycin (5%) and Thyme oil (5%) were the allergens most frequently encountered. Photoaggravated allergic contact dermatitis to Rosemary and Thyme was described in a 62-year-old woman who presented with several episodes of itchy hand, forearm and face dermatitis after picking Rosemary on sunny days. Skin-specific IgE testing with Rosemary leaves was initially negative, but a reaction was detected at the prick site 2 days later.

The patient was shown to be allergic to Thyme as well.

Allergic contact dermatitis due to thymol (a main component of Thyme oil) was reported in a 70-year-old woman 6 weeks after initiating treatment of 4% thymol once daily to a chronic paronychia; and similarly, in a report, pruritic contact dermatitis was noted to a topical application of the combination antiseptic solution Listerine to a chronic parenchyma of the toe. Patch testing with the individual ingredients revealed selective allergic hypersensitivity to thymol. Although massage with essential oils, including those manufactured from Thyme, may alleviate symptoms of eczema, individuals sensitised to Thyme may experience exacerbation of their eczema.

In a group of 47 Thyme farmers examined while threshing Thyme and exposed to organic dusts, 63.8% reported occurrence of work-related symptoms during this activity. The most common complaints were blocking of the nose (38.3%), dry cough (29.8%) and general weakness (27.7%). Lung function tests were normal and did not show a significant post-work decline. Positive skin hyperactivity to microbial antigens and positive precipitin reactions was demonstrated. The study concluded that Thyme farmers engaged in threshing of Thyme are at a greater risk for work-related symptoms as a result of associated organic dust. A number of molds has been associated with Thyme cleaning.

Allergic alveolitis was described in a female farmer as a result of massive exposure to organic dust contaminated with microorganisms during threshing of Thyme.