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Members of the Eucalyptus genus are quick-growing evergreen trees or shrubs. Some species can reach heights over 135 m. The stem and trunk bark of the eucalyptus may be dappled in grey, green, russet or cream, and may peel in sheets. These trees have leathery, smooth, lance-shaped leaves, which are studded with glands containing a fragrant, volatile oil with a distinctive aroma.

Eucalyptus originated in Australia. There are over 700 species of the tree, which now grows in almost all tropical and subtropical areas. It is found in north and South Africa, India, southern Europe, California, and the Mediterranean countries.

The puffball-like flowers may be red or orange. They have no petals; instead, numerous stamens arise from a capsule-like calyx, which give them their fluffy appearance. Eucalyptus flowers in the tropics almost year-round.

Eucalyptus thrives in any environment with a mean annual temperature above 60°F (15.5°C). Whole eucalyptus forests have been planted in California and around the Mediterranean to help stem erosion. Eucalyptus has been planted in swampy areas, where its fast growth demands much water and performs a draining function.

Different species of eucalyptus are grown for timber, paper, oil and gum production. The oils of the leaves and shoots are used for medicinal purposes: for example, as an ingredient in tablets for soothing sore throat. The oils may be divided roughly into 3 classes of commercial importance: (i) the medicinal oils, which contain substantial amounts of eucalyptol (also known as cineol); (ii) the industrial oils, containing terpenes; and (iii) the aromatic oils. Tannin is extracted from the bark of some varieties.

IgE-mediated reactions

Eucalyptus is known to cause cutaneous diseases through its oil or pollens, and respiratory allergic diseases through its pollens. Eucalyptus pollen may result in symptoms of asthma, hay fever and conjunctivitis.(1-3)

A 30-year-old woman with asthma and rhinoconjunctivitis was described. Her symptoms appeared to be exacerbated by eucalyptus pollens, and by ingestion of an infusion containing eucalyptus. Specific IgE was positive for eucalyptus pollens. The authors suggested that care should be taken in administering herbal medications in the case of asthmatic subjects. (1)

Eucalyptus pollen has been shown to be an important airborne allergen in Taiwan, with 76% of 195 children aged between 5 and 15 years demonstrating specific IgE to this pollen in their serum. (3) Specific assays to determine specific IgE to eucalyptus pollen have been shown to be effective. (2)

An aerobiological survey conducted in West Bengal, India, of 31 pollen types found that the pollens showing the strongest sensitizing potential included E. citriodora (lemon-scented gum). Skin-prick testing for sensitization to this tree was positive in 89/260 (34.23%). (4) Similarly, in an ‘All India Coordinated Project on Aeroallergens and Human Health’ undertaken to discover the quantitative and qualitative prevalence of pollens at 18 different centres in the country, it was found that the predominant airborne pollens included eucalyptus. (5) A more recent study of 331 Indian patients suffering from nasal allergy, nasal polyps, allergic conjunctivitis and allergic asthma were assessed for sensitization to a range of aeroallergens, and 321 patients showed significant positive results. Parthenium hysterophorus was found to be the most common (30%) followed by Prosopis julifora (25%). Approximately 20% of the group were sensitized to Eucalyptus spp. (6)

A study conducted in South India evaluating sensitization to Nile trumpet tree (Dolichandrone platycalyx) evaluated 317 patients with allergic rhinitis and asthma attending a tertiary care centre, and found using skin-prick tests that 16% of patients were sensitized to Dolichandrone pollen extract, which was the fourth-most common sensitizer after Parthenium hysterophorus, Prosopis juliflora, and Artemisia vulgaris. Approximately 7% of the subjects were sensitised to pollen from Eucalyptus globulus. (7)

An atmospheric survey of the eastern Mediterranean coast of Turkey found that the most prominent tree pollens were Cupressaceae, Eucalyptus and Pinus. The most common herb pollen was from the Chenopodiaceae family. (8) Eucalyptus pollen has also been documented in pollen studies in south-western Spain, (9) and Athens, Greece. (10)

Similarly, a study of the prevalence of aero-allergen sensitization in atopic asthmatic adults in Mexico reported that almost 7% were sensitized to Eucalyptus spp. Other prominent tree allergens included Acacia spp, Quercus spp, Fraxinus spp, Populus spp, Prosopis spp, Schinus molle, Alnus spp and Juniperus spp. (11) Similarly, a skin-prick test study conducted on 965 patients aged 16 to 78 years with allergic rhinitis in the Guadalajara metropolitan area found that 78% returned positive skin-prick tests, with 6% sensitized to Eucalyptus sp. The most frequent outdoor allergens were Quercus sp and Fraxinus sp. (11)

Contact urticaria due to eucalyptus pollen has been reported. (12)

Other reactions

Eucalyptus oil is distilled from the fresh leaves of Eucalyptus globulus and other species. It may result in toxicity when ingested orally. A 6-year-old girl presented with slurred speech, ataxia and muscle weakness progressing to unconsciousness following the broad application of a home remedy for urticaria containing eucalyptus oil. (13)

Eucalyptus oil may result in allergic contact dermatitis. (14)

Euproctis edwardsi, the mistletoe browntail moth, is a variety of hairy caterpillar widely distributed in south-eastern Australia. Itching urticarial wheals and papular eruptions can follow contact with the caterpillars or their detached hairs. Infestation with Euproctis edwardsi can be minimized by removal of mistletoe from eucalyptus trees. (15)

A 46-year-old woman with vocal-cord dysfunction precipitated by eucalyptus odour exposure was described. (16)

No allergens from this plant have yet been characterized.

Cross-reactivity

A high degree of cross-reactivity between the different eucalyptus species can be expected, and occasionally also between these and species of the genus Melaleuca. (17)

Last reviewed: April 2022