Melaleuca species: Melaleuca

Family

Myrtaceae

Common Name

Parebark, melaleuca, Tea tree (Melaleuca alternifolia), broad leafed paperbark, punk tree (M. quinquenervia), the latter species being a noxious weed in Florida. Melaleuca are closely related to the genus Eucalyptus.

Plant Description

A large genus of 150 shrubs and trees, Melaleuca are indigenous principally to Australia and Southeast Asia. Some species have a papery bark that peels off in sheets. Leaves are generally simple, leathery, and either flat or cylindrical. Inflorescences are profuse, bottlebrush-like spikes, with flowers having showy stamens and colors of white, pink, purple, or red (Fig. 263 and Fig. 264).

Figure 263. Melaleuca quinquenervia.

Figure 264. Melaleuca quinquenervia flowers.

Toxic Principle and Mechanism of Action

Melaleuca alternifolia contains an essential, pungent smelling, colorless or light yellow oil similar to Eucalyptus oil.

Toxicity is due to the presence of cyclic hydrocarbon terpenes, sesquiterpenes, and various oils that are readily absorbed through the skin and mucous membranes [1]. The mechanism of toxicity has not been determined. Melaleuca oil has antibacterial and antifungal properties and has been used topically on dogs and cats to treat skin infections and repel fleas. The oil is also toxic to people as it is readily absorbed through the skin or if ingested [2-4]. Skin absorption is increased if the melaleuca oil is applied with organic solvents such as alcohol or dimethylsulfoxide (DMSO).

Melaleuca have not been associated with cyanide poisoning that has been encountered in sheep, cattle, and goats eating the leaves from recently felled trees of Eucalyptus cladocalyx (sugar gum) and E. viminalis (manna gum) [5,6]. Eucalyptus oil if ingested is toxic.

Risk Assessment

Animals are unlikely to eat the leaves of the plant because of the strong pungent odor of the leaves. Most animal poisoning from Melaleuca arises from the application of the oil to the skin and hair coat as a means of cleaning the hair or as a treatment for various dermatologic diseases including ectoparasites.

Clinical Signs

Topical application of melaleuca oil or products containing the oil can cause ataxia, incoordination, muscle weakness, hypothermia, depression and behavioral abnormalities [1,[7]. Severely affected animals may have elevations in liver enzymes, and blood urea nitrogen. Recovery can be expected if the animals can be bathed using mild soap to remove residual oil on the hair and skin. Activated charcoal should be administered orally to prevent further absorption of melaleuca oil that may have been ingested after animals have licked there haircoat. Melaleuca oil or other allergens in the plants are capable of inducing atopic allergic dermatitis in animals [8].