Rhododendron species: Rhododendron, azaleas

Family

Ericaceae (Heath family).

Common Names

Rhododendron, azalea, mountain rosebay, red laurel, rosebay laurel, great laurel, and many regional names such as Cascade rhododendron, California rosebay.

Plant Description

With as many as a 1000 species, and as many or more cultivars, rhododendrons are native to the acidic soils of the temperate northern hemisphere and southeast Asia. In general, the name azalea is given to the deciduous species, while the evergreen species are called rhododendrons.

Ranging from small shrubs to erect, branching small trees with rough or smooth bark, Rhododendron species have alternate, glabrous or hairy, dark green, revolute, deciduous or evergreen, elliptic to lanceolate leaves. Inflorescences are terminal umbellate, racemose or corymbose clusters of showy flowers. Consisting of 5 fused sepals and 5 fused petals, the flowers are generally funnel-shaped, in virtually all colors except blue (Fig. 337, Fig. 338, Fig. 339 and Fig. 340). Fruits are hard or soft capsules.

Figure 337. Rhododendron (Azalea).

Figure 338. Rhododendron species.

Figure 339. Rhododendron exbury (Azalea).

Figure 340. Rhododendron cultivar.

Toxic Principle and Mechanism of Action

All species of the family Ericaceae contain varying quantities of toxic diterpenoids collectively known as grayanotoxins I and II (formerly andromedotoxin, rhodotoxin, and acetylandromedol) [1]. As many as 18 grayanotoxins (I - XVIII) have been identified, the greatest number being found in the Leucothoe species (fetter bush) [2,3]. Tannins and other compounds are also present in varying amounts. All parts of the rhododendrons including the flowers and the nectar are toxic, although there may be considerable variation between species and even amongst plants of the same species depending on the growing conditions.

Grayanotoxins act to increase sodium channel permeability of cells by opening the channels to sodium, which enters the cells in exchange for calcium ions, thus rendering the channels slow to close so that the cell remains depolarized [4,5]. Other neurologic mechanisms may also involve a cholinergic response seen clinically as bradycardia and excessive salivation.6 The cardiac effects can range from bradycardia, sinus arrest, and arrhythmias.

Other members of the Ericaceae that contain grayanotoxins include:

Andromeda polifolia         Andromeda, bog rosemary

Kalmia spp.                        Laurels

Ledum spp.                        Labrador tea

Leucothoe spp.                  Fetter bush, dog laurel

Lyonia spp.                         Maleberry

Menziesia spp.                   Rusty menziesia

Pieris spp.                           Pieris, Japanese pieris

Risk Assessment

Rhododendron species are commonly grown as showy garden plants, and azaleas are frequently sold as flowering potted plants for indoor use. Household pets, while not commonly poisoned, are at risk of poisoning if they eat the leaves and flowers. Livestock are poisoned more often where rhododendrons are accessible to the animals, especially in winter time when the evergreen leaves are an attraction. Honey made by bees feeding on the nectar of rhododendrons has long been known to be toxic to people who eat the "mad-honey" [7].

Clinical Signs

Excessive salivation, increased nasal secretions, vomiting, abdominal pain, bloat, and irregular respirations develop several hours after rhododendron leaves are ingested [8-10]. Projectile vomiting may be noticeable. Hypotension, tachycardia, and respiratory depression may also develop. Weakness, partial blindness, and seizures have been reported in severe intoxications. Neurologic signs may persist for several days before the animal recovers. Weight loss may be notable.

Treatment is primarily directed at relief of the more severe clinical signs. Activated charcoal given orally (2 - 8 g/kg body weight) is helpful if given shortly after the rhododendron is consumed. Atropine is useful in countering the cardiovascular effects.