Toxic Fungi of Western North America

by Thomas J. Duffy, MD

Isoxazole poisoning from Amanita muscaria & Amanita pantherina (pantherine syndrome)

Two Amanita species (Amanita muscaria (L.:Fr.) Pers. and Amanita pantherina (DC.:Fr.) Krombh.) are responsible for a quite different type of poisoning caused by the toxins ibotenic acid and muscimol. Other toxins appear to be present as well. Such additional toxins may explain, for example, why vomiting is more likely with Amanita muscaria as compared to Amanita pantherina (the most toxic over-all). Both of these amanitas have several variants. (91)

Amanita muscaria is present in large quantities along the coasts of Northern California through British Columbia, most commonly in the variety muscaria and in the subspecies flavivolvata. The former is not uncommon in California, but gives way to the latter in the PNW and in Colorado.

Amanita muscaria
Amanita muscaria photo © Michael Wood

Amanita muscaria var. muscaria Pers. has a red cap whose surface has white warts (easily removed), white gills, a white ± persistent ring and a white stipe. The volva consists of 2-3 (4) ± complete rings just above the swollen base of the stipe. This mushroom is widely distributed in Western North America, including Mexico. The red cap fades to reddish orange in sunlight. The more detailed description below for subspecies flavivolvata holds in general for this and the other varieties of Amanita muscaria, except for the coloration and some variation in size.

Amanita muscaria subsp. flavivolvata Singer has removable creamy to yellow warts on a deep golden tan, reddish orange to red cap, ± viscid, smooth, 5-17 (39) cm across. Bright red caps in full sun quickly fade to a more orange color. The cap begins as a convex to almost hemispheric structure and becomes plane to depressed in age, sometimes humping slightly in the center. The margin has short striations in maturity. The flesh is white with yellow to reddish tissue just below the cuticle. The white to pale cream gills are attached then free, broad, crowded with numerous shorter gills. The edges of the gills are smooth to rough. The stipe is white to cream, tan when handled or bruised, 6-16 (20) cm long and 1-3 (4) cm wide at the top, enlarging to a ± bulbous base. The surface is smooth above the ring and fibrillose to scaly below. The ample ring is persistent, membranous, white with a ± yellow edge and often a yellowish tint to the undersurface of the ring. The volva is poorly formed and presents more as a series of fragmented concentric rings. Found from California originally; this variety is uncommon there, but common in the PNW and in the Rocky Mountains. Its range also extends from New Mexico and Arizona down into Mexico's Sierra Madre Orientale.

Amanita muscaria var. guessowii Veselý [=Amanita muscaria var. formosa sensu. auct. amer.] has a yellow-orange, yellow or yellow-tan cap when young. It is found in the PNW, California and Mexico, but is uncommon. In California, what is probably this subspecies, is found in the mountains from 3500 feet up to the subalpine zone in the Sierra Nevada and Cascade mountains. There is one low-land collection from Napa county. (93) In the PNW, it is found occasionally in both montane and sea-level areas. In Mexico, the range of this species is restricted to the higher elevations.

Presumably, other varieties of Amanita muscaria are toxic and, possibly, also the closely related Amanita breckonii Ammirati & Thiers.

Amanita muscaria var. alba Peck is a white to yellowish-buff form in the PNW. There are 2 California collections of this form from the northern coastal forests of Humboldt and Del Norte counties. (93)

Jenkins noted the presence in Alaska of what he thought was identical to a Scandinavian taxon—Amanita muscaria var. regalis. However, this mushroom is now considered to be a separate, but closely related species--Amanita regalis (Fr.) Michael). (94) Dr. Rodham Tulloss and Dr. Joe Ammirati have also compared this taxon with the Scandinavian one and agree they are not the same.(95) The cap is yellow to golden brown with yellowish or light tan floccules.

Amanita pantherina (DC.:Fr.) Krombh. resembles Amanita muscaria with its warty cap and ±persistent ring; but the cap color is different, usually a yellow brown to dark brown. The bulbous stipe base also differs in that the volva adheres to it at the base with an abrupt inrolled collar at its apex; sometimes there are 2-3 volval rings from the universal veil just above the major portion of the volva. Amanita pantherina is quite common in California and Oregon, but is found less frequently in southern California and Baja California.

Amanita pantherina
Amanita pantherina photo © Michael Wood

There is enough variation in Amanita pantherina that it is best to give a standard description. The description chosen is that of the San Francisco Bay area mushroom.

The cap is 4-15 cm broad with ground color dark brown or tan, sometimes buff yellow, although often with a darker apical area. The shape of the cap is convex to subglobose, becoming plano-convex to plane, depressed or umbonate in age. The margin of young caps is incurved to decurved becoming uplifted in senescence and is striate to tuberculate-striate. The striations are 6-10 mm long and less evident in age. The surface is viscid to subviscid, usually with flat to pointed, floccose, loosely attached universal veil remnants, which are whitish to buff, sometimes arranged in concentric rings. The cap is often glabrous in age. The flesh is white except for a pale yellow next to the cuticle, firm, 3-14 mm thick; odor and taste mild. The gills are bluntly attached, usually adnexed but becoming deeply notched to free. The gills are close to subdistant, white becoming buff; margin fimbriate then smooth. The stipe is 5-15 cm long, 8-20 mm broad at the apex, equal to tapering or clavate, bulbous at base. The stipe surface is moist to subviscid, pruinose to appressed fibrillose and longitudinally striate above the superior skirt-like ring, which collapses in age. The color of the stipe is white to buff, staining buff or brown when bruised. The volva is white, consisting of one to several rows of irregular tissue, which is inrolled at the bulb apex forming a ± distinct collar, occasionally disappearing with age. The identification depends primarily on the distinctive arrangement of the universal veil remnants on the cap along with the inrolled volva. The mushroom is commonly found under Monterey pine in the San Francisco Bay area.

Amanita pantherina var. pantherinoides (Murrill) Dav.T.Jenkins, having either a free-limbed short portion of the volva or an abrupt collar, occurs at least in Oregon and Washington. This variety is usually brown at the apex, but the rest of the cap is a warm pale tan or honey yellow.

A form of Amanita pantherina in which the young buttons are white is found in Colorado, but apparently has not been studied extensively. (91)

Both Amanita muscaria and Amanita pantherina occasionally have specimens low in ibotenic acid/muscimol. (96) Dr. Joe Ammirati notes that one or another variety of Amanita pantherina may be found in every month of the year.

Isoxazole poisonings have been occasionally reported with what were thought to be Amanitas intermediate in character between Amanita gemmata and Amanita pantherina. However, Amanita gemmata may be a complex of closely related species, ±yellow or yellow tan, whose volvas are not always the typical membranous sheath. The intermediate forms have also been thought to be “hybrids” in Northern California and in the PNW. (97)