THE MERCK VETERINARY MANUAL
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Overview of Poisonous Mushrooms

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Mushrooms are the fruiting bodies of a variety of fleshy fungi that are found worldwide. They typically appear suddenly from growing vegetative (mycelium) portions, and they contain spores as reproductive units. Fungi lack chlorophyll; their nutritional requirements are met by utilizing organic material from a saprophytic, parasitic, and/or mycorrhizal life cycle.

Exposure is by ingestion. Although most mushrooms are edible and safe, a few contain diverse secondary metabolites (cyclopeptides, monomethylhydrazine, orelline/orellanin, muscarine, ibotenic acid and muscimol, psilocybin, and unknowns) that on ingestion/absorption result in mild to severe illness and even death. The mushroom species most frequently implicated in human/animal mushroom fatalities globally is Amanita phalloides. Most Amanita species are typically identified by their physical characteristics: veil (universal/partial), cap or pileus (with scales—remnants of universal veil), lamellae (gill-like, spore-bearing structure under surface of pileus), spores (reproductive structure—white to black and other shades of color), stipe/stalk (cap support), annulus or ring (remnant of partial veil on stipe below pileus), volva (remnant of universal veil at base of bulb), and mycelium. Other characteristics helpful in the identification of some poisonous mushrooms are listed in Table 1: Physical Characteristics of Common Poisonous MushroomsTables.

Table 1

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The time lapse (latent period) between mushroom ingestion and observed clinical signs in exposed animals largely dictates the prognosis. A long delay time is synonymous with fatality (Table 2: Poisonous Mushrooms: Onset of Action and Organs TargetedTables). However, short latent periods do not always indicate survival, because the animal may have ingested a combination of non-lethal/lethal mushroom species growing in the same location (dilution effect, or dose determines toxicity). In mushroom species with clinical signs that appear <3 hr after ingestion, effects are generally self-limiting and not life-threatening. Those with clinical signs that appear >6 hr after ingestion are life-threatening.

Table 2

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The sudden appearance of mushrooms and their short lifespan within the environment, coupled with the indiscriminate eating habits of many animals, creates diagnostic challenges. History and time after ingestion at which clinical signs are seen determine the treatment approach and prognosis. Establishing the time of ingestion may be difficult to impossible. With no proven antidotes to treat mushroom poisonings, treatment is primarily directed at decontamination, mushroom identification when possible, and intensive supportive care.

Last full review/revision August 2014 by Cecil F. Brownie, DVM, PhD, DABVT, DABFE, DABFM, FACFEI

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