The Begonia is a genus of perennial flowering plants in the family Begoniaceae. Begonia is one of the largest flowering plant genera and contains more than 1,500 species and an extraordinary number of hybrids, cultivars, and ornamental forms. Because there are so many species and cultivated varieties, it is impossible to provide a single, one-size-fits-all physical description that accurately covers the entire genus.

In general, the majority of Begonia species are terrestrial, meaning they grow in the earth, although some are known to grow from other plants, trees, or similar structures as epiphytes. The plants are typically considered herbs or undershrubs and are native to subtropical and tropical moist climates of South America, Central America, Africa, and southern Asia. In the wild, terrestrial species are commonly upright-stemmed, rhizomatous, or tuberous. In most species, the fruit is a winged capsule containing numerous minute seeds, although berry-type fruits are also known. The leaves, which are often large and variously marked or variegated, are usually asymmetric or unequal-sided.

There are two potentially toxic agents of concern in the tuberous portion of Begonias: insoluble calcium oxalates and cucurbitacin B. Calcium oxalate crystals are one of the more common plant toxins and can be found in hundreds of popular houseplants and ornamentals. When consumed, calcium-oxalate-containing plants may cause an intense burning sensation of the mouth, throat, lips, and tongue; excessive drooling; choking; gagging; and potentially serious swelling of the throat that may cause difficulty swallowing or the inability to swallow, also known as dysphagia. Symptoms can occur immediately or within approximately two hours after ingestion, and irritation may continue intermittently after the exposure.

Plants containing calcium oxalate crystals contain special cells called idioblasts. Found in a number of plant species, both poisonous and non-poisonous, idioblasts differ from neighboring cells because they contain non-living substances such as oil, latex, gum, resin, tannin, pigments, or minerals. In this case, one of those substances is raphides, or bundles of needle-like crystals of calcium oxalate that tend to be blunt at one end and sharp at the other. The crystals are packed in a gelatinous substance that also contains free oxalic acid.

When an animal chews on the plant, the tip of the idioblast is broken, allowing saliva from the animal or sap from the plant to enter the cell. This causes the gelatinous material to swell, forcing the raphides, or needle-like calcium oxalate crystals, to shoot out from the cells into the surrounding tissue. The calcium oxalate crystals then penetrate and embed themselves into the tissues of the mouth, tongue, throat, and stomach, causing immediate discomfort and aggravation, much as would be expected when millions of microscopic needles become lodged in the mouth and throat.

The idioblasts may continue to expel raphides for a considerable amount of time after ingestion, allowing the crystals to embed themselves not only in the mouth and throat, but also in the lining of the stomach and intestines. This may cause additional gastrointestinal upset, including vomiting, gagging, dry heaving, diarrhea, abdominal discomfort, or refusal to eat.

The second toxic compound, cucurbitacin B, is a biochemical compound produced by the plant as a defensive mechanism to discourage feeding. Cucurbitacins are generally cytotoxic, meaning they can promote cell injury or cell death, and they may be extremely poisonous to some animal species in sufficient quantity. Cucurbitacins are also known for providing plants with an extremely bitter and displeasing taste, which further discourages prolonged chewing or large ingestion.

In the vast majority of cases, clinical signs will present immediately or within two hours of ingestion. Clinical signs include obvious pain and irritation, often expressed by violently shaking the head, drooling excessively, pawing at the mouth, gagging, vomiting, or dry heaving. The animal may also whine, bark, meow, or yelp in an unusually hoarse or weak-sounding voice. Other clinical signs may include depression, diarrhea, inappetence, and swelling of the mouth, throat, and tongue.

In some cases, swelling may be severe enough to restrict oxygen intake, resulting in dyspnea, shortness of breath, or gasping for air. Irritation of the stomach and intestinal tract may also result in diarrhea. Because the plant is both bitter in taste and acts as an immediate irritant to the mouth, large ingestions are uncommon. Additionally, the toxins of greatest concern are concentrated in the underground portions, or tubers, and the overall toxin amount is relatively low. For that reason, potentially life-threatening or serious intoxication from Begonia, while theoretically possible, is basically unheard of in ordinary companion-animal exposures.

It may be that, like the tomato, the potential danger posed by Begonia has been somewhat exaggerated in some sources. In regard to the plant’s toxicity toward humans, the University of Nebraska Cooperative Extension in Lancaster County has listed Begonia as non-toxic, with the notation that these plants have shown no more toxicity than an equal amount of vegetable matter and that symptoms are unlikely. This does not mean Begonia is harmless to pets, particularly if a tuber is chewed, but it does support the conclusion that most exposures are expected to be irritant and self-limiting rather than life-threatening.