What Are Insoluble Calcium Oxalate Crystals?

First described in 1675 by Dutch microscopist Antonie van Leeuwenhoek, insoluble calcium oxalate (CaOx) crystals are the most abundant biomineral in plants that are present throughout the tissues of 200 plant families and are a potential cause of toxicity in pets.

Calcium oxalate crystals are found in five morphologies

  • Raphides (needle-shaped bundles)
  • Druses (spherical aggregate of individual crystals)
  • Styloids (elongated crystals with ridged or pointed ends)
  • Prismatic (four or more sides similar in length and width)
  • Crystal sands (minute crystals in a single cell).

Raphides pose the greatest risk due to their shape which allows them to penetrate tissues.

Calcium oxalate crystal shapes
A vintage image of some calcium oxalate shapes

The reaction to insoluble calcium oxalate-containing plants is classed as mechanical irritant contact dermatitis, due to the physical injury it causes to the oropharynx.

What is the function of insoluble calcium oxalate crystals?

The function of insoluble calcium oxalate crystals varies according to the plant species and the type of crystals present.

  • Protect the plant against herbivory. When an insect or animal chews on a CaOx-containing plant, the needle-sharp crystals penetrate the oropharynx causing intense pain and burning. Some crystals also have also barbs and grooves that anchor insoluble calcium oxalate crystals to the wound, allowing proteolytic enzymes to penetrate the tissues, intensifying pain and burning.
  • Regulation of free calcium in the tissues.

What happens when a plant containing insoluble calcium oxalate crystals is consumed?

The severity of signs depends on the concentration and type of oxalate crystals present in the plant tissue. When an insect or animal chews a plant containing raphides, the needle-sharp crystals are released from the cells and penetrate the delicate tissues of the lips, mouth and throat causing pain, burning, swelling, and difficulty swallowing.  Spinach contains insoluble calcium oxalate crystals with no ill effects, due to their block shape.

Needle sharp raphides

Symptoms of insoluble calcium oxalate exposure include pain, burning, swelling, drooling, and loss of appetite.

Dieffenbachia (dumb cane), is a popular house plant with a dark history. This plant was used by Caribbean sugar plantation farmers to punish Jamaican slaves by rubbing their mouths with cut stalks which caused painful swelling, blisters and an inability to speak. This is where the name dumb cane originated, and why the scientific name Dieffenbachia should be used in its place. Nazi Heinrich Himmler proposed the use of Dieffenbachia seguine to mass sterilise prisoners, however, he was unable to obtain the plants from Latin America.

Plants that contain insoluble calcium oxalate crystals

Common plants that contain insoluble calcium oxalate crystals, this list is by no means extensive, and it is always recommended you research the toxic properties of any plant you bring into the home.

  • Monstera species (Swiss cheese plant, Monstera adansonii etc.)
  • Aglaonema (Chinese evergreen)
  • Anthurium species (Flamingo plant)
  • Dieffenbachia species
  • Philodendron species
  • Rhaphidophora species (Mini monstera)
  • Spathiphyllum (Peace lily)
  • Caladium species
  • Zantedeschia species
  • Syngonium species
  • Epipremnum (Devil’s ivy)
  • Alocasia species (Elephants ears)
  • Caladium species
  • Brassaia (Umbrella Tree)
  • Parthenocissus quinquefolia (Virginia creeper)
  • Rheum rhabarbarum (Rhubarb)

First aid

Because pain is almost immediate, most children and animals will not eat a significant amount of plant material. However, in some cases, swelling can occur in the throat, resulting in airway obstruction which requires immediate medical care. There is a single case of a dog fatality after consuming Dieffenbachia. Large ingestions of plant matter that contains CaOx can lead to gastrointestinal disturbances, but this is rare due to the immediate pain experienced upon exposure.

If a child or pet consumes a plant containing insoluble calcium oxalate crystals, carefully remove any remaining plant matter from the mouth and offer something cool, such as milk, yoghurt, or onion-free stock (for pets).

Should you keep plants with insoluble calcium oxalate crystals?

There is always a risk that if a child or animal ingests any part of a plant that contains insoluble calcium oxalate crystals they will experience pain. This is usually enough to stop further ingestion and is generally considered mild and self-limiting. Some children and dogs will consume anything and therefore should not have access to insoluble calcium oxalate-containing plants.