The cactus family comprises perhaps 2000 species in 50–15 genera (the classification of the family is more than usually controversial). The plants are distinctive, often spiny, succulents most of which occur naturally in the arid regions of the Americas. Several species of Opuntia Mill. have been introduced into Australia, South Africa, India, and the Mediterranean region where they have naturalised and have often become troublesome as weeds.
Cacti are popularly grown and cultivated as house and greenhouse plants. They may easily be confused with certain cactiform species in the family Didiereaceae and more importantly with some Euphorbia L. species in the family Euphorbiaceae. Members of the genus Euphorbia differ from cacti in not having areoles at the spine bases (see below) and in having a white milky latex which in many cases has powerful skin irritant properties. These Euphorbia species are, furthermore, commonly grown in collections of succulents alongside cacti. The use of the term cactus in describing a cactus-like plant should therefore be accepted with scepticism, especially if an irritant (but not mechanical) dermatitis is suspected.
The red food colouring, cochineal, is obtained from certain beetles (Dactylopius coccus O. Costa and related species, fam. Dactylopiidae) that are cultivated on Opuntia species.
The dumpling cactus (Lophophora williamsii J. Coulter, syns Anhalonium williamsii Rümpl, Echinocactus williamsii Lemaire) is the source, when dried, of peyote or mescal buttons. These have been utilised in American Indian ceremonies for very many years as orally active hallucinogens. Mescaline is the main active constituent; it also occurs in some other species of cactus.
The fruits of certain Opuntia species, including Opuntia vulgaris Mill. (syn. Opuntia opuntia Karsten) and Opuntia ficus-indica Mill., are edible. They are known as prickly pears and Indian figs respectively, although the former name is often used loosely in place of the latter.
Many, but not all, cacti are capable of inflicting mechanical injury. This may be the result of penetration of the skin either by one or more erect spines, or by glochids — tufts of short, barbed or hooked hairs arising from pin-cushion like structures named areoles, from which the larger spines, if present, also arise. Eye injuries have also been recorded (Biger & Abulafia 1986). Glochids are a hazard of genera belonging to the tribe Opuntieae in particular (including Pereskiopsis Britton & Rose, Pterocactus Schumann, Tacinga Britton & Rose, Opuntia Mill., and Nopalea Salm-Dyck), and especially the spineless species. There is reason to believe that an allergic reaction may occur when glochids or spines enter the skin, possibly from fungal contaminants. No attempt has been made to list here all species that may be capable of causing mechanical injury.
This species is listed as a cause of mechanical injury (Oakes & Butcher 1962). Other species doubtless share this property.
The plant is used in Mexico as a rubefacient (Díaz 1976).
Wimmer (1926) referred to the spines of this species as a possible cause of mechanical injury.
The glochids from this species can produce "sabra dermatitis" (Shanon & Sagher 1956). See also Opuntia ficus-indica Mill.
The spines, especially the glochids, may produce infected sores of the face, mouth, tongue, throat, and gastric mucosa in man and grazing animals. The glochids are minute and barbed, and produce considerable irritation and itch on penetrating the skin. If an attempt is made to suck out the glochids from the skin, they are likely to attach themselves to the tongue. They may be removed by spreading an adhesive plaster over the area and ripping it off quickly (Watt & Breyer-Brandwijk 1962). Melted wax may also be used (Shanon & Sagher 1965).
In Africa, the plants are placed outside houses as "burglar alarms" (Irvine 1961).
The spines from this and related species of Opuntia are frequently found in the tongues of range cattle in the southeastern USA, often predisposing the tongue to bacterial infection. A similar disorder has been reported in sheep (Migaki et al. 1969).
Occupational dermatitis from gathering the fruit of this species has long been recognised (Rossiter 1924, Cavallucci 1930). An outbreak in the Mediterranean region was described by Shanon & Sagher (1956a, 1956b, 1965, 1969). Glochids entering the skin of workers engaged in gathering the fruits of the plant produced lesions of the skin between the fingers. Any part of the skin could be affected, producing a condition resembling scabies. The glochids were transferred to the workers' clothing and thence to other individuals. The hard palate and tongue were occasionally affected. The prickly pear should be picked only when wetted, and picking should be stopped when it is windy since the glochids can become airborne. Shanon & Sagher (1956) could reproduce in the laboratory the effects of the glochids on the skin. Patch tests showed that allergenicity was not involved. A further case has been reported by Banerjee (1977).
One of the authors (AJR) has seen extensive sabra dermatitis of the back and thighs in a holiday maker who fell into a "prickly pear" (an unidentified Opuntia Mill. species) in Majorca.
According to Behl et al. (1966), the sap of the plant is irritant and can produce eczematous dermatitis.
This species is popularly grown in homes and greenhouses. The glochids cause intense irritation (Behl et al. 1966). Two cases of dermatitis and keratoconjunctivitis caused by the glochids were described by Whiting & Bristow (1975).
The name Opuntia macrodasys is occasionally encountered. This is of no botanical standing, and appears to be a misspelling of Opuntia microdasys.
Wimmer (1926) refers to the glochids of this species as a possible cause of mechanical injury.
The glochids produced an eruption of the skin on the left arm and left thigh. The forced warm-air system of the house blew the glochids into the clothing covering these sites (Kirsch 1958).
In its natural habitat, this tropical American epiphytic species grows on "ant gardens" within which nest various species of biting and stinging ants (Wheeler 1921).
Application of the crude drug to the skin caused pruritus, pustules, and excoriations (Piffard 1881). In Cuba, the juice of the stems is used as a vermifuge and as a vesicant (Morton 1981).
Cactus spines can cause mechanical injury. The Mexican Aztecs of about 1200–1600 A.D. ritually impaled victims on the tops of spiny cacti (Rowley 1960). Deep penetration of a cactus spine can produce a mass resembling sarcoma (Costa 1943) or pseudotuberculous lesions of bone (Warthin & Davies 1924, Barney 1925). Buhr (1960) noted that cactus spines can produce foreign-body granulomas that may requires surgical attention. Gougerot & Eliascheff (1930) described a chronic subcutaneous nodule that was produced by an unidentified spine. Other local toxic effects from cactus spines were described by Hauffe (1926), Löwy (1926), Glass (1927), and Stein (1928). Sutton, in discussion of Lain (1918), described a disorder that was comparatively common in the arid regions of the western United States, most of those affected being gasoline tractor drivers. The eruption was confined to the skin of the limbs, and resulted from spines of cactus plants breaking off under the skin; secondary infection and skin ulceration were common. Karpman et al. (1980) have suggested that contaminating fungal material may be responsible for the often observed inflammatory reactions following cactus spine injuries.
Glochids entering the skin can also produce granulomatous reactions (Winer & Zeilenga 1955) which may be toxic and allergic. Since such reactions are uncommon compared with the large number of cactus injuries suffered in Arizona, USA, an allergic mechanism was postulated by Schreiber et al. (1971). They observed positive delayed skin test reactions to intradermal injections of extracts of glochids in two-thirds of affected patients.