Pistachio is a green, edible seed obtained from the Pistachio tree, which belongs to the Anacardiaceae (Sumac) family, along with Mango and Cashew nut. The Pistachio tree grows up to 10 m high and has deciduous pinnate leaves up to 20 cm long. It is native to the mountainous regions of central and south-western Asia. Pistachio is now cultivated in other parts of the world, particularly in California and Australia.
The plants are dioecious, with separate male and female trees. The flowers have no petals and are borne in panicles. Pistachio harvests are heavier in alternate years. The fruit is a drupe (in culinary but not botanical terms a nut), containing an elongated seed with a hard, whitish shell around 2 cm long and a striking light green kernel. It has a very characteristic flavour. When the fruit ripens, the shell splits partway open with an audible pop, but the nut remains enclosed and is usually marketed in this state as a snack. The shell of the Pistachio is naturally a beige colour, but it is sometimes dyed red for commercial sale.
Pistachio is often confused with some of the other 9 species in the genus Pistacia, such as P. terebithus and P. lentiscus. These species have a different distribution, in the Mediterranean and south-west Asia, and have much smaller nuts, lacking the hard shell of P. vera.
Pistachio nuts are widely used in the catering industry in ice creams, cakes and other confectionary, mortadella (a sausage) and oriental dishes. They are also eaten roasted as a popular snack.
Oil is processed from the seed and may pose a threat to patients with allergy, depending on the method of manufacture and processing (1).
Pistachio nuts are reported to be highly flammable when stored in large quantities, and are prone to self-heating and spontaneous combustion.
A number of allergens have been isolated, and some characterised (2).
In 3 adults with anaphylaxis to Pistachio, allergenic proteins of 34, 41, 52 and 60 kDa were found (3). In a similar study of 3 patients who had experienced anaphylaxis to Cashew nuts and were sensitised to the close family member Pistachio, both nuts were shown to have several IgE binding protein bands, the strongest of these bands having similar molecular weights of 15, 30 and 60 kDa (4). In an investigation of 2 children who had experienced anaphylaxis to Pistachio, IgE binding was found to be strongest in 3 bands between 30 and 41 kDa, the strongest at the 34 kDa band (5).
The following allergens have been characterised:
- Pis v 1, a 2S albumin (2,4).
- Pis v 2, an 11S globulin (6).
- Pis v 3, a vicilin-like protein (6).
- Pis v 4, a magnesium superoxide dismutase (6).
A lipid transfer protein (LTP) may be present, as suggested by a study of lipid transfer proteins in Rosaceae members which found that the Rosaceae LTP reacted to a broad range of foods, including Pistachio (7).
Preliminary evidence for the presence of a 2S albumin has been reported (4).
The close relationships among the Anacardiaceae suggest cross-reactivity, and this is supported by studies demonstrating cross-reactivity between Cashew and Pistachio (8). In a study of 42 children with Cashew allergy, 7 had an associated food allergy to Pistachio (9).
By means of serum from 2 children with Pistachio nut allergy, both were shown to be reacting to several Pistachio and Cashew allergens with common antigenic determinants. Cross-reactivity was also found between Pistachio nut and Mango seed, but not Mango pulp (5).
In a study of 3 individuals who experienced anaphylaxis to Cashew nut, all demonstrated IgE antibodies to Cashew and Pistachio. Evaluation of the allergen found that the strongest IgE-binding bands had similar molecular weights and that a 15 kDa protein band may have been a 2S albumin panallergen (4). Whether further cross-reactivity occurs as a result of other foods containing the 2S albumin has not been evaluated, but cross-reactivity has been demonstrated between Pistachio, Peanut, Walnut and Sunflower seed (3), all known to contain the albumin; and similarly, cross-reactivity has been reported to occur among allergens in Sesame seed and allergens in other foods, including Hazel nut, Rye, Kiwi, Poppy seed, Black walnut, Cashew, Macadamia, Pistachio, and Peanuts (10).
Cross-reactivity among Pistachio, Mango (14) and Artemisia (11) has been suggested. Sensitisation to Pistachio is common in Parietaria allergy (12). Cross-reactivity may also occur between Pellitory and Pistachio (13).
Cross-reactivity with other lipid transfer protein-containing foods is possible (6).
As Pistachio contains an 11S globulin (Pis v 2) and a vicilin-like protein (Pis v 3), cross-reactivity with other foods containing these allergens is possible, but it has not been elucidated as yet.
Cashew nut, and possibly Pistachio nut, allergy may be associated with pectin allergy, and the possibility of pectin allergy should be considered in Cashew- or Pistachio-allergic patients who have unexplained allergic reactions. A study describes a 3 1/2-year-old boy who developed anaphylaxis after eating Cashew nut and later after eating a pectin-containing fruit "smoothie". The child had skin reactivity to pectin and a high IgE antibody level to Cashew nut and Pistachio nut, as well as a low levels to Grapefruit, to which he had previously also reacted. The pectin in the smoothie was confirmed to be of citrus origin (14).
Pistachio may uncommonly cause symptoms of food allergy in sensitised individuals. Adverse reactions are similar to those seen with other tree nuts and include symptoms of oral allergy and food allergy, cutaneous manifestations, angioedema and severe anaphylaxis (3,5,15-16). Most of these reports have concerned small children. The IgE-mediated mechanisms have been confirmed with SPT and IgE antibody testing (5). As more exotic foods are introduced into more countries, more reactions are likely to be seen (17).
Two children were described as having allergy to Pistachio nut. Skin reactivity for Pistachio and Cashew were detected in both, and for Mango seed in one. IgE antibodies for Pistachio and Cashew nut were found in both (5).
Two uncommon cases of oral allergy syndrome after eating Pistachio nuts were reported, involving a 54-year-old man and a 3-year-old girl. Both were shown to have skin reactivity for Parietaria and Pistachio nut. A double-blind, placebo-controlled food challenge was performed on the adult patient and was negative, but a positive intraoral reaction was noted when the oral mucosa was slightly scratched. It was suggested that breaking the shells with his teeth had slightly injured the oral mucosa, which had enhanced the local response and resulted in symptoms (14).
Anaphylaxis to Pistachio has been described in 3 individuals who were allergic to both Mango and Pistachio nut. The individual history of each is informative. One, a 3-year-old boy, developed facial oedema and inspiratory stridor within minutes after eating Pistachio ice-cream. After eating a single Pistachio nut on a subsequent occasion, he developed generalised itching, hives and facial angioedema. A 28-year-old woman developed generalised urticarial eruptions on 2 separate occasions after ingesting Paprika, Curry and Celery. Three months later, she complained of a burning sensation in her mouth, swelling of the lips, face and tongue, and nausea and abdominal cramps immediately after eating a Mango. A 28-year-old man experienced episodes of generalised itching and hives, profuse sweating, abdominal pain, nausea and vomiting immediately after eating Peanut, Peach, Paprika, Hazel nut or Mango, on different occasions. Two of these patients were found to have skin reactivity to Pistachio. All were found to have skin reactivity to fresh Mango but not to Mango extracts of peel or pulp (15).
Anaphylaxis has also been reported in 3 other adults (3).
Food-dependent exercise-induced anaphylaxis to Pistachio has been described in a 16-year-old boy; it occurred 30 minutes after he began to play football after having ingested Pistachio nuts. SPT evaluation using roasted Pistachios was negative. No allergen-specific IgE antibodies were detected to Pistachio, Cashew Nuts or Mango. An open oral challenge test with Pistachio in resting conditions was negative. A specific food exercise challenge using a treadmill ergonometric stress test was positive 60 minutes after the patient had ingested 50 g of Pistachio nuts, with the onset of mild diffuse erythema and small wheals on his face and thorax (18).
In a study of 42 French children with Cashew allergy, 7 were found to have an associated food allergy to Pistachio. The authors stated that the increase in Cashew allergy was of concern because it affects young children who may have a reaction without ever having been exposed to Cashews. One could surmise that this may be valid for the close family member Pistachio as well (8). Indeed, it has been recommended that Pistachio be included in the list of foods to be tested in the investigation of anaphylactic reactions of unknown origin in small children (5).
Compiled by Dr Harris Steinman, email@example.com
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