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f201 Pecan nut

Whole Allergen
Code f201
LOINC 6208-3
Family Juglandaceae
Genus Carya
Species Carya illinoinensis
Route of Exposure Ingestion
Source Material Shelled & heated nuts
Latin Name Carya illinoensis/Carya illinoinensis
Other Names Hickory nut
Categories Food Of Plant Origin, Seeds & Nuts

Summary

Pecan is a deciduous monoecious nut tree belonging to the genus Carya of the family Juglandaceae. Carya illinoinensis (pecan nut) is the most widely cultivated economical edible nut crop. The pecan tree is native to North America, however, it is also cultivated in other countries, such as Australia, Brazil, China, Israel, Peru, Argentina and South Africa. The prevalence of pecan nut allergy is increasing with its existence in the United States, Canada, Australia, Europe, Israel, etc. Oral ingestion of pecan nuts can lead to IgE-mediated allergic reactions like oral food allergy and even severe reactions like anaphylaxis. Three allergenic components, i.e., Car i 1 (2S albumin), Car i 2 (vicilin-like protein) and Car i 4 (legumin-like protein) have been identified for pecan nut allergy. There is a high prevalence of co-allergy between walnuts and pecan nuts and is found to be strongly correlated since the allergens from both have high sequence identity. Patients with walnut allergy are at a very high risk of having pecan nut allergy too. Further, cross-reactivity with other tree nuts (e.g., hazelnut and macadamia) and tree pollens (e.g., birch) have also been reported.  

Allergen

Nature

Pecan tree is an economically productive, heterodichogamous, wind-pollinated, deciduous, monoecious nut tree, that can reach an utmost height of 55 m with a trunk diameter of >2 m. It has a different root system that consists of taproot (grows downwards), lateral roots (grows horizontally) and fibrous roots (grows in all directions from lateral). It has alternate and compound leaves with 9-17 leaflets, that differ in length and color. Usually, male (staminate) flowers appear 1-2 years earlier than female (pistillate) flowers in sexually mature pecan trees (1, 2). The male flowers known as catkins are arranged in a group (each group with 3 catkins) of 2-8 and appear on last season’s shoot. The female flowers on the other hand occur as cluster inflorescence and appear at the end of the current season’s shoot (1, 3). Pecan fruit is a nut having a kernel closed in a shell. It serves as a storage organ and usually matures in the autumn season. The green husk of the nut becomes dry as soon as it becomes physiologically ripe and cracks open to expel the kernel. They are collected manually or mechanically and are processed to be used in the food industry (1).

Habitat

Pecans usually grow along low-lying river bottomlands due to their high requirement of water, preferably in loamy and well-drained soil, between 30° and 42° N latitude. They get adjusted to a different types of climates, ranging from mild to extremely harsh winters, and from humid to semiarid temperatures (2, 4). The seasonal water requirement of pecan trees is around 1000-1300 mm which is achieved by rainfall of around 1300 mm in humid regions and 660 mm being supplemented with flooding or runoff from elevation in semiarid regions (2). However, the growth of pecan tree roots may be limited due to drought, too much moisture in the soil temperature below 18℃ (3).

Taxonomy

The genus Carya consists of about 25 species, among which Carya illinoensis is the most widely cultivated economical edible nut crop (1). The other species cultivated for its edible nut is Carya cathayensis, also known as Chinese hickory, grown in East Asia (4).

Taxonomic tree of Pecan  (1,5)  
Domain Eukaryota
Kingdom Plantae
Phylum Spermatophyta
Subphylum Angiospermae
Class Dicotyledonae 
Order Juglandales
Family Juglandaceae
Genus Carya
Species Carya illinoinensis

 

Tissue

Pecan seeds nut proteins are primarily composed of 2S albumin, 7S vicilin, and 11S legumin which have been reported to cause allergic reactions (6).

Epidemiology

Worldwide distribution

In United States (US), the prevalence of total tree nut allergy in the general population by a telephone survey was found to be 0.6%, with pecan nut allergy reported in 31% of tree-nut allergic individuals (7). Another telephone survey was conducted in 10 Canadian provinces and the results showed that the self-reported prevalence of tree nut allergy was found to be 1.73% in children and 1.07%  in adults (8).

In a cross-sectional study conducted in Australia, the prevalence of food allergies in 116 school-going children (10-14 years) was assessed. The results reported that the prevalence of pecan nut allergy in these children was found to be 0.2% (9).

The Pronuts study was a multicenter, European study (London, Geneva and Valencia) that reported the pecan nut allergy in 31.9% nut allergic individuals, with the maximum pecan nut allergy found in Valencia (66.7%). The overall consumption of pecan among all centers was only 6% and was not considered as index nut (10). Furthermore, in a cross-sectional study conducted in Mexico (n=1126), the probable pecan allergy was reported to be 0.3% (11).

A prospective cohort study conducted on 83 tree nut allergic patients in Israel found 57 patients sensitized to pecan, with the clinical allergy to pecan nut reported to be 59.6%  (12). Further, a study was conducted to assess the cross-sensitization profiles of tree nuts in birch endemic areas in Finland. The results reported that the prevalence of pecan nut allergy in tree nut allergic subjects with and without birch sensitization was found to be 21% and 8%respectively (13).

Risk factors

Pecan and walnut allergy are found to be strongly correlated since the allergens from both have high sequence identity. A high prevalence of co-allergy between walnuts and pecan nuts have been reported. In a study evaluating 60 multi-allergic pediatric patients, only 3 out of 32 walnut allergic patients could tolerate pecan. The Jaccard similarity coefficient for the co-occurrence of walnut and pecan nut allergy was found to be 0.91 (14). Further, the NUT-CRACKER (NUT Co Reactivity—ACquiring Knowledge for Elimination Recommendations) study, which was a prospective cohort study (n=83) reported that two-third of walnut allergic individuals were allergic to pecan also. Moreover, walnut-pecan allergic patients usually suffer from more gastrointestinal reactions on oral challenge test with walnut (12). 

Environmental Characteristics

Worldwide distribution

Pecan tree is native to North America. However, it is also cultivated in Australia, Brazil, China, Israel, Peru, Argentina and South Africa (1, 3). In 2016, the worldwide production of pecan nut was mainly obtained from Mexico (47%) and US (46%), constituting of >90% of the production from these countries. The other countries where pecan nut is produced, include Australia (4%), China (1%), and South America (1%) (1, 4).

Fresh pecans exhibit distinct aroma, flavor and taste and are thus consumed without roasting them. The oil extracted from it is used in foods as well as for the production of medicines and essential oils (1). Pecans are used in bakery products, confectionery items, such as chocolates, muesli mixtures as well as gourmet products (15).

Route of Exposure Section

Main

Oral ingestion of pecan nuts can lead to immunoglobulin (IgE)-mediated allergic reactions like oral food allergy and even severe reactions like anaphylaxis (16). 

Clinical Relevance

Oral allergy symptoms and Anaphylaxis

In a study conducted on 25 double-blind placebo-controlled food challenge (DBPCFC) positive pecan allergic patients, oral allergy symptoms were depicted as urticaria (36%) and gastrointestinal reactions (24%) (17). In another study conducted on 60 multi-allergic pediatric patients, DBPCFC was performed on 30 patients for pecan, of which 29 showed oral food allergy symptoms. The oral food challenge reported skin reactions, gastrointestinal reactions and respiratory reactions in 93%, 41% and 3% respectively (14). Anaphylaxis due to walnut-pecan allergy was reported in 53.1% of 49 patients subjected to oral food challenge in a prospective study conducted in Israel. In the same study of 49 walnut-pecan allergic patients, the oral food challenge depicted skin reactions in 67.3%, gastrointestinal problems in 46.9% and upper and lower respiratory tract infection in 49% and 20.4% (16).

Allergic rhinitis

Allergic rhinitis was reported in 32% of pecan allergic patients confirmed by DBPCFC in a study.

Asthma

In a cross-sectional study conducted in 1126 individuals in Mexico, the history of asthma was found to be significantly associated with probable allergy to pecan nuts (p=0.009) (11).

Asthma was reported in 24% of pecan allergic patients confirmed by DBPCFC in a study (17).

In a prospective cohort study conducted in Israel, asthma was reported in 44.9% of walnut-pecan allergic patients (16).

Atopic Dermatitis

A prospective cohort study reported atopic dermatitis in 59.2% of walnut-pecan allergic patients in Israel (Elizur, Appel (16).

Other diseases

Only two cases have reported the development of contact dermatitis as type I hypersensitivity reactions due to prolonged contact with pecans (18). 

Molecular Aspects

Allergenic molecules

The World Health Organization and International Union of Immunological Societies’ (WHO/IUIS) Allergen Nomenclature Subcommittee have registered three allergenic components, i.e., Car i 1 (2S albumin), Car i 2 (vicilin like protein) and Car i 4 (legumin like protein) for pecan nut. (19, 20). 

 

Name of allergen Isoallergen Biochemical name Molecular weight (kDa)    Allergenicity
Car i 1 Car i 1.0101 2S albumin 16

Major allergen.

IgE binding was found in 79% of 28 sera and 42% out of 12 sera of pecan allergic patients (19, 21).

88% sequence identity between Car i 1 and walnut allergen Jug r 1 (22).

Car i 2 Car i 2.0101 Vicilin-like protein 55

IgE binding was found in ~30% of 27 sera and 24% of 25 sera of pecan allergic patients (17, 19).

92% sequence identity of Car i 2 with walnut allergen Jug r 2 and 44% with Jug r 6 (22).

Car i 4 Car i 4.0101 Legumin seed storage protein Subunit of hexameric protein:55.4 kDa

Major allergen.

IgE binding was found in 57% of 28 sera of pecan allergic patients (19).

95% sequence identity between Car i 4 and walnut allergen Jug r 4 (22).

 kDa: kilodaltons, IgE: Immunoglobulin E

Evidence has revealed that dietary lipids coexisting in pecan nuts may serve as adjuvants for sensitization to pecan allergens (Car i 1, Car i 2 and Car i 4) by demonstrating coordinated accumulation of allergens and lipids during embryological development of pecan nut (23). 

Biomarkers of severity

The major allergens Car i 1 and Car i 4 were found to be very stable post blanching for 10 mins, roasting at 148° C for 30 min/172° C for 12 min and autoclaving for about 5 mins. This suggests the presence of heat-stable allergens in pecans (24). Further, Car i 1 (2S albumin) was found to be very stable upon pepsin digestion and able to bind IgE immunologically in a similar way as undigested protein (21).

Cross-reactivity

In the Pronuts study, walnut and pecan were found as highly correlated nut allergies, where 97% of pecan allergic individuals were found to be allergic to walnuts, while only 75% of walnut allergic children were allergic to pecans (10). The NUT-CRACKER and NUT-CRACKER-PS study was conducted in Israel to study coincidences of allergies among tree nuts and predict walnut-pecan allergy severity respectively. A total of 134 of 183 patients (47 from NUT-CRACKER and 87 from NUT-CRACKER-PS study) were found to be walnut allergic. Pecan allergy was co-existing in 79.9% of walnut allergic patients (3.6-27.3 years), while 100% of pecan allergic patients were allergic to walnut (12, 25). This suggests the co-allergy to be unidirectional. The high association between walnut and pecan allergies (spearman coefficient=0.96) could be attributed to high sequence identity among the pecan allergens (Car i 1, Car i 2 and Car i 4) with their counterparts in walnut (Jug r 1, Jug r 2, Jug r 4) respectively (19, 22). Pecan allergy was also found to be significantly correlated with hazelnut (Odd’s ratio [OR]=14.9, p<0.0001, Pearson correlation coefficient = 0.47) and macadamia nut (OR=15.2, p<0.0001, Pearson correlation coefficient=0.36), while it was found to be inversely related with peanut allergy (Pearson correlation coefficient = -0.25) (10). Further, 66.7% of peanut-allergic patients showed negative results to specific skin prick tests for pecan (26). In a single-center, retrospective analysis of serum specific IgE levels to coconut, 51% of co-sensitization was reported between pecan and coconut but were not found to be significantly associated (27). A study conducted in Finland to determine cross-reactivities between birch pollen and tree nuts reported that 21% of subjects sensitized to birch pollens were sensitized to pecan also (13).

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Magnus Borres

 

Last reviewed: December 2020

References
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