Pecan or Hickory is a deciduous tree and belongs to the Juglandaceae family. It grows to a height of 180 m and needs sufficient rainfall or watering with a humid to dry climate for cultivation. It is wind-pollinated, with the flowering season starting in spring. Typically, its pollen size is around 50 µm. A significant correlation of pecan pollen count with the onset of allergic symptoms in sensitized patients has been observed in studies. Pecan pollen is allergenic and has been reported to induce allergic rhinitis, conjunctivitis, and asthma in allergic patients. No pecan allergens have been identified yet. Cross-reactivity has been reported between genus Carya and Juglans species. 

Worldwide distribution

Pecan is considered a major cause of allergic rhinitis in Northern America(7).

In a study performed in the United States in 209 children to test sensitization rate in rhinitis, 27.8% of children showed positive skin prick test towards pecan (8). Another study conducted on 371 allergic patients in New York City to analyze the tree pollen sensitization rate, observed hypersensitivity for pecan (hickory) in 27.1% of these patients (9).

In an Israeli study of 395 participants comprising 78.2% of a rural community and those atopic to 1 or more allergens, symptoms were seen in 50.3% had symptoms. 11.6% showed positive skin prick test to pecan, and 76.1% were atopic to pecan (p< 0.005). Of the individuals atopic to pecan, clinical symptoms coinciding only with the Pecan pollen season was observed in 65.2% of the atopic patients, and an additional 10.9% had perennial symptoms. The measurement of Pecan tree pollen during this period, May, showed that the pollen grains comprised 70% of the total airborne grains  (7). Another study in Israel confirms the same moderate correlation of pecan pollen count with the onset of allergic symptoms in sensitized patients. Pecan pollen count was observed to be 95% of total pollen count when pecan flowering season peaks in mid-May. In pecan allergic patients (n=26), around 58% (n=15) were symptomatic, most patients suffered from bronchial symptoms, and a few patients reported nasal or ocular symptoms(10).

In a study in Mexico, 4.8% of allergic patients (total n =247)  were found sensitive to pecan pollen (11). 

Worldwide distribution

Pecan is used in agricultural commodities in some parts of Mexico and the US (12). Pecan is also found in countries like Australia, Brazil, Israel, and South Africa (1).

Main

The main route of exposure for pecan pollen is through the airway (7).

Pecan tree pollen is considered highly allergenic, resulting in allergic rhinitis, conjunctivitis and asthma in pecan sensitized individuals(7). The clinical symptoms such as nasal and bronchial onset were observed to be induced in pecan pollen sensitized individuals at an average concentration of 10-20 pollen per m3(10).

Allergic Rhinitis (AR) and Allergic Rhino-conjunctivitis (ARC)

A study was conducted in Israel to analyze the prevalence of pecan pollen allergy among 395 individuals. 11.6% were skin-prick test positive to Pecan tree pollen, constituting 25.4% of the atopic population. The part of individuals atopic to Pecan pollen was 76.1%, whereas 58.7% of the Pecan-atopic participants had AR (hay fever), 43.5% had asthma, and 31.5% had both AR and asthma. (7).

Asthma

Importance should be given to analyze the manifestation of pecan allergy and its symptoms in atopic children population (aged 4-17 years) as asthma prevalence has been found significant among pecan atopic individuals in such young population(7).

Other diseases

Walnut trees, especially black walnut (Juglans nigra) trees produce a highly toxic chemical called juglone. In the walnut tissue, juglone is present in non-toxic and colorless form hydrojuglone. When exposed to air or soil compounds, hydrojuglone is oxidized into juglone. Hydrojuglone is stored in leaves, stems, fruit hulls, inner bark, and roots. It is also used as an additive for food, preservative for wood and wood products, and an anti-bacterial or anti-fungal agent. Minute quantities can sicken, sedate, or kill living beings like other plants, animals, and humans(13).

Prevention strategies

Avoidance

Preventive measures such as taking a shower after coming home, removing clothes used at work outside the home, and abstaining from going outside when the pollen count is high, closing windows at home can help prevent exposure to pecan pollen (14). 

Allergenic molecules

To date, no pecan tree pollen allergens have been identified. Though pecan food allergens, Car i 1, Car i 2 and Car i 4 have been identified and listed in the WHO/IUIS database(15). 

Cross-reactivity

Cross-reactivity could be expected between the genus Carya species and, to a moderate degree, to those of the genus Juglans, e.g., Walnut tree. Moderate to severe cross-reactivity has been demonstrated between shagbark hickory, pecan and black walnut(16, 17). 

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: November 2020