Route Of Exposure

Allergen Exposure

Mesquites belong to the Fabaceae (Legume) family. There are about 44 species of mesquite found throughout the world. They occur most frequently in warmer and semi-arid regions of North and South America (especially Argentina), Africa, India, and south-western Asia. Three of the most common mesquites are native and characteristic shrubs of the south-western US and Louisiana: honey mesquite (P. glandulosa), screwbean mesquite (P. pubescens), and velvet mesquite (P. velutina). Mesquite (P. juliflora) is native to Mexico, South America and the Caribbean. It has been introduced into Asia, Australia and elsewhere. All 4 are deciduous and have characteristic bean pods that have long been used by humans, wildlife and livestock as a food source.

Mesquite is a deciduous shrub or small tree typically reaching a height of 5-10 metres, and characterised by 22-centimetre bean-like pods. Mesquite bark is dull red and somewhat rough. The green-brown stem is sinuous and twisted, with axial thorns situated on both sides of the nodes and branches. The leaves are bi-pinnately compound and nearly hairless. The leaflets have short to non-existent stalks, and are dark green. Straight 2.5 cm spines arise in pairs from leaf axils. Seedpods are 10-22 cm long, yellow-green, flattened and narrowed between the seeds, ending in a point, and either straight or curved; they occur in drooping clusters.

Mesquite flowers in early summer. Its flowers are small, green-yellowish spikes without any particular fragrance, and yet are attractive to bees. Honey mesquite’s flowers are creamy-yellowish to green and sweet-smelling. They are densely packed in cylindrical clusters. In India, P. juliflora flowers twice a year, in February-March and August-September. Pollen may be produced in large quantities. Pollination is by both insects and wind.

Mesquite in several varieties has been used for reclamation of desert lands and as a wood resource, with the end result that its easily dispersed and far-travelling pollen is an abundant and significant source of allergens.

This tree can be found in warm climates in the desert, alongside desert washes and streams, on plains and hillsides, and often in thickets. Mesquites are widely used as ornamental shade trees throughout the south-west US because they need little or no watering and can survive on limited rainfall.

Mesquite wood is used chiefly for firewood. The wood is also used for fence posts, tool handles, furniture, and to create aromatic charcoal for barbecuing. Liquid smoke for meat flavouring is manufactured from honey mesquite sawdust. Honey mesquite provides an excellent source of nectar for honey bees. Native Americans used the seedpods, with or without the seeds, as flour, the root fibre for rope, and the wood for utensils, posts, and fuel.

Clinical Relevance

Allergen Description

Mesquite pollen has been shown to contain at least 13 antigenic proteins. Early studies reported antigens varying from 10 to 81 kDa, with the most allergenic fraction being a 20 kDa protein. Other proteins of 13, 20, 27.5, 41, 55.5 and 81 kDa have been detected. Other authors, in evaluating pooled serum from 10 mesquite-allergic individuals, detected allergenic bands of 11, 16, 17, 18, 20, 27, 30, 36, 44, 56, 71, and 99 kDa. An additional IgE response by a 64 kDa band not present on the gel was detected on the blot, resulting in a total of 13 blot bands. The remaining kDa gel band produced only a slight image when blotted. In an Indian study of 38 mesquite-sensitised individuals, the patients' pooled sera demonstrated 16 IgE binding components, with components of 24, 26, 29, 31, 35, 52, 58, 66 and 95 kDa recognised by more than 80% of individual patients' sera.

To date, only one mesquite allergen has been partly characterised:

Using serum IgE of mesquite-allergic individuals, 59 and 66 kDa proteins were isolated from the pollen, wood and wood smoke of mesquite. The authors commented that more allergens of mesquite pollen were present but did not give further descriptions.

A more recent study purified a 66 kDa protein from mesquite pollen extract, which was shown to be recognised by 90% of mesquite-sensitised patients in skin test and ELISA, as well as to induce significant histamine release in allergic patients' blood.

Potential Cross Reactivity

Cross-reactivity among members of the legume family (Fabaceae) may be expected, but has not yet been investigated. Significant cross-reactivity between members of the genus is probable.

Cross-inhibition assays have shown a close relationship between Mesquite pollen, Ailanthus excelsa, Cassia siamea, Holoptelea integrifolia, Putranjiva roxburghii and Salvadora persica. IgE binding components of 14, 41, 52 and 66 kDa were shared allergens, whereas those of 26 and 29 kDa were specific to P. juliflora.

In a study investigating cross-reactivity of mesquite tree pollen with plant-derived foods, in vitro cross-reactivity of mesquite pollen with Lima bean was evaluated. Of 110 patients with asthma, rhinitis or both who were tested intradermally, 20 showed marked positive reactions with mesquite pollen extract. Of these, 12 patients showed elevated IgE antibodies to mesquite pollen extract alone, and 4 to both Lima bean and the pollen extract. Lima bean extract could inhibit IgE binding to mesquite in a dose-dependent manner. Immunoblot and immunoblot inhibition demonstrated the presence of 20, 26, 35, 66 and 72 kDa shared IgE-binding components between the 2 extracts. Histamine release, peripheral blood mononuclear cell proliferation, and interleukin IL-4 levels also suggested allergenic cross-reactivity. Whether Mesquite is cross-reactive with other Phaseolus species was not investigated.

A 66-kDa protein isolate from mesquite pollen extract was shown in inhibition assays to have a close allergenic relationship with Ailanthus excelsa, Cassia siamea, Salvadora persica pollen and Phaseolus lunatus (lima bean).

Clinical Experience

IgE-mediated reactions

Mesquite pollen may induce asthma, rhinitis and conjunctivitis in sensitised individuals.

Mesquite is a major cause of allergic disease in the south-western United States, Mexico, Saudi Arabia, Namibia and South Africa, Kuwait, the United Arab Emirates (UAE), and India.

Mesquite pollen is a potent allergen capable of evoking immediate hypersensitivity reactions in a susceptible population remote from the plant source. One study reports that 62% of 100 consecutive patients attending an allergy clinic were shown to be positive to mesquite pollen, as confirmed by IgE antibody tests, despite the absence of the plant within a 50-mile (80-kilometre) radius. Asthmatics may experience a dual response: an acute immediate (IgE-mediated) response as well as a late response.

In Tucson, Arizona, a city that has a high prevalence of allergic rhinitis and asthma, pollen from the mesquite tree has been noted to be among the major causes. In a report on sensitisation to mesquite pollen in Charleston, South Carolina, USA, 1 598 out of 4 361 patients were said to be sensitised to mesquite pollen, even though most of them had no known direct exposure to mesquite. A Texas study compared serum-specific IgE testing to skin-prick testing for 53 inhalant allergens using subjects > or = 18 years old with chronic rhinitis and who had at least 1 positive skin-prick test to a 53-inhalant allergen panel. Skin-prick test was positive in 34% for mesquite, compared to 19% for serum-specific IgE. In another study, the lead author examined aeroallergen sensitisation rates in military children in Texas undergoing skin testing for rhinitis. Twenty-nine per cent were positive for mesquite pollen.

An earlier study conducted in Tucson assessed the incidence and remittance of positive skin-test reactions to individual allergens in 828 children aged 6-11 years, and the remittance rates for individual allergens were high and variable (19-49%). The perennial allergens Bermuda and Alternaria were early sensitisers and had low remittance rates. For mesquite, the percentage of positive skin-prick tests at 6 years of age was 11%, and 24% at 11 years of age.

In a study in Comarca Lagunera, Mexico, skin-prick tests on 101 patients with asthma reported sensitisation to mesquite tree in 57%. A study of the prevalence of aeroallergen sensitisation in 181 atopic asthmatic adults in Guadalajara and Veracruz, Mexico, reported that 20% were sensitised to mesquite tree pollen. In a study aimed at describing the skin-prick test sensitivity patterns in the different climatic zones in Mexico, 13-24% were sensitised to mesquite.

The practices that ‘green’ the desert have some unintended consequences, including the promotion of allergen sensitisation to imported plants. In a study from Kuwait on 706 patients aged 6 to 64 years who had allergic rhinitis, specific IgE to mesquite tree was found in 50.3%. In a study of 553 asthmatics in Kuwait with sensitisation to common aeroallergens, as measured by serum-specific IgE, it was found that the 3 most prevalent sensitising pollens were from Chenopodium (70.7%), Bermuda grass (63%), and mesquite tree (63%), all of which are horticultural plants imported for the purpose of ‘greening’ the desert.

In a Saudi Arabian study of 473 allergic patients suffering from bronchial asthma in 4 geographical regions, a total of 76% in Qassim, 37.5% in Gizan, 29% in Abha, and 11% in Hofuf reacted positively to mesquite pollen extract. In a study in Saudi Arabia of 84 children with allergic rhinitis, skin testing with common allergens found that cat fur, Bermuda grass and mesquite were the most common allergens. Symptoms of recurrent rhinosinusitis, otitis media with effusion, and tonsil and adenoid infection were commonly noticed among them. Similarly, in a study in the Eastern Province of Saudi Arabia of 1 159 patients with positive skin-prick tests to inhalants, 46% of natives and 32% of North American expatriates were positive to this allergen. Mesquite tree pollen may also be carried in sandstorm dust, as demonstrated in a study conducted in Riyadh, Saudi Arabia, which reported that mesquite tree pollen was one of the most abundant aeroallergens. Among 263 United Arab Emirates nationals with a respiratory disease suspected of being of allergic origin, 23.5% were sensitised to pollen from mesquite trees. In a study of 327 adult patients with respiratory, dermatologic and ophthalmologic diseases of suspected allergic origin who attended a hospital based in the United Arab Emirates, and who were evaluated for aeroallergen sensitisation, skin-prick testing showed that 45.5% were sensitised to mesquite. A more recent study found mesquite to be the most prevalent sensitisation in patients in the Riyadh region of the Kingdom of Saudi Arabia, with skin-prick tests being positive in 72%.

A study conducted in the Kingdom of Saudi Arabia (KSA), the United Arab Emirates, and Sudan, evaluated 492 consecutive patients, mean age 30 years. Indigenous raw material for pollen grains was collected from the desert near the capital city of Riyadh, KSA. The highest sensitisation to indigenous pollens was detected to Chenopodium murale (32%) in Khartoum (Sudan) and Salsola imbricata (30%) and P. juliflora (24%) in the Riyadh region.

In Oman, an evaluation of the records of 689 patients who had undergone an allergy skin-prick test with a diagnosis of asthma, allergic rhinitis or rhinoconjunctivitis found that 36% were sensitised to mesquite (P. glandulosa).

A study reviewing perennial and seasonal aeroallergen trends in the Middle East, and their effect on military personnel serving in the area, reported that most of the countries have significant grass and weed pollen seasons, and that mesquite tree is as relevant among these personnel as it is in the USA.

In India, where the mesquite tree is common, a Delhi study reported that mesquite pollen was the most significant pollen sensitising agent, with 35% of patients being sensitised to it. Of the 166 patients who skin-tested positive for mesquite, 50 sera were evaluated for serum-specific IgE for mesquite, of which 60% were positive. An earlier study employed intradermal tests with mesquite pollen extract on respiratory allergy patients from the Bikaner (arid) and Delhi (semi-arid) areas in India, and elicited positive skin reactions in 71/220 of the patients. In a recent study of 331 Indian patients with nasal allergy, nasal polyps, allergic conjunctivitis and allergic asthma, 321 patients showed significant positive results to skin-prick tests. Dust and pollen allergens were positive in most of the patients, compared with insect antigens. Among the pollen antigens tested, those of Parthenium hysterophorus were found to be the most common (30%), followed by mesquite (25%). An earlier study evaluating sensitisation to Nile trumpet tree (Dolichandrone platycalyx) was investigated in all patients with allergic rhinitis and asthma attending a tertiary care centre in South India, and found that of 317 subjects, 18% were sensitised to mesquite pollen. In100 patients attending a clinic in Jaipur, India, who were skin-prick tested, 16% were positive for mesquite.

A study cautioned that when commercial mesquite tree pollen extract results in an isolated positive skin test result, this may be a false positive result, and that in some patients the allergen extract apparently has the ability to cause direct mast cell degranulation.

Immunotherapy for mesquite pollen allergy has not been extensively evaluated to date.

Other reactions

Individuals with marked hypersensitivity to mesquite pollen should be aware that honey might contain mesquite pollen, as demonstrated by a report of a patient who was extremely sensitive to mesquite pollen and experienced anaphylaxis after ingesting honey containing this pollen.

Mesquite wood charcoal has been widely promoted for the unique taste it imparts to broiled food. In a study comparing 13 mesquite charcoal cooks with 17 gas-flame broiler cooks, the prevalence of respiratory symptoms among workers exposed to mesquite charcoal smoke was greater to a statistically significant degree. Unidentified high-molecular-weight saturated and unsaturated aliphatic hydrocarbons were present in air samples from the environments of the mesquite broiler cooks, but not in air samples from the environments of the gas-flame broiler cooks.