Other Topics

Hackberry (C. occidentalis), a tree native to the USA, grows 10-13 m in height. Southern hackberry, also native to the USA and the largest hackberry, grows 20-25 m in height. There are 5 native North American tree species and 2 shrubs. Nettle tree (C. australis) is a tree or shrub found in Europe and is used for its edible fruit, a yellow dye made from the bark, and wood for whip-handles and walking-sticks. (1)

The trees have broad crowns with ascending, arching branches. The leaves are single and alternate, 5 to 15 cm long, serrated and asymmetric at the base. The bark is scaly and coarsely ridged. Both male and female flowers are found on the tree (monoecious reproduction). The flowers are inconspicuous and appear about the same time as the leaves. Male flowers are longer and fuzzy. Female flowers are greenish and more rounded. Pollen anthesis is from April through May in the northern hemisphere. Pollen is shed in large amounts.

The fruit is a berry-like, pitted drupe 6-10 mm in diameter, on a single stalk, persisting in winter, and dispersed by birds and mammals. (1) It is edible in many species, with a dryish consistency but a sweet taste, reminiscent of a date.

Older taxonomies classified the suborder Urticineae as a separate order, Urticales, which contained elm in Ulmaceae, mulberry in Moraceae, nettle in Urticaceae, and hemp and hops in Cannabaceae. Hackberries used to be included in Ulmaceae along with elm, but are now regarded as more closely related to Urticaceae and have therefore been placed in a separate family, Celtidaceae. (2) However, in the APG II system Hackberry has been placed in the hemp family (Cannabaceae).

The closely related species C. sinensis was introduced to the south-eastern region of Queensland, Australia, and has had a destructive effect on indigenous plant communities. Its pollen has been identified as an allergen source. (3)

Tala (C. tala) is a tree or shrub widely distributed in the central part of Argentina, flowering from September through December and reported to be an important cause of hay fever, affecting up to 44% of patients. (4, 5) Asthma has not been described in connection to it. (1)

In Italy, in a study evaluating Ulmaceae airborne pollen at the Modena Geophysical Observatory monitoring station, 5 pollen types were identified: C. australis, Ulmus glabra, Ulmus laevis, Ulmus minor, and Ulmus minor/laevis. However, the family contributed little pollen to the air (about 1% of the total pollen recorded), Ulmus and Celtis contributing 0.9% and 0.04%, respectively. The highest levels of Celtis were recorded during the afternoon. (6)

IgE-mediated reactions

Although C. tala was reported over 4 decades ago to be an important cause of hay fever in Argentina, affecting up to 44% of patients, (5, 6) very few studies have evaluated the significance of this tree pollen in allergy. C. occidentalis and other species of Celtis are generally included in decades-old lists of hay fever plants or plants potentially causing hay fever. (7,8) The most important hackberry for hay fever was reported to be C. occidentalis. (9) Asthma has not been described in this connection. (1)

A study that examined aeroallergen sensitisation rates in allergic rhinitis-affected military children in Texas found that of 345 children tested using a 51-allergen panel, 80.3% had at least 1 positive test result, and the average number of positive test results was 11.4. The most common allergens were grasses, Alternaria, and cottonwood. Thirty-two of 51 allergens were positive in 20% or more children. Almost 30% were skin-prick test-positive for hackberry. (10) The same authors investigated the prevalence of positive skin-test responses in 1 137 symptomatic military patients aged 4-79 years old, using a skin-prick testing panel of 53 aeroallergens. At least one positive skin test was recorded in 81.6% of patients. Approximately 26% were sensitised to hackberry. (11) In a more recent study by the lead author and others, serum IgE testing was compared to skin-prick testing for 53 inhalant allergens using subjects > or = 18 years old with chronic rhinitis, and who had at least 1 positive ST to a 53-inhalant allergen panel. The mean number of positive ST and CAP results were similar. For hackberry, 43% were sensitised according to skin-prick tests, versus 25% with serum-specific IgE testing. (12)

Research has indicated a high prevalence of hackberry pollen in Argentina, (13,14) but the studies were not correlated with clinical reports.

The closely related species C. sinensis was introduced to the south-eastern region of Queensland, Australia, and its pollen has been identified as an allergen source. (15)

No allergens have yet been characterised.

Cross Reactivity

Allergic reactions may be expected, due to the extensive cross-reactivity that has been posited between the different species of the genus Ulmus (represented by white elm tree). (16) However, there is very little cross-reactivity data on elm and hackberry. (17)

Last reviewed: May 2022