Red mulberry

Further Reading

Mulberry t70

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Code: t71
Latin name: Morus rubra
Source material: Pollen
Family: Moraceae

Allergen Exposure

Geographical distribution
Trees and shrubs of Moraceae are primarily tropical, with temperate trees represented by Morus, Broussonetia, and Maclura (1).

The Mulberries, comprising about 10 species, are monoecious or dioecious trees or shrubs. They bear the staminate and pistillate flowers on different branches of the same tree or on different trees; these flowers are minute, and the staminate ones are in elongated cylindric spikes (2). The origins of the Mulberry tree (M. alba) are in China, Japan, Thailand and Malaysia. The tree is now found throughout the world. Only 2 species are native to North America, but several others have been introduced.

The Red mulberry (M. rubra), closely related to the Mulberry or White mulberry tree (M. alba), is native to eastern North America, from southernmost Ontario and Vermont south to southern Florida and west to southeast South Dakota and central Texas (3-4). The White mulberry (M. alba) is a native of China, where it is grown for feeding silkworms. It is also widely cultivated in the United States and Canada, occurring in fruit-bearing and ornamental varieties (2). Paper mulberry, B. papyrifera is native to Asia but grown elsewhere, including the USA. M. alba and B. papyrifera have a predilection for warmer regions, extending to the US West Coast (1).

Red mulberry is a medium-sized deciduous tree, growing 10-15 m tall and with a trunk up to 50 cm in diameter. It may, rarely, reach 20 m in height. The leaves are variable but usually alternate, 7-14 cm long, 6-12 cm broad, simple, broadly cordate, with a shallow notch at the base, unlobed on mature trees, often with 2-3 lobes on young trees, and with a finely serrated margin (1,3). Red mulberry leaves are a deeper bluish green, and M. alba a lighter green (1.2).

Female and male flowers are found on dense catkins, on the same or separate trees. Pollination for most Mulberries is in April and May, beginning in March in the southern states, and later, extending into June, in the northern states (1,4-5). Each male flower has 4 stamens bent inward in the bud but straightening elastically and becoming exserted at maturity. They shed large amounts of pollen, which is occasionally the cause of hayfever.

The fruit is a compound cluster of several small drupes, similar in appearance to a Blackberry but somewhat elongated, 2-3 cm long, red ripening to dark purple, edible and very sweet. The fruit ripens in June or July.

Allergens
No allergens have yet been characterised.

Potential cross-reactivity

An extensive cross-reactivity among the different individual species of the genus could be expected (6) but is largely unstudied (7). Morus members are felt to be essentially identical based on skin test similarities, and B. papyrifera pollen was reported to ”interact almost perfectly” with other Mulberry pollens (1-2).

Clinical Experience

IgE-mediated reactions
Mulberry tree was reported to induce severe hayfever, but asthma was not reported (2,8). Anecdotal evidence suggests that asthma and hayfever are possible following exposure to pollen from this tree; however, no specific studies have been reported to date.

Mulberry has been reported to be a major problem in Tucson, Arizona, a city that has a high prevalence of allergic rhinitis and asthma (9).

Compiled by Dr Harris Steinman, harris@zingsolutions.com.

References

  1. Weber RW. Red mulberry. Ann Allergy Asthma Immunol 2003;90(5):A6
  2. Wodehouse RP. Hay fever Plants, 2nd ed. New York, NY: Hafner 1971:107-10
  3. Wikipedia contributors, ”Morus rubra”, Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Morus_rubra&oldid=220472864 (accessed July 9, 2008)
  4. Lewis WH, Vinay P, Zenger VE. Airborne and Allergenic Pollen of North America. Baltimore, MD: Johns Hopkins University Press, 1983:64-70,199-200
  5. Bassett IJ, Crompton CW, Parmalee JA. An Atlas of Airborne Pollen Grains and Common Fungus Spores of Canada. Hull, Quebec: Printing and Publishing, Supply and Services Canada, 1978:207-8
  6. Yman L. Botanical relations and immuno-logical cross-reactions in pollen allergy. 2nd ed. Pharmacia Diagnostics AB. Uppsala. Sweden. 1982: ISBN 91-970475-09
  7. Weber RW. Cross-reactivity of plant and animal allergens.
    Clin Rev Allergy Immunol 2001,21:153-202
  8. Wodehouse RP. Pollen Grains. New York, NY: McGraw-Hill, 1935:387-8
  9. Sneller MR, Hayes HD, Pinnas JL. Pollen changes during five decades of urbanization in Tucson, Arizona.
    Ann Allergy 1993;71(6):519-24

As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.