w45 Alfalfa

Fabaceae (Leguminosae), a family of 630 genera with about 18,000 species, consists of 3 subfamilies: Mimosoideae, which includes Acacia, Mimosa, and Mesquite; Caesalpinioideae, which includes Honey locust; and Faboideae, which includes Peanut, Soybean, and Alfalfa.

Alfalfa is a perennial legume native to Europe, and now introduced as a forage plant to many other parts of the world. It is best suited to temperate and warm-temperate regions and thrives in semi-arid areas under irrigation. It is a common weed of roadsides, fence-lines, and waste areas. It can become a weed in cultivated crops in areas previously used for forage. (1)

Alfalfa is one of the most valuable forage plants. Alfalfa is the type species of the medicks. Other introduced medicks are annuals and include spotted medick (M. Arabica), California bur-clover (M. hispida), and black medick (M. lupulina). (1)

Alfalfa is a curled or twisted small leguminous plant. The plant grows to a height of up to 1 metre and has a deep root system. It has ascending stems up to a meter long, with stems arising from a thick woody crown. The leaves are alternate and compound, comprised of three hairy ovate leaflets that are 10-35 mm long. The terminal leaflet has a short stalk, while the lateral leaflets do not. (1)

Alfalfa flowers are blue to purple, 5-11 mm long, and borne in globe-shaped terminal clusters or in leaf axils. The fruit is a spirally-coiled brown pod, 5-8 mm long, with several seeds. In the northern hemisphere, flowering is from May to October. Alfalfa is entirely entemophilous. It is suggested that since Alfalfa is cut while in bloom, pollen dispersal may be facilitated by drying. (1)

Alfalfa is widely grown throughout the world as forage for cattle, and is most often harvested as hay, but can also be made into silage, grazed, or fed as greenchop. (2) It is also a commercial source of chlorophyll. Alfalfa seeds are small and kidney shaped, and usually used in sprout form for cooking.

Natural flavourant, for cola, liquor, and maple-flavoured beverages and cordials. A commercial source of chlorophyll. Alfalfa and alfalfa extracts are used as herbal medicines.

Allergen Description

No allergens have been characterised.

A pathogenesis-related (PR) class 10 protein isolated from Astragalus mongholicus had a 73.3% identity with the PR-10 protein PR10.2 from Alfalfa. (3) Bet v 1 homologues, which are panallergens, are PR-10 proteins. The clinical relevance of the Alfalfa PR10.2 has not yet been determined.

The minor Pear allergen, Pyr c 5, an isoflavone reductase (IFR), has an 80% amino acid sequence identity with Bet v 6, a minor Birch tree pollen allergen, and 59% with the IFR from Alfalfa. (4) The clinical relevance of the Alfalfa IFR has not yet been determined.

Potential Cross Reactivity

Cross-reactivity could be expected between species of the genus Medicago.

Clinical Experience

IgE-mediated reactions

Alfalfa is entirely entemophilous naturally, yet sensitization does occur, with markedly positive skin test, resulting in asthma, hayfever and allergic conjunctivitis in susceptible individuals, primarily among farm workers. (1, 5, 6, 7)

In 327 adult patients with respiratory, dermatologic and ophthalmologic diseases of suspected allergic origin attending a hospital in the United Arab Emirates, 22.9% were skin prick test positive to Alfalfa. (8)

In a study in Saudi Arabia 1,159 patients (806 Saudi Arabs and 241 Western expatriates (mainly North Americans)) attending an allergy clinic were skin prick test positive to one or more inhalants. Alfalfa was one of the 10 most frequent positive skin test positive allergens affecting 36% of Saudi Arab patients and 24% of North American expatriates. Other common allergens in the Saudi Arab patients were: Goosefoot (Chenopodium album) 53%, Kochia 51%, Mesquite tree 46%, Cottonwood tree 38%, dust mite-Dermatophagoides farinae 36%, Cockroach 35%, House dust 31%, Bermuda grass 29%, and Acacia tree 29%. For North American expatriates living in the area, the 9 other allergens were: dust mite-Dermatophagoides farinae 43%, House dust 41%, Alternaria 36%, grass mix 34%, Bermuda grass 33%, Mesquite 32%, Cat 31%, Kochia 28%, and Goosefoot 24%. (9)

An examination of the pollen content of the atmosphere of Montpellier, southern France, and compared with pollinosis of patients born and living in and around Montpellier, found that some patients had positive skin tests to Alfalfa pollen though these pollens were almost absent from the counts. The authors suggested that in a few cases local sources of these pollens could account for the positive skin tests but this could be attributed to cross-reactive mechanisms. (10)

In a survey of Ohio cash grain farmers, wheezing was associated with Alfalfa hay. Whether this was as a result of the production of hay, Alfalfa pollination, irritants or allergens, was not clarified. (11)

Other reactions

Farmer’s lung as a result of exposure to mouldy organic dust from Alfalfa hay has been described. (12, 13)

Rhinitis, asthma, and urticaria from alfalfa plants or milled powder in farmers, millers, or store workers have been described. (7)

Alfalfa sprouts has been associated with drug-induced lupus erythematosus, a lupus-like illness. (14)

Last reviewed: June 2022.