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Congress grass: why Parthenium causes a rash as well as sneezing
- Parthenium hysterophorus — congress grass, gajar ghas — arrived in India in the 1950s in imported grain and now grows on almost every vacant plot.
- It causes two different problems by two different mechanisms: a skin reaction that builds over days, and nasal and chest symptoms that come on within minutes.
- It is the commonest cause of plant dermatitis in India, and is reported as responsible for close to 40% of patients attending contact dermatitis clinics.
- It affects men who work outdoors far more often — around five times more — and is frequently mistaken for a fungal infection or for "sun allergy".
- The skin form is diagnosed by patch testing, the airway form by skin prick testing. They are not the same test.
A weed with a history
Parthenium is not native to India. The Indian Journal of Dermatology, Venereology and Leprology records that it was introduced into Asia in cereal and grass seed shipments from the United States during the 1950s, and that parthenium dermatitis was first reported in India at Pune in 1968. In the decades since it has spread across the subcontinent with remarkable success, and around Hyderabad it is everywhere: roadsides, railway sidings, the edges of building sites, and every plot waiting to be developed.
The plant has small white flowers and finely divided leaves, and looks unremarkable. The relevant chemistry is in the sesquiterpene lactones it contains, principally parthenin, carried on trichomes on the leaf and on airborne particles.
Two problems, two mechanisms
The skin reaction
This is a delayed, cell-mediated reaction — a contact dermatitis — and it builds over hours to days rather than minutes. It classically appears on the exposed parts: face, eyelids, neck, the V of the chest, forearms and the backs of the hands. The skin becomes itchy, thickened, darkened and leathery, and in established cases the whole face can be involved.
Two mistakes follow from this pattern. Because it affects sun-exposed areas it is very often called "sun allergy" or a photodermatitis. And because the skin is thickened and dark it is often treated for months as a fungal infection with antifungal creams that do nothing. Neither is right, and patch testing is what settles it.
The airway reaction
Separately, Parthenium pollen is an aeroallergen, producing immediate IgE-mediated symptoms: sneezing, a running then blocked nose, itchy eyes, and in sensitised asthmatics a tight chest and wheeze within minutes of exposure. Indian work published in Clinical & Experimental Allergy established immediate hypersensitivity to Parthenium and described parthenium rhinitis.
The same person can have both, or either alone. They are diagnosed differently, and this is the practical point of the article: a patch test does not detect the airway allergy, and a skin prick test does not detect the contact dermatitis.
Who gets it
The published Indian series describe a strong skew towards middle-aged and older men who work outdoors — farmers, construction and roadside workers, gardeners, drivers — with a male-to-female ratio of roughly five and a half to one. In an expanding city that is a very large group of people, many of whom have been treating the rash themselves for years with whatever the pharmacy suggested.
What helps
- Avoidance, as far as is realistic. Clearing the weed around the house and workplace before it flowers, and not burning it — burning aerosolises the allergen and makes things worse for everyone nearby.
- Barrier and clothing. Full-sleeved cotton, a cap, and washing exposed skin as soon as you come in. Changing out of work clothes before sitting on furniture keeps the allergen out of the house.
- Treating the skin properly — which usually means topical steroids of appropriate strength, sometimes short systemic treatment for severe flares, and stopping the antifungal that was never going to help. Severe, persistent disease is managed with a dermatologist.
- Treating the airway — intranasal steroid, antihistamine, and asthma treatment where the chest is involved.
- Immunotherapy, where testing confirms Parthenium sensitisation and symptoms justify it. This is one of the allergens for which extracts are available in India and where a three-year course can genuinely change the disease.
- At this clinic: skin prick testing including Parthenium, assessment of the airway, and immunotherapy where indicated
- Referred: patch testing for the contact dermatitis, and dermatology input for severe or long-standing skin disease
Next step
Get the right test for the right problem
Tell us whether the trouble is mainly the skin, mainly the nose and chest, or both — it determines which test is arranged.
Sources
Parthenium dermatitis in India: past, present and future. Indian Journal of Dermatology, Venereology and Leprology.
Sriramarao P et al. — immediate hypersensitivity to Parthenium hysterophorus and parthenium rhinitis. Clinical & Experimental Allergy, 1991.
General information, not a substitute for consultation. Reviewed by Dr. Radhika Sharma, September 2026.