The Scourge of India

Santa Maria feverfew (Parthenium hysterophorus)

How often we meet a plant that is so poisonous, it harms humans in a scale of epidemic proportion? Behold the Santa Maria feverfew (Parthenium hysterophorus, Asteraceae), also known as the scourge of India or the worst invasive weed of the 21st century. If we can combine the poison ivy and hogweed into a super badass plant, it’s this one! It can literally poison you from afar in the air you breathe. 

 

Parthenium hysterophorus belongs to the Asteraceae or sunflower family. If you examine its ‘flower’ carefully, the flower-head is actually a composite of numerous small flowers, even the ‘petals’ are individual flowers! Every flower can produce a seed, which is propagated by air. The genus Parthenium is so named because it resembles a medicinal herb called feverfew (Tanacetum parthenium), which is related to wormwood and chrysanthemum. The species name hysterophorus translates to womb-bearing, because the plant’s fruit looks like a human reproductive tract. P. hysterophorus is native to tropical America and it was described by Swedish naturalist Carl Linnaeus more than two centuries ago in 1753.

During the 1950s, seeds of P. hysterophorus contaminated cereal or grass seeds shipment from the USA, introducing the species to almost all tropical countries across the world, particularly in India. Since then, epidemics of skin allergy (contact dermatitis), hay fever (allergic rhinitis) and asthma ripped through the country. The cause was quickly established as exposure to P. hysterophorus, but it was already too late. The species was fully naturalised in all of India, and in some areas forming huge expanse of mono-colonies. Even today, P. hysterophorus is responsible for 40% of all skin allergy cases in India, affecting numerous people beyond count. Parthenium allergy  or dermatitis (inflammation of the skin) presents itself in two forms, i.e., acute and/or chronic. In acute disease, it causes skin lesion similar to those induced by poison ivy (urushiol). The acute dermatitis is unpleasant but it can be treated by topical corticosteroid and avoidance of allergen. However, it is the chronic disease that makes Parthenium dermatitis hard to manage. If a patient is continuously exposed to P. hysterophorus, the allergy can manifest itself in chronic hay fever or asthma, and dermatitis that leads to abnormal tissue growth all over the body (lichenoid eruptions, nodules and/or vesicular eczema), which is disfiguring and debilitating. Worse still, chronic Parthenium dermatitis can be induced by photosensitisation, similar to hogweed toxicity. For reasons yet unknown, sun exposure can intensify dermatitis caused by P. hysterophorus. Chronic Parthenium dermatitis greatly reduces quality of life and it requires long term treatment with expensive prescription drugs. For people living in poverty, inability to work due disfigured limbs is a death sentence.

 

All parts of P. hysterophorus, particularly dried hairs (trichomes) can induce allergy in humans. Even the airborne pollen is implicated in seasonal contact dermatitis, which is most prevalent during summer. In fact, it is the airborne allergen that tends to give chronic systemic (full-body) disease. This makes P. hysterophorus one of the few poisonous plants that can poison humans from a great distance away! Just when you think things can’t get any worse, it actually does! Removal of P. hysterophorus is a great challenge, the toxic allergens are known to penetrate latex or rubber gloves. If one’s allergic to the plant, gloves may not offer adequate protection, pulling out the weed is unsafe. Even plant materials that stain clothing are known to cause secondary allergy to humans. The current medical consensus is to wash the body and/or clothing frequently (hourly!) in vicinity where P. hysterophorus is prevalent. Burning the plant is not an option too because the allergens can get aerosolised in burning smoke, poisoning even more victims. Presently, chemical control with powerful herbicides is the only effective solution, and scientists are seeking novel biological control methods to eradicate P. hysterophorus.

 

Parthenium hysterophorus, dense mono-colony

Can it get worse still? Absolutely! P. hysterophorus is a highly adaptable and vigorous species. It is not just toxic to humans, but also to plants. This species is well-known to produce toxic chemicals from its roots that will seep into the ground and poison nearby plants. This is a phenomenon called allelopathy, and it enables P. hysterophorus to kill its competitors (mostly native plants) and colonise a given habitat. In areas where P. hysterophorus is prevalent, often no other plants can be found. This just adds to the production of allergens in a vicious cycle.

 

P. hysterophorus is undoubtedly one of the most harmful plants that I have featured here, yet it remains largely unknown to the public (especially Western countries). A first world plant that became a third world problem, who cares? That’s the sad fact of humanity. But is it the fault of the plant? Or is it human greed and ignorance? What solutions lie in the future of managing Parthenium dermatitis? For that we will have to look into the chemistry of its toxin in [Science].