Invasive 'famine weed' could escalate the spread of malaria

An invasive weed could escalate the spread of malaria in East Africa after scientists found new evidence that it creates a favourable breeding ground for female mosquitoes, which transmit malaria.

In a study published in Scientific Reports last month, researchers found that the plant popularly known as "famine weed" releases chemicals called terpene from its roots that have a "distinct blend of mosquito-attractive fragrances".

This weed which is generally known to be toxic to both humans and livestock is readily ingested by malaria mosquitoes for sugars as a source of energy and it also tolerates its key toxin called parthenin."

Baldwyn Torto, aco-author of the study and head of the behavioral and chemical ecology unit, International Centre of Insect Physiology and Ecology (icipe), Kenya

He adds that as adult female mosquitoes feed on the weed known scientifically as Parthenium hysterophorus, they acquire sugars, a vital energy resource for flight to find a mate for reproduction, and seek humans to bite for a blood meal required for developing their eggs.

Torto tells SciDev.Net that female adult mosquitoes emerging from breeding sites contaminated with famine weed chemicals live a week longer than normal, increasing their likelihood to transmit the malaria parasite.

Researchers assessed whether gravid (pregnant) female mosquitoes would prefer to lay their eggs in distilled water or water treated with a blend of terpenes extracted from famine weed roots. The weed that thrives in warmer climates is a native of north-east Mexico and is endemic in America.

"Gravid females were more attracted to lay eggs in the root exudate-treated water… than in the distilled water control," the study says.

Eunice Anyango Owino, a medical entomologist at the School of Biological Sciences, University of Nairobi, Kenya, says that this weed could be considered as "the most destructive" globally.

"The findings that it favours the breeding of malaria-transmitting mosquito has only exacerbated the already bad situation considering that malaria remains a major cause of mortality and morbidity, especially in children below five years old and pregnant women," she explains.

Owino says that the weed with its roots could be removed physically from water bodies besides using herbicides but she cautions that increased use of chemicals could be dangerous to the environment, costly to purchase and apply them.

Torto adds that the fragrance produced by famine weed can be exploited to develop trapping tools to target egg laying females as part of disease surveillance and control.

Torto explains that natural enemies such as stem-boring weevil (Listronotus setosipennis) and the leaf-feeding beetle (Zygogramma bicolorata), which targets the vegetative and reproductive parts of the weed under different environmental conditions and habitats, could help curb its spread.

According to a report published in May by the International Association for the Plant Protection Sciences, the Kenyan government has imported these two insects for biological control of the weed that has now invaded at least 48 countries globally including Ethiopia, Kenya, Mozambique, South Africa, Tanzania, Uganda and Zimbabwe.

But Owino cautioned that the control of the invasive weed could be problematic as the plant's rapid growth rate and numerous but small seeds make it very hard to control.

"The seeds can easily be transported to new areas by even the wind, animals and water, are resistant to adverse weather, and remain viable for germination over a long time," she tells SciDev.Net.