Invasive Mosquito Tied to Malaria Outbreak in Ethiopia

The species can survive the dry season and infect residents of urban areas

A close-up view of a mosquito biting a human with a droplet of blood emerging from the mosquito's abdomen.
An Anopheles stephensi mosquito feeds on a human.  Smith Collection / Gado / Getty Images

An outbreak of malaria has hit Ethiopia’s fourth-most populous city, Dire Dawa. In 2019, the city had only 205 reported cases of the life-threatening, flu-like disease, but that number soared to more than 2,400 in the first half of 2022 alone, according to Science’s Gretchen Vogel.

Scientists presented evidence Tuesday tying this outbreak to an invasive mosquito species native to Asia, called Anopheles stephensi, at a meeting of the American Society of Tropical Medicine and Hygiene.

In Africa, malaria is common during the rainy season or in rural regions, according to NPR’s Ari Daniel. But the invasive A. stephensi can survive through the dry season and thrive in densely populated residential areas.

The outbreak earlier this year “was the first urban malaria outbreak in Ethiopia during the dry season,” Sarah Zohdy, an entomologist with the U.S. President’s Malaria Initiative who contributed to the new findings, tells NPR.

“Instead of two or three months, [malaria season] will be 12 months a year,” Ayman Ahmen, a researcher at the University of Khartoum in Sudan who was not involved in the study, tells New Scientist’s Corryn Wetzel. “Catastrophe is coming.”

A mosquito sits on a window net inside a house in India.
A female Anopheles stephensi mosquito inside a house in West Bengal, India.  Soumyabrata Roy / NurPhoto via Getty Images

Malaria is caused by parasites spread through mosquito bites. In 2020, there were an estimated 241 million cases of malaria worldwide and 627,000 deaths caused by the disease, according to the World Health Organization. In Africa, more than half a million people die from malaria each year, most of whom are children under five, Science reports.

Continent-wide, most malaria cases in Africa are spread by the mosquito species Anopheles gambiae. But A. stephensi, which is native to India and the Persian Gulf and has been a major spreader of malaria there, is beginning to establish itself along the eastern coast.

The invasive species was first spotted in Africa in Djibouti in 2012 and has subsequently been detected in Sudan, Somalia and Nigeria, as well as in Yemen, according to Maria Cheng of the Associated Press (AP).

When the malaria outbreak began in Dire Dawa earlier this year, “there was a huge increase in cases, but there was no formal investigation of what caused the increase,” Fitsum Tadesse, a molecular biologist at the Armauer Hansen Research Institute in Ethiopia and one of the researchers behind the new findings, tells New Scientist. “So, we decided to jump in and investigate.”

In the study, which took place between April and June 2022, the researchers contact traced malaria patients in Dire Dawa and searched for mosquitoes near their homes. Ninety-seven percent of the adult mosquitoes the researchers caught were the invasive kind, while none of the captured non-invasive mosquitoes carried the disease-causing parasites, writes Science News Jake Buehler.

The findings confirm that the invasive insects caused the increase in malaria cases, Marianne Sinka, who studies malaria-transmitting mosquitoes at the University of Oxford in England and was not involved in the research, tells Science.

“This new evidence is terrifying,” Thomas Churcher, an epidemiologist at Imperial College London in England, who did not contribute to the work, tells the AP. “If these mosquitoes get a toehold in Africa, it could be phenomenally bad.”

Most African mosquitoes lay eggs in rainy-season puddles, writes Science. But the invasive species “prefers to breed in water storage containers that are typically common in rapidly expanding urban settings,” bringing the insect vectors closer to city residents, Tadesse tells Science News. The researchers found that households with wet habitats nearby were 3.4 times as likely to have a resident test positive for malaria than households without a nearby water source.

The invasive mosquitoes are also capable of evading control tactics such as bed nets and indoor spraying, since they typically bite people outdoors, writes the AP. These invasive insects have largely been resistant to the most common insecticides used in Africa, per Science.

“If we keep doing the same thing, we won’t be successful in targeting this mosquito,” Tadesse tells NPR. “We need to be innovative.”

Since A. stephensi feed on cattle, one strategy could be treating livestock with insecticides, Martin Donnelly, an evolutionary geneticist at the Liverpool School of Tropical Medicine (LSTM) in England, tells Science. Tadesse tells NPR that covering necessary water storage containers and removing unneeded ones could also help. And last year, the WHO recommended a malaria vaccine for children in countries with high transmission rates.

“We’re waiting to see the impact of new tools like pesticides and vaccines,” Anne Wilson, an infectious diseases expert at LSTM, says to the AP. “But if this mosquito starts to take off, we may be out of time.”

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