Study calls for robust surveillance to detect unusual outbreaks in African countries

Study calls for robust surveillance to detect unusual outbreaks in African countries

A new study has linked an invasive insecticide-resistant mosquito native to South Asia to an unprecedented urban malaria outbreak in Ethiopia, an Africa with previously low disease rates.

The report, presented today at the annual meeting of the American Society of Tropical Medicine and Hygiene (ASTMH), follows reports of a suspicious 40-fold increase in malaria cases in neighboring Djibouti since 2013, with new evidence that this mosquito, which was not confirmed in Africa until 2012, poses a significant threat to the goal of reducing malaria on the continent eliminate.

Malaria in Africa is typically associated with rainy seasons in rural areas, but this mosquito caused a 10-fold increase in malaria infections in just three weeks in an urban area during a dry season. Unlike the mosquitoes that typically transmit malaria parasites in Africa, this one is best known for its ability to thrive in artificial water storage tanks like those seen in rapidly growing urban neighborhoods.”

Fitsum G. Tadesse, molecular biologist, Armauer Hansen Research Institute

The outbreak occurred in Dire Dawa, a city of about 500,000 people in eastern Ethiopia that typically sees only about 200 cases a year. But between January and May 2022, when there is little rain and infections are particularly rare, about 2,400 cases were reported. A study by Tadesse and his colleagues found that the increase was caused by a mosquito called Anopheles stephensi. They also found that the mosquito was resistant to the insecticides most commonly used to control malaria via treated bed nets and indoor insecticide spraying.

Most malaria in Africa is caused by a species of mosquito known as Anopheles gambiae, whose populations rise and fall with rainy seasons that swell rural waterways. Anopheles stephensi mosquitoes have long been the primary vectors of malaria in urban areas of India and Iran. But they were never seen in Africa until 2012, when they were reported in Djibouti, a tiny East African country of about 1 million people that was then on the verge of eradicating the disease. A. Stephensi mosquitoes likely arrived in shipping containers, Tadesse said, but they did not appear to pose a significant threat until 2020, when they were suspected to be the cause of the steadily rising number of malaria infections in Djibouti since 2013.

Now there is new evidence that they were the source of the Dire Dawa outbreak - along with the recent evidence from An. Stephensi mosquitoes thousands of miles away in Nigeria, West Africa - is fueling concerns about a new front opening in Africa's long-running battle against malaria. Africa suffers 95 percent of the world's 627,000 annual malaria deaths, and most victims are children under 5 years old. In recent years, progress in combating the disease has stalled.

“This is not like any other malaria-carrying mosquito we have seen before in Africa,” said Sarah Zohdy, PhD, a disease ecologist and An. Stephensi is an expert at the US Centers for Disease Control and Prevention (CDC) working with the US President's Malaria Initiative (PMI), a key partner for the Ethiopia study. “This mosquito’s ability to survive in dry seasons and urban environments has the potential to change the landscape of malaria in Africa,” she said. “It could result in malaria expanding from a predominantly rural disease to a rural and urban challenge, also affecting Africa’s fast-growing and densely populated cities, where infection rates have been comparatively low.”

Meanwhile, a separate study presented at the ASTMH annual meeting reported new evidence that An. Stephensi mosquitoes are ubiquitous in and around households in nine Sudanese states. Zohdy said the concern about An's detection. Stephensi mosquitoes in Sudan - and also in Nigeria - is that they were able to follow the pattern of Djibouti, where several years passed between the discovery of the mosquito and its connection to a wave of malaria infections.

Zohdy said CDC and PMI, which are jointly implemented by USAID and CDC, are actively working to mitigate the threat of An. stephensi through the use of improved vector and disease surveillance. PMI and CDC are also working with the World Health Organization, which just last month launched an initiative to stop the growing spread of An. Stephensi in Africa. Additionally, PMI guides the use of interventions informed by An. Stephensi's unique biology, including the development of methods to limit breeding sites in urban areas.

What the outbreak in Ethiopia tells us about the new threat of malaria in Africa

Tadesse said malaria experts in Ethiopia were alerted to the potential threat of An following outbreaks in neighboring Djibouti. Stephensi mosquitoes. He said Dire Dawa was a railway junction connected to Djibouti. When malaria infections suddenly spiked earlier this year, he said he and his colleagues "immediately started thinking about 'Stephensi'" and "we jumped in to determine the causes of the change." Driven by urgency, the team, including Tadele Emiru and Dr. Your Getachew completed his field work in just over two months.

In addition to linking An. stephensi on malaria infections, Tadesse and his colleagues conducted extensive testing in water sources in households and neighborhoods where cases occurred. They found evidence of malaria-carrying An. stephensi in nearby water tanks, confirming what was already known from extensive work in India, where An. Stephensi is best known as a “container breeder”. But he said they also found plenty of An. Stephensi mosquitoes on the edge of streams and nearby rivers.

“We should be careful to think of it as just a container grower,” Tadesse said. "It can survive in artificial or natural water sources. It can also feed on animals or humans. Basically, our results suggest that it can survive anywhere, so we have to look for this mosquito in places where there were no humans before and expect to find it."

For example, he said projections that 126 million additional people in Africa could be at risk of An malaria. Stephensi mosquitoes may be underestimating the threat if Dire Dawa's evidence of the mosquito's adaptability is confirmed elsewhere.

“We are fortunate that this team acted quickly to confirm in just a few months that An. stephensi mosquitoes were the source of an unusual dry season urban outbreak,” said ASTMH President Daniel Bausch, MD, MPH&TM, FASTMH. “Robust surveillance to detect and investigate unusual outbreaks in African countries is essential to developing strategies to prevent this invasive mosquito from derailing Africa’s efforts to eradicate malaria.”

Source:

American Society for Tropical Medicine and Hygiene

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