Mosquito on the move

Located at a critical crossroads of international trade between Africa, Asia, and the Middle East, the bustling port of Djibouti welcomes hundreds of ships every day. In the not-too-distant past, a dangerous stowaway may also have disembarked there, and its arrival in Africa could potentially derail anti-malaria efforts across the continent.

This unwanted intruder is the mosquito species Anopheles stephensi. Native to parts of South Asia and the Arabian Peninsula, An. stephensi mosquitoes are very good at transmitting malaria to humans and, unlike other vectors of the disease in Africa, flourish in urban settings.

In 2012, when An. stephensi was first detected in Africa, Djibouti reported only 27 presumed or confirmed malaria cases and seemed well on its way to eliminating the disease. But by 2020, the country’s malaria caseload had reached over 73 000.

Number of reported malaria cases in Djibouti, 2010–2020

Source: World malaria report 2021

“Stephensi is very competitive and efficient in malaria transmission,” says Colonel Dr Abdoulilah Ahmed Abdi, a health advisor to Djibouti’s president who was instrumental in sounding the alarm about this deadly new malaria vector. “At the same time, it’s very adaptable to urban areas.”

Continued invasion in Africa

Worryingly, the invasive mosquito has continued to spread to other countries in the African continent, with cases reported in Ethiopia and Sudan (2016), Somalia (2019), and Nigeria (2020).

“Native [malarial] mosquitoes are thought to breed mainly in rural settings,” notes Dr Fitsum Tadesse, a lead scientist at the Armauer Hansen Research Institute who has been monitoring An. stephensi’s spread in Ethiopia and helped draft a five-year national response. “But this new mosquito can breed in water storage containers,” he adds. “These are typically common in rapidly expanding urban settings that have poor infrastructure and sewage systems, where people have to store water everywhere in containers.”

Compounding the problem, many traditional anti-malaria tools are harder to implement against An. stephensi. “Its resting and feeding behaviour are quite different from other vectors,” notes Dr Delenasaw Yewhalaw, an entomologist at the Tropical and Infectious Diseases Research Center of Jimma University in Ethiopia.

“Most of the conventional intervention tools like IRS [indoor residual spraying] or long-lasting insecticidal nets may not work or may not be as effective to contain this species.”  An. stephensi also exhibits an alarming resistance to most insecticides, he adds.

Taken together, An. stephensi could pose a significant threat to Africa’s efforts to control and eliminate malaria. One study projected that the species could put an additional 126 million people at risk of the disease if allowed to spread unchecked. At the same time, much more research is needed to determine when, where, and how An. stephensi is moving across the continent.

For one, it’s still unclear how and when this species originally arrived in Djibouti, and whether this was the only country where the vector entered the continent. And, while its presence strongly correlates with the rapid increase in malaria cases both in Djibouti and in the nearby Ethiopian city of Dire Dawa, the extent to which An. stephensi might drive malaria surges in other African cities is unknown.

New WHO initiative

To help find answers to these questions, strengthen research and vector surveillance, and encourage a comprehensive response to this threat, WHO is launching a new initiative to stop the further spread of An. stephensi in the region and determine if it can be eliminated from the areas it has already invaded.

One critical component of this work will be to encourage collaboration across sectors and borders. “There has to be multisectoral and cross-border collaboration,” argues Dr Yewhalaw. “This is an urban malaria vector. Ministries of Agriculture, Health, Education, Environment, Sanitation, Water Resources and Municipalities/City Administrations – all need to work together.”

Another important part of this initiative, notes Dr Yewhalaw, is boosting awareness of An. stephensi in the cities and communities most at risk, especially those where conventional surveillance methods and interventions are hard to implement. “People should be aware that the water they use for domestic purposes could be a good breeding ground for this invasive species.”

A better understanding of the role and invasive reach of An. stephensi may also help scientists and public health officials track and trace its local mosquito relative Aedes aegypti, the species that is the primary vector of arboviruses like dengue fever, chikungunya and yellow fever. Like An. stephensi, Ae. aegypti also travels far from home and shares the ability to breed in small pockets of standing water.

“Whenever we look at the breeding sites for Anopheles stephensi , we’ve also seen that Aedes aegypti  is there,” notes Dr Renee van de Weerdt, WHO Country Representative for Djibouti. “And that’s why, for us, in support of the Ministry of Health and other relevant parts of the Djiboutian Government, having joint vector control interventions is important.”

For Dr Ghasem Zamani, a malaria advisor with the WHO Eastern Mediterranean Regional Office, understanding and grappling with the rapid spread of An. stephensi will be critical to protecting against disease outbreaks, particularly in urban settings in the coming years.

“These vectors have been expanding their range for many reasons. Rapid ecological change, which includes human and environmental change, rapid urbanization, population movement, security issues, and increasing trade between countries – all of these are changing the movement of goods, people, and vectors. It is not just An. stephensi coming to Africa, it’s also Ae. aegypti invading more areas than ever before.”

With 40% of the population of sub-Saharan Africa living in cities, continued traveling by An. stephensi is especially worrying. “Djibouti is a gate, and An. stephensi will spread very quickly,” notes Colonel Abdi. “We have large cities in Africa and it’ll be a big, big threat. And with climate change, I'm sure it will reach some other Western countries too. We should have a global policy.”

Read more on the new WHO initiative to stop the spread of An. stephensi and the Global vector control response.