Almost a decade ago, the Republic of Djibouti seemed on the verge of malaria eradication. In the small country on the Horn of Africa there were only 27 cases in 2012. But between February and May 2013, the disease broke out 1,228 times, followed by another wave of more than 2,100 cases in November 2013. Strangely, many people in the capital, Djibouti City, fell ill. In Africa, malaria is primarily a rural problem.
The outbreak is not an anomaly, but rather marks the arrival of a new threat: the invasive mosquito Anopheles stephensi, which appears to have made its way into Africa just before breaking out from Asia. Djibouti had more than 73,000 cases of malaria last year, at least partly due to this mosquito. Recently, a study was presented at the annual meeting of the American Society for Tropical Medicine and Hygiene (ASTMH) in Seattle, linking an unusual outbreak of malaria in an Ethiopian city earlier this year to the same pathogen. On. stephensi has also been sighted in Sudan, Somalia and Nigeria, and may be lurking in other countries as well.
“This is one of the largest movements of a malaria vector in the last 50 years,” says Seth Irish, a medical entomologist at the World Health Organization (WHO). There is increasing evidence that malaria, which kills half a million people in Africa every year, most of them children under the age of five, is being exacerbated. To better assess the threat, the WHO has launched a new initiativeto increase surveillance of the species and study its habits.
Anopheles stephensi does not only thrive in the rainy season like its African relatives
in one 2014 in Acta Tropical published article The outbreak in Djibouti was first reported that An. stephensi, native to southern Asia and the Arabian Peninsula, made the short leap to Africa. The article also sounded alarm about what this might mean: Unlike most African mosquitoes that transmit malaria parasites, An. stephensi in cities — which could make the mosquito’s arrival a “significant future health threat” for Africa, the researchers warned.
On. stephensi is known to be an efficient vector of malaria in its native habitat, particularly in cities. The mosquito can carry both of the parasites that cause most cases of malaria in humans: Plasmodium vivax and the more deadly P. falciparum. It thrives in artificial water sources such as cisterns and even deep wells, allowing it to stay active even during dry seasons. Their African relatives, like An. gambiae and An. funestus, prefer more rural environments and lay their eggs in puddles, which in many countries only occur during the rainy season, so they are spared the disease the rest of the year.
Proven protective measures such as mosquito nets may not be very effective against Anopheles stephensi.
(Photo: OLYMPIA DE MAISMONT/AFP)
A 2020 study estimated that 126 million people could be at increased risk of malaria, if the mosquito spreads uncontrollably. “However, we don’t know much about where it is and what contribution it really makes to transmission,” says Jan Kolaczinski, a medical entomologist at the WHO. Epidemiologist Anne Wilson, who with her colleagues from the Liverpool School of Tropical Medicine (LSTM) studied the spread of An. stephensi in Sudan and Ethiopia notes that in some areas where An. stephensi was found, the cases of malaria have not increased.
To better understand the role of the mosquito, molecular biologist Fitsum Tadesse of the Armauer-Hansen Research Institute in Addis Ababa, Ethiopia, and his colleagues tracked malaria cases in Dire Dawa, a city in the east of the country that was hit in the first half of this year An unusual outbreak of more than 2,400 cases occurred during the dry season last year. In all of 2019, there were only 205 cases in the city. They tested household members of 80 malaria patients and compared them to households of 210 people who did not have the disease. People living with people with malaria had a 5.6 times greater risk of infection. The team found that more mosquitoes breed within 100 meters of infected households – and 97 percent of adult mosquitoes were An. stephensi.
Standard mosquito control measures may be ineffective against the invasive species
The study provides the clearest evidence yet that the invasive insect can cause a rise in malaria cases, says Martin Donnelly, an evolutionary geneticist at LSTM who was not involved with the study. “It’s a big step forward.”
At the ASTMH meeting, data from the University of Khartoum entomologist Hmooda Kafy showed that An. stephensi is found in 39 out of 61 surveyed areas across the country. In some areas, the mosquito has been detected in or near nine out of ten households. The mosquito species has not yet been detected in Kenya, but researchers are reviewing archived samples there. At the same time, the country has stepped up its surveillance efforts. Tadesse suspects that the mosquito has spread more widely than previously thought, possibly by hitchhiking in shipping containers: “It’s likely that you can find them in all corners of the continent,” he says.
Although to. stephensi is well adapted to city life, but it also breeds in cisterns or wells in the country, notes Sarah Zohdy, an entomologist at the CDC. “We call it an urban vector, but it’s actually everywhere,” she says.
The tribes of An found in Africa. stephensi are largely resistant to the most commonly used insecticides. They may prefer barns or sheds to dwellings, and will sting people when they are outside. As a result, standard mosquito control measures such as insecticide-treated bed nets and indoor spraying with insecticide may not be very effective.
One control measure is to cover water reservoirs to prevent adult mosquitoes from laying their eggs, but this often proves difficult to do. Another option is to add an insecticide to the water that targets the immature mosquitoes in their larval stage. Both approaches also help fight Aedes mosquitoes, which transmit viral diseases like dengue and chikungunya.
More research on An. stephensi is urgently needed so that the WHO can fine-tune its recommendations to combat it, says Seth Irish. “It’s a big problem, but we need to invest time and effort to understand the real implications.”
This post comes from the science magazine Science. It is not an official translation of the Science-Editorial staff. In case of doubt, the English original, published by the AAAS, applies. German editing: cvei