Anopheles stephensi puts urban populations at risk

Publicação: 9 de March de 2022

The vector has already become a global issue and can spread to Europe and the Americas, beyond Africa

Probably An. Stephensi has already spread beyond our current knowledge, says researcher

In 2012, an unusual outbreak of urban malaria was reported in the city of Djibouti (capital and largest city of the Republic of Djibouti, a country located in East Africa), and since then increasingly severe outbreaks have been reported annually due to an Asian mosquito called Anopheles stephensi, species known to proliferate in urban environments. Now, the main vector of malaria in India has become abundantly present in several locations in Africa. In addition to resistance to insecticides, An. stephensi is an urgent and dangerous threat to recent progress in malaria control. Compared to endemic African mosquitoes, the An. stephensi is one of the few species of anopheles found in urban environments.

An article recently published in the journal Parasites & Vectors BMC entitled Emergence of the invasive malaria vector Anopheles stephensi in Khartoum State, Central Sudan confirms the geographical expansion of the An. stephensi. Dr. Bashir Salim, one of the authors, recalls that this was the first peer-reviewed journal report on the emergence of this invasive vector of Asian malaria. “The An. stephensi in Sudan is very alarming. The emergence and spread of this efficient vector poses a threat to global health, particularly in sub-Saharan countries where the burden of malaria is greatest and the majority (more than 95%) of cases and related deaths are reported,” he points out.

According to the researcher, the An. stephensi has already become a global issue because, in addition to its rapid spread in Africa, it also emerged in Sri Lanka in 2017, a year after the country received the World Health Organization (WHO) certification in 2016, as malaria-free. This suggests that the vector may also spread to Europe and the Americas if effective measures are not taken. Dr. Salim points out that the invaded countries, their neighbors and those at risk need to strengthen control, surveillance and remain alert to the emergence of vectors of invasive diseases and impose the implementation of the International Health Regulations.

“Unless very drastic measures are taken, the spread of An. stephensi in Africa is inevitable. It is very likely that it has already spread beyond our current knowledge,” warns Dr. Salim. Measures include improving the capability of vector surveillance systems and incorporating advanced molecular and sequencing tools to monitor and track their spread, something similar to the European Program for the Monitoring and Control of Invasive Disease Vectors. In addition, the global health community should invest more in operational research to generate updated knowledge about bionomy and susceptibility of these vectors to insecticides used in the region.

To have an idea, in Dijibouti, infections increased 2,800 times between 2012 and 2020. About 60 thousand people from a population of 800 thousand contracted the disease in 2020. In 2021 there was a drop after a strong government campaign to spray areas where the mosquito was prevalent. Unfortunately, the increase in the number of malaria cases related to An. Stephensi is undeniable. “Unless effective action is taken immediately in the affected countries and those at risk, a terrible situation, especially significant malaria epidemics in densely populated areas, is inevitable,” he regrets. And can the increase in malaria cases in populated places on the African continent lead to the emergence of super mosquitoes? According to Dr. Salim, it’s the other way around. The unusual and unexpected increase in malaria cases is a strong epidemiological indicator of change in vector composition (emergence of competent invading vectors) or development of insecticide resistance among endemic vectors.

In addition, these vectors pose risk by spreading local malaria parasites. “The ability of An. stephensi to transmit Plasmodium falciparum and P. vivax was confirmed in laboratory and field investigations, therefore, its dissemination in a given area will change the epidemiology of malaria, resulting in a major threat to the disease control program”, warns the researcher.

Factors that influence the dissemination of An. Stephensi in Africa

Dr. Salim explains that there are several risk factors and at the top is high adaptability and acclimatization to new environments and climatic variability. The second is climate change that increases the adequacy of the environment. In third place there is unplanned urbanization in Africa, where people live in densely populated environments without drinking water supplies, forcing them to store in artificial containers, which are the An. Stephensi preferred breeding sites. stephensi. In addition, the presence of animals nearby, which increases survival capacity and population growth by providing sustainable sources of blood supply, which are essential for the maturation of eggs in the body of females.

The researcher also cites the biological and behavioral resistance of the vector to insecticides used in the region and local vector surveillance and control systems in Africa, which have reduced capacity in terms of financial support, technically qualified personnell and advanced molecular and sequencing tools to monitor changes in vector composition and its distribution. “That is, vector surveillance and control systems in Africa are not equipped to detect, report and control An. stephensi in a timely manner, as evidenced by the fact that all initial reports on the presence of An. stephensi in Djibouti in 2012, in Ethiopia in 2016, and in Sudan in 2019 and 2021 were accidental,” he said.

Regarding the reduction of damage to the environment and biodiversity, Dr. Salim suggests that health stakeholders such as regional ministries of health, their partners such as WHO and the Global Funds support the implementation of non-insecticidal and environmentally friendly base vector control interventions. This can also help reduce the economic burden on endemic countries and prevent the commercialization of global health, dependent on the insecticide industry. The researcher also says that special attention is needed at international entry points, such as land border crossings, airports and seaports, as they are the routes for the introduction of this vector from endemic areas, already invaded, to currently free areas.

Six big challenges of An. stephensi

– It is a competent vector for both Plasmodium falciparum and P. Vivax;

– It is resistant to local vector control tools in Africa;

– Rapid ability to disseminate and expand its geographical distribution, in addition to being ableto quickly adapt to new environments.

– Limited information on their bionomy in Africa, which hinders the ability of local surveillance and vector control systems to find, identify and control;

– It prefers to feed on animals that are not commonly the target of public health interventions in vector control programs;

– Preference for reproducing in water stored in artificial containers, which means that the malaria transmission station will be expanded from the 3-5 months of the rainy season to the whole year.

Finally, Dr. Salim emphasizes that instead of considering this as an opportunity for institutes in developing countries to conduct research, they should look at it as a threat. “We would like to call on the entire global health community to collaborate and support the elimination of this invasive vector from the currently invaded areas, as well as to prevent further dissemination. Governments, donors and especially the community at risk must collaborate collectively on this urgent issue,” he concludes.

Besides Djibouti, An. stephensi has already been detected in Ethiopia, Darfur, and Sudan. Scientists also believe that it may have already spread along East African trade routes to South Sudan, Uganda, and northern Kenya. This is not the first malaria vector to invade a new continent, there are previous examples that were regional and are becoming global, such as the Asian tiger, which has invaded northern Europe.

Approximately 126 million people in danger in Africa

A study by researchers at Oxford University and the Liverpool School of Tropical Medicine entitled “A new malaria vector in Africa: Predicting the expansion range of Anopheles stephensi and identifying the urban populations at risk estimates that about 126 million people in African cities may be in danger.