Authors: Jeremy Herren, Scientist, International Center for Insect Physiology and Ecology; and Clifford Mutero, Consultant Scientist, International Center for Insect Physiology and Ecology
A species of mosquito that can transmit malaria – known as Anopheles stephensi – has invaded East Africa and is rapidly moving around the region. Moina Spooner of The Conversation Africa asked Jeremy Herren and Clifford Mutero to provide some insight into why this invasion is happening and what can be done to protect people from it.
How did Anopheles stephensi come to africa?
This species of mosquito, Anopheles stephensi, is widespread in Southeast Asia and parts of the Arabian Peninsula. It is common in India, Pakistan, Iran, Iraq, and Afghanistan. There have been increasing reports of this in Africa in recent years. It was originally reported in Djibouti in 2013.
Recent reports show that it is spreading rapidly in the Horn of Africa. It was reported in Ethiopia in 2016 and Sudan in 2017 and is likely to be spread on major transport routes. As a result, the World Health Organization has issued an increased surveillance alert to track the spread. We expect it to be found in other major African cities at some point.
Monitoring data is required to confirm further dissemination. However, based on the timetable for the trip to Ethiopia and Sudan, we speculate that it would have taken three years to reach Kenya and Tanzania. You are now within that risky time frame.
Kenya and Tanzania may be at particularly high risk due to their proximity to the Horn of Africa. They also have large coastal cities whose weather conditions (warmer and wetter) are similar to the mosquito’s native range. Other more distant cities, including some in West Africa, also have suitable environments for Anopheles stephensi.
In general, the spread of mosquitoes to new areas is facilitated by humans by ground, air and sea transportation systems. Increasing international travel and migration of people mean that vectors and pathogens appear or reappear in regions where they were removed or eradicated.
How is this mosquito different from those on the continent?
There are over 100 species of Anopheles mosquito in Africa, but only six are considered to be the “primary” vectors of malaria.
Anopheles stephensi is very effective in transmitting malaria. What is worrying is that, unlike the various African Anopheles species, it also thrives in urban areas.
Anopheles mosquitoes need water to complete their life cycle: a female mosquito lays her eggs on the surface of a water source, where they hatch and eventually develop into adult mosquitoes. Female mosquitoes suck blood from humans and other hosts so they can lay eggs. It is blood nutrition that enables mosquitoes to transmit parasites – like malaria – from one person to another.
Typically, the main African Anopheles vector species are found in rural landscapes – which is why most cases of malaria in Africa are also in rural areas. They breed in various aquatic habitats such as puddles, footprints, and hoof prints on the edges of ponds and irrigated farmland. These habitats are also found in some urban areas but are often polluted and less suitable for these mosquitoes. However, reports suggest that some African anopheles are increasingly adapting to these conditions.
In contrast, while Anopheles stephensi also thrive in rural areas, they thrive in urban areas – for example, in plastic and cement containers that hold water. This means that this species is a threat in both cities and rural areas.
What new challenges does it bring with it?
The main problem is that if the invasion becomes widespread, malaria could become more common in African cities and this would put many more people at risk of infection. As a result, malaria control efforts across the continent will be even less developed as malaria expands into cities.
There are already many challenges – such as a lack of resources. Around half of the $ 6.5 billion needed to meet the 2030 malaria targets is currently available. There were an estimated 219 million cases of malaria in 2017, 92% of which occurred in the WHO African region. The funding bottleneck is likely to increase as Anopheles stephensi increases malaria cases in Africa.
Another major challenge is that both Anopheles stephensi and African malaria vectors are developing resistance to some of the insecticides used against them. These insecticides are used on bed nets or used for indoor spraying.
Finally, the Anopheles stephensi presents a new challenge as it is more difficult to gain access to mosquito breeding and resting places in urban areas and to take control measures. In particular, it is more difficult to identify and map breeding grounds in urban areas, making larval control difficult. In addition, spraying interior debris is less easy due to the high density of apartments and the difficulty of accessing all apartments.
Has such a mosquito invasion happened in the past? If so, what happened then?
The spread of Anopheles stephensi is reminiscent of a similar invasion by Anopheles gambiae, a species of mosquito common in Africa that spread to Brazil in the 1930s and 1940s, causing devastating malaria outbreaks. For example, there have been 150,000 cases of malaria and 14,000 deaths in just eight months. This has been recognized as one of the most serious health threats in America and an aggressive eradication campaign has been launched.
The Brazilian government responded with an integrated control program. Insecticide sprays targeted larvae and adult insects. Cars or trucks leaving endemic areas have been sprayed and massive efforts have been made to improve drainage and remove the stagnant water that provided the hatcheries. This concerted effort is a key example of successful vector control and resulted in the species becoming extinct in South America in the 1940s.
Is there anything that can be done to stop the spread?
Immediate action is needed to contain the spread of Anopheles stephensi. The longer we leave it, the harder it becomes to contain and, unfortunately, there seems to have already been a significant spread of reports from across the Horn of Africa. Although mosquitoes can travel great distances and spread at high altitudes in wind, the pattern of Anopheles stephensi’s spread suggests the importance of human transportation routes.
Vector monitoring is key. We need to know where Anopheles stephensi has spread, and then quickly and strategically focus resources on limiting the spread and locally eliminating Anopheles stephensi before it takes hold. Monitoring should be carried out through national vector and malaria control programs with the support of research institutions.
Overall, a combination of environmental management to eradicate larval habitats and environmentally friendly biopesticides to control adult and larval stages of the mosquito is considered to be the most effective strategy to control Anopheles stephensi.
Other appropriate measures include: using long-lasting insecticide-treated nets, checking homes to prevent adult mosquitoes from entering, properly covering water storage tanks to prevent mosquitos from laying eggs in them, and removing unused water tanks that may be suitable Breeding ground for mosquitoes.
Read the report on the interview