INTRODUCTION TO VECTOR CONTROL, INSECTICIDES AND INSECTICIDE RESISTANCE
1.3
Malaria: a vector-borne disease
Malaria is a vector-borne disease caused by protozoan parasites from the genus Plasmodium.
The symptoms of malaria can include fever, tiredness, anaemia and headaches, more severe cases can lead to seizures and death.
The Plasmodium parasite
Five species of Plasmodium are known to infect humans, with Plasmodium falciparum and P. vivax being the most important. P. falciparum is the major species found in sub-Saharan Africa, while P. vivax is the most important in other parts of the world.
Symptoms of malaria may appear ten to 15 days after being bitten by an infected mosquito. If left untreated, malaria caused by P. falciparum can rapidly develop in severity, and even lead to death within a single day. The most vulnerable to malaria are young children and pregnant women. Travellers who have not been exposed to malaria before are also vulnerable.
The World malaria report 2021 estimated that in 2020, there were 241 million cases of malaria in 85 countries and territories, an increase from 227 million cases in 2019. The vast majority of these cases were in Africa. The report also estimates that in 2020, 627,000 deaths could be attributed to malaria.
A complex life cycle
Plasmodium parasites have a very complex life cycle which requires two hosts. In the case of human malaria, the parasites need both humans and certain species of Anopheles mosquitoes to complete their life cycle.
When an Anopheles mosquito bites an infected person, it will ingest some of the Plasmodium parasites that are in the bitten person’s blood. The parasite then ‘infects’ the mosquito and continues to develop. Depending on the temperature, and other factors, it can take between eight to 14 days before the parasite has developed into a life stage that migrates to the mosquito’s salivary glands. From there, the parasite is ready to be injected into the people that the mosquito subsequently bites.
However, this means that the mosquito has to stay alive for at least eight days after biting an infected person. If the mosquito dies before that, then the parasite won’t have time to develop and migrate to the salivary glands, and therefore can’t be passed on, or vectored, to anyone else. Female Anopheles mosquitoes can live for three or four weeks. This means that only older mosquitoes will have lived long enough to have acquired the parasite, and for the parasite to have developed into the stage where it can infect humans.
Anything that reduces the average life expectancy of female mosquitoes will also limit the number of mosquitoes that are able to be vectors of the Plasmodium. It will also reduce the number of times an infectious mosquito can bite, and therefore pass on the parasite.
You cannot get malaria from everyday contact with a malaria-infected person, such as talking with or sitting next to them.
The good news
The good news is that malaria is preventable and curable. Prevention of malaria is primarily based on protection against mosquito bites. Chemoprophylaxis with anti-malarial drugs is also used in some situations. Since October 2021, WHO also recommends broad use of the RTS,S/AS01 malaria vaccine among children living in regions with moderate to high P. falciparum malaria transmission. The vaccine, whilst not offering 100% protection, has been shown to significantly reduce malaria, and deadly severe malaria, among young children.
To quote Dr Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization, in the World malaria report 2021: “Malaria has afflicted humanity for millennia. We have the tools and strategy now to save many lives – and with new tools, to start to dream of a malaria-free world.” Let’s share this dream!
Other vector-borne diseases
Malaria is just one vector-borne disease. There are others that collectively account for 17% of all infectious diseases, together causing more than 700,000 deaths each year. They can be caused by parasites, as in the case of malaria, but also by bacteria and viruses.
Dengue is the most prevalent viral infection which is transmitted by the Aedes mosquito species. Other viral diseases, including chikungunya fever, Zika virus fever, yellow fever, West Nile fever, and Japanese encephalitis, are also transmitted by mosquitoes. In some cases, there are effective vaccines, but in all cases protection against mosquito bites is a key preventative measure.
Author: Konstantina Boutsika
References
World malaria report 2021, WHO