Mosquito-borne diseases cause millions of deaths annually
| Sep 8, 2016
Not sharks. Not snakes or lions either. When it comes to animals, mosquitoes are the most dangerous in the world.
Mosquitos may look small and fragile, but their ability to carry and transmit diseases causes millions of deaths each year worldwide. Malaria alone caused 438,000 deaths in 2015, according to data from the World Health Organization (WHO). Other animals may be big, strong and scary, but they don’t cause even a third of mosquitoes’ casualties: Snakes claim about 125,000 fatalities worldwide each year, lions claim 200 victims, and sharks account for just 55, according to WHO data.
Several viruses are transmitted by mosquitoes and other arthropods – the arboviruses. The best known are the Aedes and Culex mosquitoes. The list of diseases that can be transmitted is huge and ranges from the most common such as dengue, chikungunya and Zika to the more exotic filariasis and West Nile fever, not to mention malaria, yellow fever and leishmaniasis. Some cause no more than a discomfort that resembles the common cold, but many can be fatal.
Small, but dangerous
The Aedes mosquito is one of the smallest – only 7 millimeters – but also one of the most dangerous. It is responsible for transmitting various diseases, including dengue, Zika, chikungunya and yellow fever.
Dengue has the highest number of cases: a recent estimate published by Bhatt and colleagues in Nature indicates 390 million infections by the disease each year, of which 96 million were manifested clinically. According to WHO, dengue is endemic in more than 128 countries, with about 3.9 billion people at risk.
The Americas recorded more than 37,000 confirmed cases of chikungunya in 2015 by the Pan American Health Organization (PAHO). Zika had more than 91,000 probable cases recorded up to April this year, according to an epidemiological bulletin of the Ministry of Health of Brazil. Both diseases have been documented in more than 60 countries in Africa, Asia, the Americas and Europe.
Despite the huge numbers, most of the time these diseases are mild or moderate. However, some cases may be serious. Classic dengue rarely has serious consequences, but hemorrhagic dengue can lead to death. Severe cases of the disease are very rare, and the mortality rate is relatively low – about 40 deaths per 100,000 cases recorded (0.04%).
The main risk of Zika is the infection of pregnant women, which may cause microcephaly and other complications in babies. Furthermore, the Zika virus is also associated with Guillain-Barré syndrome (a rare disease in which the immune system attacks peripheral nerves cells, causing muscle weakness and loss of sensation).
This year's first dengue vaccine is being distributed through the private healthcare system in Brazil, and the government is studying the possibility of their distribution in the public healthcare system. A vaccine against Zika is also expected in the near future.
Yellow fever is an acute viral hemorrhagic disease endemic in tropical areas of Africa and Central and South America. Since the launch of the "Initiative against Yellow Fever" in 2006, significant progress was made in combating this disease in Africa, and more than 105 million people were vaccinated in mass vaccination campaigns, significantly decreasing the number of cases. However, a recent outbreak of the disease – considered the worst in decades – has killed more than 345 people in Angola.
According to WHO, about 130,000 cases of yellow fever are reported annually, and the disease causes 44,000 deaths worldwide every year. There is a vaccine for yellow fever that is effective, but it is not for everyone, only those living in exposed areas or traveling to endemic locations.
Of these diseases, malaria may be the most dangerous. Malaria is the leading parasitic cause of morbidity and mortality worldwide, especially in developing countries where there are serious economic and social problems.
In 2015, more than 3.2 billion people were at risk, according to WHO. The organization estimates that there were 214 million new cases of malaria worldwide in 2015. The disease, transmitted by the Anopheles mosquito, is endemic in 107 countries. If not treated early, it can lead to convulsions, delirium, anemia, renal failure, pulmonary edema, coma and death. There is no vaccine for malaria, but treatment with antimalarial agents is generally safe and efficient – when performed quickly and properly.
Fighting these diseases is essential but not easy. Most endemic areas is in less-favored regions, which must overcome many social and economic challenges to address public health issues. Another important issue is the ecological factor to fight mosquitoes.
Tackling these diseases is a difficult task because the main problem is the transmission by mosquitoes and ecological imbalances are putting more and more people in contact with them. The ease of travelling also contributes to the rapid global spread of diseases; Zika, for example, is believed to have been introduced in Brazil during the World Cup two years ago and quickly spread across the country and the continent. In the case of malaria, mosquito resistance to larvicide is also considered a factor contributing to the increase in the number of cases.