They are known as the most lethal animal in the world. And for good reason: it is estimated that each year there are over 700 thousand deaths caused by these insects in all corners of the globe. More than the number killed by snakes, scorpions, wolves, bears and sharks combined (and pretty much any other dangerous animal you can think of). What makes them dangerous are not claws, sharp teeth, or a deadly venom, but something much smaller and more banal, but equally lethal: the diseases they can transmit with their bites. Insidious pathogens such as the dengue virus or the protozoan that causes malaria, which we hear about more and more often even in our latitudes, thanks to climate change that is rapidly expanding the range of many species of mosquitoes, and therefore of their microbial tenants. Here are some of the most widespread and lethal diseases that can be transmitted by these annoying, and dangerous, insects (perhaps the only animal whose extinction science is seriously evaluating whether and how to induce).
West Nile
It is a disease caused by an RNA virus of the Flaviviridae family, the West Nile virus. It was identified for the first time in Uganda in 1937, and since then it has spread to many areas of Africa, the Middle East, North America, Asia and Europe. In Italy its presence has been confirmed since 1998, and since 2002 monitoring has been active throughout the national territory. Epidemics in our country are periodic, also linked to climate trends.
The disease is often asymptomatic, or causes mild flu-like symptoms, but in the forms that affect the brain, called neuroinvasive, it can cause high fever, severe headaches, muscle weakness, disorientation, tremors, vision problems, numbness, convulsions, paralysis and coma, and can be fatal. This year, 171 cases have been reported in Italy since May, 101 of which had neuroinvasive manifestations. And six deaths in people with a confirmed West Nile infection. The natural reservoir of the virus is birds and other wild animals (but it can also affect domestic or farm animals, such as dogs, cats and horses), while the vector is mainly common mosquitoes, i.e. those belonging to the Culex genus.
You are used to
The disease is caused by a flavivirus, similar to the one that causes West Nile disease. It is often asymptomatic, but can cause fever and skin rashes, and can be dangerous, especially in immunocompromised people, affecting the brain and causing encephalitis and meningocephalitis. The virus is under special surveillance due to the risks it could cause in the event of mutations that improve its invective capacity or lethality, but for now the cases reported in humans remain relatively few. In Europe it has caused periodic epidemics in wild birds since at least 1996, which are usually followed by a limited number of human infections. In recent days it has been talked about because of a positive mosquito captured in the Macerata area, as part of the national monitoring program for this virus. But from May to today, only one case of human infection has been reported, in the province of Modena. As with West Nile, the vector of the Usutu virus is the Culex mosquito.
Eastern equine encephalitis
Another virus that has been making headlines in recent days is Eastern equine encephalitis, a pathogen first identified in horses in 1830 (hence the name), which can infect various mammal species, including humans. The infection is usually asymptomatic, but in about 30% of cases it can cause encephalitis, which is often fatal and can leave serious health consequences even after recovery. The disease is endemic along the eastern coast of the United States, and a human infection was recently reported in the town of Oxford, southwest of Boston in Massachusetts, prompting authorities to impose an evening curfew for the population.
Dengue
It is a disease transmitted by four closely related viruses (Den 1, 2, 3, and 4) of the Flaviviridae family. It is transmitted by mosquitoes, mainly the Aedes aegypti species, after they bite another infected human. Dengue has been known for over two centuries, and is mainly widespread in tropical and subtropical areas of Africa, Southeast Asia and China, India, the Middle East, Latin and Central America, Australia, and several areas of the Pacific. In recent decades, however, it has begun to cause cases also in Europe and other more northern regions, mainly in people returning from travel to areas where the disease is endemic.
The disease causes fever within 5-6 days of the mosquito bite, with very high temperatures, accompanied by acute headaches, pain around and behind the eyes, severe muscle and joint pain, nausea and vomiting, skin irritations that can appear after 3-4 days. More rarely, the disease can present itself in the form of hemorrhagic fever, causing hemorrhages in various parts of the body that can also be fatal. This year in Italy 324 cases of dengue have been recorded, all imported. The fear, however, is that in the future climate changes could also make our territory suitable for the transmission of the virus, and therefore transform it into an endemic disease. Fortunately, a vaccine is available against these viruses, which is recommended for all travelers who wish to go to areas where there is sustained transmission.
Malaria
Malaria is a disease that we know well in our parts, since it was endemic in several areas of Italy until a few decades ago. It is caused by protozoa belonging to the Plasmodium genus, and is transmitted to humans by the bite of mosquitoes of the Anopheles genus. Symptoms usually consist of fever, often very high, headache, vomiting, diarrhea, sweating and shaking chills, all symptoms, at least initially, common to any flu syndrome or other infections. The severity can depend on the species of Plasmodium that causes the infection and the general state of health of the patient, and the disease can be fatal, even if today there are several extremely effective therapies. Today it is mainly widespread in countries in tropical and subtropical areas, particularly in Africa where infections and deaths are currently concentrated.
Zika
Another disease caused by a Flavivirus. The vector is usually Aedes albopictus, or the tiger mosquito, also widespread in Italy. It is mainly widespread in some areas of the African continent, in Southeast Asia and in South America. In 80% of cases, and more, the infection is asymptomatic, and when the symptoms appear they are those of a normal flu. What makes it insidious, however, is the fact that the disease seems to increase the risk of developing some peripheral neuropathies such as Guillain-Barré syndrome and acute polyradiculoneuritis, supported by autoimmune mechanisms. And because, as emerged during a Brazilian epidemic in 2016, it causes a dramatic disorder: an increase in the incidence of hydrocephalus in children born to women who contract the virus during pregnancy. The cases of confirmed infection in Italy in 2024 were 4, all in people returning from trips to areas where the virus is endemic.
Chikungunya
First identified in Tanzania in 1952, the Chikungunya virus is now widespread in over 60 countries in Asia, Africa, Europe and the Americas. In Italy, the spread is due to the tiger mosquito, and 8 cases were recorded in 2024. Symptoms include fever, shooting joint pain, muscle pain, headache, fatigue and skin rash. The course is usually benign, but in some cases the joint pain can last for months.
Yellow fever
It is an acute disease caused by a virus of the Flaviviridae family. Symptoms include fever, chills, loss of appetite, nausea, muscle pain, particularly radiating to the back, and headache. They tend to improve after about five days, but in a minority of cases they recur, with fever, abdominal pain and liver damage, which cause jaundice (hence the name, since patients take on a yellowish color) and can cause hemorrhages and kidney failure. In these severe forms, the disease is fatal in 30-60% of cases, and there are currently no effective treatments. The disease is present in central Africa and northern areas of South America.