|August 21, 2018|
The most dangerous mosquito-borne diseases
|Malaria, dengue fever, zika --- mosquitoes aren't just annoying when they buzz around your ear at night, they can also infect us with countless illnesses. On World Mosquito Day, here are some of the most dangerous ones.|
You can hardly see sandflies with the naked eye. These mosquitoes are dangerous because they transmit the infectious disease leishmaniasis. They are at home in southern countries, such as the Mediterranean or the tropics. But they have also been seen in Germany. Only fertilized females suck blood. Non-fertilized females and male sandflies pose no risk. There are 30 different Leishmania species. Animals, such as dogs, are most often infected by the parasitic protozoa, but ten of the Leishmania species can also infect humans.
First symptoms sometimes only appear after weeks or months. It starts with fever and headaches. The lymph nodes swell up. The infected person feels tired and weak and may lose a lot of weight.
Leishmaniasis affects various parts of the body. The mucocutaneous form of the illness affects the nose and throat area. In the beginning, a skin ulcer usually develops. If the disease is not treated, it spreads to cartilage and connective tissue and destroys it. This can lead to holes in the nasal septum, for example. But it can get worse, too: you could develop intestinal leishmaniasis. It attacks important internal organs like liver and spleen.
Around 12 million people worldwide contract leishmaniasis every year. The disease is curable, but people with the symptoms described should see a doctor as soon as possible. Without appropriate treatment, leishmaniasis can lead to death. There is no effective vaccine.
The nocturnal mosquitoes of the genus Culex transmit the so-called Sindbis virus. It is most common in African regions, but scientists have also found it in mosquitoes and humans in European countries.
At first, the affected person has flu-like symptoms and a fever. In some cases the body temperature can rise to more than 40 degrees Celsius (104 degrees Fahrenheit). Brain inflammation can develop. After the first phase, the joints become inflamed. The inflammations get stronger over the course of the disease. They occur mainly at the wrists, finger joints and ankles and are later accompanied by skin rashes.
If the disease is not treated properly, it can become chronic. This results in constant joint pain. But that only happens in the most extreme cases. Usually the human immune system is able to cope with the virus. The symptoms of the disease recede after a few weeks without any after-effects. There is no vaccine.
The tiger mosquito, as well as some other mosquitoes of the genus Aedes, are responsible for yellow fever, which is a so-called flavivirus. The danger of being stung by the mosquito and contracting yellow fever exists in 34 countries in Africa and 13 countries in South and Central America.
Scientists distinguish three different transmission pathways: In the so-called sylvatic cycle, mosquitoes normally transmit the virus between monkeys in the rainforest. If the odd human gets infected, he or she can carry the virus into town.
This leads to the second transmission, known as urban cycle. In dense urban settlements, mosquitoes carry the virus from one person to another. That's how yellow fever becomes endemic in cities.
The third yellow fever cycle affects people in peripheral areas between forests and savannah, where mosquitoes, monkeys and humans live in close proximity. There, infections from animals to humans and back take place again and again.
Fever and flu-like symptoms occur first. Those affected are sick and have to throw up. If yellow fever isn't treated, it can lead to meningitis and even to vital organs being severely damaged or failing completely.
The good news: There's a vaccine against yellow fever! Some affected countries list a yellow fever vaccination as one of the entry requirements for travelers, for example Uganda and Sierra Leone. The immunization lasts a lifetime.
Aedes aegypti are also the vectors transmitting dengue fever. Popular vacation destination Thailand, for example, is home to the tiger mosquito, which spreads dengue. In fact, the virus is found in large parts of Southeast Asia --- from Afghanistan to northern Australia, but also in sub-Saharan Africa, Central and South America and some southern states of the US.
When bitten and infected, the symptoms range from muscle and joint pains to headaches and fever. Even people who have survived an infection with dengue fever aren't safe. A second infection is much worse than the initial infection.
96 million people contract dengue fever every year. This makes it the disease most frequently transmitted by mosquitoes. There is now a vaccine. However, this is subject to certain restrictions, because even a vaccination can trigger deadly reactions in the body.
For example, in endemic areas where there is a high transmission rate, only children aged nine years and older as well as seropositive people, i.e. those who have already been infected once, should be vaccinated. The World Health Organization recommends using the vaccine only in countries where more than 70 percent of the population have developed antibodies against dengue.
Aedes aegypti, the yellow fever mosquito, and Aedes albopictus, the Asian tiger mosquito, transmit the Zika virus. It occurs in the tropical regions of Africa, in South and Central America, but also in Florida and Texas. Areas in Southeast Asia are also affected. These include Thailand, Vietnam and the Philippines.
In 2015, many babies with severe disabilities were born in Brazil. The children have a skull reduction called microcephaly. Women in the first trimester of their pregnancy are particularly at risk of contracting the virus and passing it on to their babies, especially as the transmission often goes unnoticed.
In adults, only about one in five infected persons develops symptoms: a rash, conjunctivitis, joint pain and fever. Unlike with newborns, there are usually no long-term consequences for adults. The symptoms will simply disappear after a while. Researchers are still working on an effective vaccine.
West Nile fever
This virus can be dangerous for the elderly or for people with a weakened immune system. It can cause meningitis and myocarditis.
The virus is native to African and Asian countries, but scientists have also found it in southern France, the US and Canada. It is currently spreading to Italy, Greece and several South-Eastern European countries and has already claimed several lives.
Signs of infection in humans are similar to many other mosquito transmitted diseases: shaking, cold chills, fever, headaches and dizziness, followed by a skin rash. Not every infected person shows these symptoms. They only appear in about one in five. There is no vaccine, nor are there any prophylactic drugs to date.
The Chikungunya virus is widespread in southern and eastern Africa, but also on the Indian subcontinent, on the islands in the Indian Ocean and in Southeast Asia. Like many diseases transmitted by mosquitoes, it causes fever.
But this usually only lasts for about three days. It is followed by a period of severe joint pain that lasts one to two weeks before it subsides again. Some of the infected patients develop itchy, punctiform skin hemorrhages as well as mucosal ones, for example from the nose. There is no permanent damage. Once the disease has been overcome, the patient is immune for the rest of their life. There is no vaccine.
Malaria is probably the best-known tropical disease. The Anopheles mosquito transmits the parasite, a unicellular organism called a Plasmodium. Around forty percent of all people worldwide live in areas with a heightened malaria risk.
The disease progresses in fever episodes. There's a mix of symptom: headaches and pains in the limbs, sweating and chills, but also diarrhea.
Over the course of the disease, severe damage to the nervous system can occur. Every year about half a million people die malaria, most of them children. The Anopheles mosquito feels very comfortable in warm areas. It needs heat to transmit the disease. Only females are active --- they are the dangerous bloodsuckers. Although research is in full swing, there is still no reliable vaccine, but you can take prohpylactic drugs to lower your risk of contracting the disease.
Protection against mosquitoes
The general recommendation: use mosquito-repellent sprays. In warm regions, mosquito nets are a good measure, too, as is light-colored clothing that should cover the whole body.
But there is no such thing as fail-safe protection, unless a vaccine against the respective pathogen exists. Unfortunately, this is not the case for all diseases transmitted by mosquitoes --- yet.