Mosquitoes and vector transmission
In part four of our journey through unpleasant beasts we come to the mosquito family. We will discuss their habits, the diseases they carry and what you can do to protect yourself.
Mosquitoes are an evolutionary success story. There are over 3,500 species, which tend to live in hot, equatorial regions around standing water sources. Three subtypes - Anopheles, Aedes and Culex - may sound familiar. Their young grow underwater and fly in adulthood. They are typically found in Africa, Asia, South America and, on occasion, may spread further afield. As global warming takes hold, we expect their domain to spread, and with it disease.
These insects bite using a complex apparatus of serrated jaws hidden within a slithering proboscis. They have a preference for the loud, pregnant or overweight. They hunt by heat or chemical detection. Their jaws ooze a chemical cocktail which prevents blood clotting, platelet action and inflammation. Sometimes they may trade blood for disease, some of which are pretty nasty. Welcome to the curious world of vector transmission and symbiosis.
Vectors are organic vehicles for microbial passengers. A virus, parasite or bacteria will hitch a ride inside an insect or animal, and spread to others when their host begins to feed. Often these stealthy diseases will lay dormant and only create symptoms once reaching a critical population, which aids in their infiltration. Stopping disease means breaking the reproductive cycle.
Mosquitoes carry a number of diseases, some of which are discussed below.
Malaria is a term given to disease caused by a family of microscopic parasites , the worst of which is Plasmodium falciparum. Other strains tend to be less serious, but may hang around from months to years. Falciparum is carried by the Anopheles mosquito and is endemic to equatorial regions, principally Africa, Asia and South America. Incidence has been declining in recent years due to good preventative measures.
Symptoms of infection can occur within two weeks, and include a high, swinging fever, aches, exhaustion and headaches. Shock may occur in severe infection, where organ failure can present with collapse, jaundice, anaemia and confusion. Severe disease may progress to a rare complication where runaway clotting causes severe bleeding and death.
Treatment is supportive, ranging from simple fluids and analgesia to full intensive care depending on the family of parasite and severity of infection. Prevention with antimalarial medications is readily advised when visiting endemic areas.
2. Yellow Fever
Named due to the yellowing of the skin and eyes caused by liver infection, yellow fever is a haemorrhagic (bleeding) viral illness carried by the Aedes and Anopheles mosquito. Yellow fever is endemic to Africa, Central and South America and causes in excess of 30,000 deaths a year. The disease replicates in our bodies lymphatic vessels and other defensive cells, and in severe forms can invade the liver.
Symptoms occur from three to six days post infection and include fever, back pain, headache and loss of appetite. Severe infection will lead to liver disease, severe bleeding and can progress to organ failure. Treatment is supportive, with a low mortality in mild disease. A vaccine has been trailed in endemic areas, but the global mainstay of treatment is prevention of infection.
3. Dengue fever
Dengue fever is a viral infection consisting of four distinct families. It is present mainly in Asia and Latin America thanks to the Aedes mosquito. It is usually a mild and self-limiting (temporary) illness, but severe infection can be life-threatening.
Symptoms can occur anytime from two to 10 days post-infection, and include a high fever, flu-like illness, headache, pain behind the eyes, nausea, vomiting, swollen glands and, in some cases, a rash in the skin creases. Swelling of the liver may cause tummy pain. Dengue haemorrhagic fever is a serious complication, where the body leaks fluid and clotting is prevented, which can lead to bleeding and shock. This form may have a lower fever and must be suspected early.
In most cases, treatment is supportive with fluids, antipyretics (to bring down fever) and analgesia. Severe forms will require intensive care admission. Rarely the disease can affect the heart, brain and other organs.
4. West Nile virus
West Nile virus is carried by the Culex mosquito and is a more worldwide disease, which may be found from Africa, the Middle East and even Europe. It is found in birds, transferred to a mosquito and then to us. Eighty per cent of those infected experience a disease so mild they have no symptoms. Severe cases tend to occur in the very old, very young and immunocompromised (those with weak immune systems, such as those with HIV and undergoing chemotherapy.)
Symptoms occur three to 14 days after infection and include a fever headache, nausea, aching and may include a rash on the trunk. Severe disease may spread to the meninges (the webbed covering of the brain and spinal cord) and cause meningitis. Symptoms of West Nile meningitis may include headache, stiff neck, disorientation, confusion, muscle weakness, paralysis, convulsions and other neurological abnormalities.
Treatment is supportive, with meningitis treated in hospital within high dependency or intensive care units. Mortality is low.
5. Elephantiasis (lymphatic filariasis)
Elephantiasis (lymphatic filariasis) is famous for its appearance (swelling of the skin and tissues caused by the blockage of the body's lymphatic drainage system). Filariasis is a parasitic roundworm of three distinct families, which are endemic to Asia and Africa. The disease may be chronic, causing progressive swelling and disfigurement over years. Adult worms block off the bodies drainage system and reproduce, making countless young which spread throughout the body. The bug is spread by the Aedes mosquito.
Infection presents with local inflammation at the site of infection, followed by either an acute (fast) or chronic (slow) course of swelling. A subcutaneous (below skin) form will present with rashes and joint pain. Treatment is a combination of medications designed to kill the young worms and prevent re-infection and disease spread. The swelling can be managed but not cured. The best treatment is to never get infected.
6. Chikungunya fever
Chikungunya roughly translates to 'contorted', describing the bizarre effects of severe joint pain caused by this virus. It is spread throughout Africa and Asia by the Aedes mosquito. Symptoms occur three to seven days post infection and include fever, severe joint pain, muscle pain and in rare cases neurological, brain or stomach symptoms. Joint pain may become chronic. Treatment is supportive and focuses of symptom relief. Serious cases occur in the very young and old.
7. Zika virus
The Zika virus is an old disease which has found new fame due to its epidemic in Brazil and association with birth defects. It is endemic to Africa, Asia and Brazil and is carried by the Aedes mosquito. It can also be spread via sexual intercourse and from mother to baby.
Symptoms present from three to 12 days post infection and include fever, conjunctivitis, rash, muscle ache and headache. Pain behind the eyes and in the tummy is rare. There is an established association between maternal infection and infant microcephaly (small, underdeveloped brain and head) and other neurological defects. The mechanism of this link is unknown at present.
Zika is rarely fatal and hospital admissions are few. Treatment is supportive and based around symptom control. Pregnant women are advised to avoid endemic areas and seek medical help if they are concerned about infection.
How to protect yourself
As you can see, the above diseases are best avoided by preventing a mosquito bite. Simple measures, such as using mosquito nets, repellent sprays and relevant prophylactic medications or vaccines reduce the risk of infection. See your doctor if you are travelling to an endemic area. It is very difficult to tell between the diseases on symptoms alone, so seeking early help is essential if returning from abroad with a fever. Blood tests are diagnostic and will help guide treatment. Most infections are mild and require little treatment, but seeking advice is essential to avoid severe complications.
Ben is a young NHS doctor in the Southwest. His interests include neurology, health communication, and medical ethics. He is also an avid advocate of compassionate care and quality improvement, running a project in the Southwest around medical humanities. Please follow and support: Dr Janaway on Facebook Dr Janaway on Twitter
The opinions expressed in this article are the author's alone and do not reflect those of the NHS or associated agencies. All facts are based on the best available evidence. The author is happy to receive questions. There are no conflicts of interest and due consideration has been given to the consequence of conclusion or interpretation.