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The World Health Organisation has declared the mosquito-borne Zika virus to be a global health emergency, suspecting a strong causal link between it and thousands of birth defects in Brazil.
Two cases of the virus have emerged in Australia, but the NSW Department of Health has said that local transmission is very unlikely. The mosquito that carries the Zika virus – Aedes aegypti – is the same species that carries yellow fever. In Australia it is mostly found in far north Queensland.
Zika is suspected of leading to complications in newborn babies and in Texas, health authorities say they have the first case of the virus being sexually transmitted in humans.
However, the real concern is the ease with which mosquitoes can transmit the disease.
Mosquitoes kill more people than any other animal on Earth, humans included.
Diseases carried by the ubiquitous insect are responsible for more than 700,000 deaths a year, according to the Bill Gates Foundation. The majority of these deaths are caused by malaria, triggered by a parasite that uses the mozzie as a host.
Common illnesses caused by mosquitoes include malaria, dengue fever and yellow fever. But there are plenty more, including exotic local illnesses in Australia such as Ross River fever, Barmah Forest virus disease and the rare Murray Valley encephalitis.
Some mosquito-borne illnesses are manageable through prevention (including vaccinations) or treatment. However, the vaccinations are not universally effective and some illnesses, such as dengue fever, have no vaccine.
It's not just a case of packing your bags, having your shots, taking your malaria pills and thinking you'll be fine. It might sound mundane, but the frontline defence for travellers, according to the World Health Organisation, is to stop the mozzies biting you in the first place.
Why mosquitoes?
The carrier of a disease is called a "vector" and mosquitoes are among the most effective vectors there are.
Mosquitoes are prolific breeders, live in most habitats favoured by humans and have the ability to get right under our skin.
Mosquitoes do not feed on blood, per se. Rather, they require a supply of animal blood for producing eggs. According to decade-old research from the University of California, Riverside, the genes that trigger yolk production in mosquito ovaries are inactive until the insect has had its first meal of blood. That is why only female mosquitoes extract blood from animals.
The female mosquito uses its needle-like proboscis to penetrate the skin of an animal to extract blood. When it bites, it injects saliva and anti-coagulants, which can be carriers of disease.
"Mosquitoes are actually the innocent party here," says Cameron Web, a medical entomologist at NSW Health Pathology and the University of Sydney. "It is the virus or parasite acting like a hitchhiker that takes advantage of the mosquito to move from host to host."
He says it's wrong to think about mosquitoes like a dirty syringe moving from person to person. The mosquito itself needs to be infected with a virus or hosting a parasite in order to transmit disease.
That is why mosquitoes do not transmit diseases such as HIV/Aids. Even if they ingest human blood carrying the virus they cannot become infected, so do not pass it on through their saliva.
Dr Webb says that mosquitoes act as vectors for diseases in two groups: viruses and parasites. This is partially why different species of mosquitoes carry different pathogens.
Vaccination
Exposure to mosquitoes can be reduced but is practically unavoidable. Preventative medicines, such as anti-malarial drugs, are not suitable for long-term treatment. And vaccination does not work for all mosquito-borne diseases.
Yellow fever is one success story for vaccination. The World Health Organisation says: "The yellow fever vaccine is safe and affordable, providing effective immunity against yellow fever within 10 days for 80 to 100 per cent of people and 99 per cent immunity within 30 days."
Development of a malaria vaccine has been under way for decades. The recent release of GlaxoSmithKline's Mosquirix vaccine has raised questions about its efficacy. Nevertheless, WHO has approved a pilot program for the vaccine.
While malaria kills more people, it is the growth of dengue fever that is of great concern to health authorities.
The virus that causes dengue comes in four distinct groups, called serotypes. A first case of dengue fever is like being hit by a severe flu – but is something that you will most likely survive. Professor Roy Hall, a virologist at the University of Queensland, says that the real danger comes with a second illness from a different serotype. He says that recovery from a first illness causes "immune enhancement", meaning dengue symptoms from a second serotype are much worse, leading to severe dengue or dengue haemorrhagic fever, which can be fatal.
This means that any vaccine for dengue must immunise for all four serotypes to be effective. A vaccine that only treats one serotype would actually leave the recipient vulnerable to contracting severe dengue fever from subsequent infection.
Mexico has this year approved a vaccine for dengue made by French company Sanofi. Trials indicate it reduces risk of contracting dengue by 60 per cent, but is less effective in children under nine. One study suggests the vaccine could cause longer term risks of severe disease.
There is no vaccine for the Zika virus. Until its likely link with birth defects, Zika was not a high priority for vaccine development. However, given the Zika virus has some similarities with yellow fever, Japanese encephalitis, dengue and the West Nile virus, there is some hope that development of a vaccine could be relatively rapid.
Zika virus disease
Region affected: Asia-Pacific, Africa, the Americas
Date identified: 1947 in Uganda
Transmission: Virus carried by Aedes aegypti
Symptoms: Mild fever, rash, conjunctivitis. Suspected link with birth defects
Number infected a year: very few
Number killed a year: zero
Treatment: rest, fluids, analgesics
Vaccine: none available
Malaria
Region affected: Mostly sub-Saharan Africa (80 per cent of cases). Also Asia, Latin America and some in Europe & Middle East
Date identified: Antiquity (mentioned in ancient China, Egypt, Rome, Greece and India)
Transmission: Plasmodium parasite carried by Anopheles mosquito
Symptoms: Fever, headache, chills, vomiting. Can be fatal
Number infected a year: 214 million
Number killed a year: More than 400,000
Treatment: Anti-malarial drugs known as artemisinin-based combination therapy. (Chinese scientist Tu Youyou discovered artemisinin and was awarded the Nobel prize for medicine in 2015 for her discovery.
Vaccine: GSK is trialling RTS,S, known as Mosquirix.
Dengue fever
Region affected: Tropical and sub-tropical regions globally
Date identified: First recorded case likely China, third or fourth century AD. World outbreak in 1780s
Transmission: Four related viruses (serotypes) carried by Aedes mosquito
Symptoms: Severe flu-like illness. First infection rarely fatal. Subsequent infections cause haemorrhagic disease (severe dengue), which can be fatal
Number infected a year: More than 100 millionNumber killed a year: 22,000Treatment: No specific treatment. Early detection lowers fatality below 1 per cent
Vaccine: Mexico has released Sanofi vaccine, Dengvaxia. Efficacy uncertain
Yellow fever
Region affected: Tropical areas of Africa and Latin America
Date identified: 17th century. Transmission: Virus carried by Aedes mosquito
Symptoms: First phase: fever, severe backache, shivers, nausea, vomiting. Second phase: Jaundice; bleeding from mouth, eyes, nose, stomach; kidney dysfunction
Number infected a year: 200,000
Number killed a year: 30,000
Treatment: No specific treatment
Vaccine: Safe, affordable, effective
Chikungunya
Region affected: Africa, Asia, Indian subcontinent
Date identified: 1952 in Tanzania
Transmission: Virus carried by Aedes mosquitoes
Symptoms: Abrupt onset fever with joint pain. Pain can last months or years
Number infected a year: 1 to 2 million
Number killed a year: Rare, less than 1000
Treatment: No specific treatment
Vaccine: Under trial.
Lymphatic filariasis
Region affected: Sub-Saharan Africa and Asia
Date identified: Antiquity.
Transmission: Parasitic worm transmitted by Culex, Anopheles and Aedes mosquitoes
Symptoms: Asymptomatic at first, followed by disfiguring swelling later in life: lymphoedema, elephantitis, scrotal swelling
Number infected: More than 100 million
Number killed a year: Not generally fatal
Treatment: Drugs to kill the parasite; exercise, elevation, lymph treatment to treat swelling
Vaccine: None
Japanese encephalitis
Region affected: South-East Asia, tropical Americas and Western Pacific
Date identified: 1871 in Japan
Transmission: Virus transmitted by Culex mosquitoes
Symptoms: Mostly mild fevers. Severe cases: high fever, disorientation, coma, seizures, paralysis, death. Survivors can suffer brain damage
Number infected a year: 68,000
Number killed a year: 16,000
Treatment: None
Vaccine: None
In Australia
Dengue fever
Number infected a year: About 1000
Deaths: very rare
Ross River fever
Region affected: Australia, Papua New Guinea, South Pacific
Date identified: 1959
Transmission: Virus transmitted by Aedes mosquitoes
Symptoms: Often asymptomatic. Fever, joint pain, rash
Number infected a year: 5000
Number killed a year: Non-fatal
Treatment: None. Infection produces immunity
Vaccine: Developed in lab but considered commercially non-viable
Barmah Forest virus
Region affected: Mostly tropics and coastal regions
Date identified: 1974
Transmission: Virus transmitted by Aedes and Culex mosquitoes
Symptoms: Fever, rash, muscle aches, joint pain
Number infected a year: About 1000
Number killed a year: Non-fatal
Treatment: None
Vaccine: None
Murray Valley encephalitis
Region affected: All states, mostly WA, NT and north QLD
Date identified: 1951
Transmission: Virus transmitted by Culex mosquitoes
Symptoms: Mostly asymptomatic, possible fever, Causes neurological impairment
Number infected a year: Rare
Number killed a year: Fewer than 50 deaths recorded
Treatment: None. Illness produces immunity
Vaccine: None
