THE ZIKA VIRUS

Zika is a member of the flavivirus family, which includes the viruses which cause dengue fever, yellow fever, Japanese encephalitis and West Nile disease. Zika is transmitted by the bite of Aedes mosquitoes, which are found throughout the Americas except for Canada and Chile where it is too cold for them to survive. If the mosquitoes drink the blood of an infected person, they can then infect subsequent people they bite. It is unclear for how long someone can transmit the virus after being infected.

The Zika virus was discovered in monkeys in 1947 in Uganda's Zika Forest, with the first human case registered in Nigeria in 1954 but for decades it did not appear to pose much of a threat to people and was largely ignored by the scientific community. But in May 2015 it was reported in Brazil and has spread rapidly. It In the past year the virus "exploded" said Prof Weaver, director of the Institute for Human Infections and Immunity, sweeping through the Caribbean and Latin America "infecting probably a couple of million people". In a statement, The Pan American Health Organization (PAHO), the regional office of the WHO, said: "PAHO anticipates that Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found." PAHO is advising people to protect themselves from the mosquitoes, which also spread dengue fever and chikungunya.

The symptoms in adults and children are similar to those for dengue fever but generally milder, including flu-like aches, inflammation of the eyes, joint pain and rashes although some people have no symptoms at all. In rare cases the disease may also lead to complications including Guillain-Barre syndrome, a disorder of the nervous system which can cause paralysis. There is some evidence that Zika can be transmitted through saliva and semen although this does not appear to be common.

Microcephaly is when a baby is born with an abnormally small head, as their brain has not developed properly. The severity varies, but it can be deadly if the brain is so underdeveloped that it cannot regulate the functions vital to life. Children that do survive face intellectual disability and development delays. It can be caused by infections such as rubella, substance abuse during pregnancy or genetic abnormalities. Brazil had fewer than 150 cases of microcephaly in the whole of 2014, but there have been more than 3,500 reported cases since October. The link with Zika has not been confirmed. But some babies who died had the virus in their brain and it has been detected in placenta and amniotic fluid too. Prof Laura Rodrigues, a fellow of the Brazilian Academy of Sciences and from the London School of Hygiene and Tropical Medicine, said some data suggested that up to one-in-fifty babies had birth defects in one of the worst hit areas - Pernambuco state in Brazil.

Rio de Janeiro is the host city for the 2016 Olympic Games from 5 to 21 August. The Brazilian authorities will be targeting the mosquitoes' breeding grounds in the run-up to the Games. The International Olympic Committee says it is in "close contact" with the Rio organisers and that Olympic venues will be inspected daily in the lead-up to and during this summer's Games. It will be to ensure puddles of stagnant water, where mosquitoes breed, are removed to minimise the risk of athletes and visitors coming into contact with the insects. However, it says fumigation will be carried out only on a "case-by-case" basis because of potential health concerns for athletes and visitors. There is also some hope there will be fewer mosquitoes in August as the month is both cooler and drier.

There is currently no vaccine or cure, and diagnostic testing is difficult. The search for a vaccine is being led by scientists at the University of Texas Medical Branch. They have visited Brazil to carry out research and collect samples, and are now analysing them in a suite of high-security laboratories in Galveston. But they warn that although a vaccine could be ready for testing in two years, it may be another decade for it to be approved by regulators. The Brazilian Health Minister, Marcelo Castro, has said a new testing kit is being developed to identify infections quickly. He also said more money was being put into the development of a vaccine. Some scientists are also trialling the use of genetically modified mosquitoes that appear to reduce mosquito populations by 90%. Meanwhile, efforts are under way to kill the mosquitoes with insecticide.

As there is no treatment, the only option is to reduce the risk of being bitten. PAHO advice is to ensure all containers that can hold even small amounts of water should be emptied and cleaned to prevent mosquitoes breeding. And that people should protect themselves by using insect repellent, covering up and keeping windows and doors closed. The director general of the WHO, Margaret Chan, said the outbreak was "extremely worrisome".

Adapted from the sites:

http://www.bbc.com/news/health-35399403

http://www.bbc.com/news/health-35370848

http://www.bbc.com/news/world-us-canada-35423288