Six cases of Zika virus in Australia last year as pregnant women warned not to travel

• Could the Zika virus be spread through sex?

Six people tested positive for the mosquito-borne Zika virus in Australia last year, but health authorities say all of them were infected overseas.

On Tuesday, a spokeswoman for the federal health department said she could not provide any further detail about the six people, except to say: "There have never been any locally acquired cases of Zika virus infection reported in Australia."

News of the imported cases came as Australian health authorities listed more than 20 "countries of concern" for pregnant Australian women to avoid amid fears about the virus, which has been linked to brain damage in thousands of babies in Brazil.

Fairfax Media understands the first imported Zika case detected in Australia was in 2012. There was one case in 2013 and 12 cases in 2014.

Zika is transmitted by some species of Aedes mosquito, particularly Aedes aegypti.Credit: AP

Professor Dominic Dwyer, a virologist at Westmead Hospital in Sydney, said he did not know more about the six cases documented in Australia last year, but he said the people involved were probably infected in South America, Central America or Tahiti where the virus was present.

He said about 80 per cent of people who get the virus do not experience any symptoms and that it usually clears from people's blood within a week.

The heightened concern about the virus this year stems from fears it is causing devastating birth defects. Infection with the virus during pregnancy appears to be linked to the development of unusually small heads and brain damage in newborns.

Professor Dwyer said although there was some uncertainty about whether Zika was responsible for this, it was possible and should "certainly be followed up".

The Australian government is advising women who are pregnant or who hope to become pregnant to consider postponing travel to any area where Zika virus transmission is ongoing, including Samoa.

Concern about Zika has triggered the most urgent global health warning since the Ebola crisis of 2014. Overnight, the foreign affairs department has listed 22 countries to avoid during pregnancy. Beyond the virus' epicentre in Brazil, the warning now extends into Central America and parts of the Carribean.

It is believed Samoa has reported Zika for the first time. Officials in Vanuatu are also on the alert having in April last year experienced a first confirmed outbreak of the virus.

Here are some answers about the outbreak.

What is the Zika virus?

A little-known infection transmitted by mosquitos, this virus appears to be related to dengue, yellow fever and West Nile virus. It is transmitted by some species of Aedes mosquito, particularly the Aedes aegypti​. The infection can occur without symptoms but in some cases it can cause fever, rash, severe headaches, joint pain and muscle pain. Illness is usually not severe and hospital treatment is not required.

Zika was first detected in a monkey in Uganda in 1947, with the first human cases identified in Nigeria in the late 1960s. Outbreaks of Zika virus have previously been reported in tropical Africa, south-east Asia, and the Pacific Islands.

It is believed the virus was introduced to South America, particularly Brazil, sometime in late 2014. On Monday, the World Health Organisation said the virus was likely to spread to all countries in the Americas except for Canada and Chile.

Recently, three Britons contracted the virus while travelling in Colombia, Suriname and Guyana. Health authorities are monitoring Samoa.

Most concern and public fear centres around a possible link between infection with Zika virus during pregnancy and birth defects including microcephaly, which causes brain damage in babies.

How does the virus spread?

Not from person to person. According to the World Health Organisation, Zika is spread by an infected Aedes mosquito, the same type of mosquito that spreads dengue, chikungunya and yellow fever. The Aedes aegypti mosquito has spread most Zika cases. They can breed in tiny water pools and usually bite during the day (especially at dawn or at dusk).

Virus expert Professor Trudie Lang, of the University of Oxford, said the virus was not like Ebola in that there was no evidence for human-to-human transmission. "Zika is not contagious," she said. "However, this is an important emerging diseases outbreak situation and we really must apply the lessons that we learnt from Ebola, because Zika could be a major public health issue [in Brazil and wider regions infected with Zika].

"There are many unknowns and so research is urgently needed to understand what is happening and how to prevent further cases."

Is a Zika virus outbreak headed to Australia?

It is possible. Transmission could happen anywhere there is dengue. Mosquitoes will play the most important role in any local transmission and only one of hundreds of mosquitoes found in Australia can transmit Zika virus, the Aedes aegypti. This species is only found in north Queensland.

For the virus to spread here, the Aedes aegypti would have to bite an infected traveller shortly after they have returned from the country where they contracted the virus.The infected mozzie would then go on to bite a local.

The chances are small but not impossible.

How will I know I've got it?

There is no widely available test. A laboratory can confirm the diagnosis by blood tests.

However, most diagnoses of Zika virus are based on people's symptoms and travel history. Symptoms are usually mild and appear a few days after a person is bitten by the infected mosquito. In many cases, infected people present with slight fever and rash. Some get conjunctivitis, muscle and joint pain. Tiredness is another common symptom. The symptoms usually last less than a week.

Is there a vaccine?

There is no cure or vaccine yet. According to WHO, symptoms can be treated with standard pain and fever medicines, rest and water.

Should pregnant women be worried about Zika?

Yes. Pretty much all the risk is for pregnant women travelling to areas in the world where Zika cases are emerging. Although much is unknown, there is a possibility that the Zika virus causes microcephaly – a condition in which babies are born with abnormally small heads. Doctors in northern Brazil noticed a surge in babies with the condition in October and are worried there is a connection.

What is microcephaly?

A condition that causes small heads and damaged brains. The brain damage is due to abnormal brain development of the baby in the womb or during infancy. Babies and children with microcephaly often have challenges with their brain development as they grow older.

According to WHO, microcephaly can be caused by a variety of environmental and genetic factors such as Downs syndrome; exposure to drugs, alcohol or other toxins in the womb; and rubella infection during pregnancy.

A Zika outbreak in Brazil is thought to be behind 4000 cases of microcephaly. Authorities in Colombia, Ecuador, El Salvador and Jamaica have advised women to delay becoming pregnant until more is known.

What can I do to protect myself? Should I not go to countries that have Zika infections?

WHO is not recommending any travel or trade restrictions related to Zika virus disease. However, the Australian government is advising women who are pregnant (in any trimester) or who plan to become pregnant to consider postponing travel to any area where Zika virus transmission is ongoing.

Areas where Zika virus transmission is ongoing are difficult to determine and likely to change over time. However, warnings now exists for countries including: Barbados, Bolivia, Brazil, Cape Verde, Colombia, Ecuador, El Salvador, French Guiana, Guadelope, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Saint Martin, Samoa, Suriname and Venezuela.

As the best protection from Zika virus is prevention, travellers should stay informed about Zika virus and other mosquito-borne diseases.

All travellers are advised to take the following mosquito bite prevention measures:

• Apply insect repellent and wear light-coloured clothes to prevent bites.
• Use screens or close doors and windows in houses, and use mosquito nets for sleeping.
• Don't leave containers that hold water (like buckets, pots) lying about.