By Siouxsie Wiles 03/03/2016

The New Zealand Ministry of Health are investigating a case of Zika in a woman who hasn’t travelled to a Zika-affected country recently. Is it a case of sexual transmission or a rogue mosquito?

Update: I talked about this story this morning on RNZ’s Morning Report and TVNZ’s Breakfast on One. Also see my latest post about a potential new mosquito vector for the virus.

Zika is a mosquito-borne virus currently infecting people in many countries in the America’s and the Pacific. The virus is causing concerns as it seems to be associated with an increase in babies being born with small brains and heads (a condition known as microcephaly).

The New Zealand Ministry of Health say they are currently investigating a case of Zika in a woman who hasn’t recently travelled to a Zika-affected country. The woman’s partner has recently returned from a country with Zika and tested positive for Zika so the Ministry say they are investigating whether the woman was infected through sexual transmission or by being bitten by an infected mosquito brought into the country in her partner’s luggage.

Rogue mosquito an unlikely scenario

As there are a few documented cases of Zika virus being transmitted sexually, and of Zika virus being present in a man’s semen after infection, I think it’s more likely that the woman was infected by her partner, rather than by being bitten by a rogue mosquito. It can’t have been just any mosquito either. It would have had to have been an infected female, as only the females bite – they need a blood meal for laying eggs.

Lots of unknowns when it comes to the sexual transmission of Zika virus

Not a lot is known about the sexual transmission of Zika virus. There have only been a few cases, and only ever from men to women. It’s not yet known if the men have to had symptoms to be infectious or how long an infected man remains infectious. It’s also not known if women can pass the virus on to men, and if Zika can be transmitted through other body fluids, like saliva.

The risk to New Zealand

However the woman was infected, the risk to New Zealand is low. If she was infected through sexual transmission, then any further cases here will be related to the number of sexual partners the infected person has. As we don’t have the right mosquitoes here, the chain of infected people is likely to end with the infected partner(s). This is because the normal route of transmission is from mosquitoes biting infected people and then going on to bite someone else. If on the other hand the woman was infected by one rogue mosquito coming in on someone’s luggage, then we might see a few cases depending on how many times the mosquito fed before it died (the mosquitoes normally only live a few weeks).

Preventing Zika infection

The advice from authorities has been clear from the beginning. The Centers for Disease Control in the US, the World Health Organisation and UK’s National Health Service all say: if you are a man who has travelled to an infected country and you have a fever or likely have Zika, you should either abstain from having sex or use a condom. This properly, repeatedly and for oral, anal and vaginal sex. What’s confusing though is that advice on how long men should wear a condom or abstain from sex for varies, from four weeks to six months. This is because it isn’t clear yet how long a man may remain infectious. Personally, I would err on the side of six months rather than four weeks.

The growing evidence of a link to microcephaly

The CDC have recently reported on Zika infection in pregnant US travellers and it makes stark reading. Between August 2015 and February 2016, the CDC received 257 requests for Zika virus testing for pregnant women. Nine women tested positive. Of these nine women, six were believed to be infected during the first trimester of their pregnancy. The report states that two of these women miscarried, two women elected to terminate their pregnancies, one women gave birth to a baby with severe microcephaly, and the sixth woman is still pregnant. Of the women infected during the later stages of pregnancy, one has given birth to an apparently healthy baby with a normal head size, while the remaining two women are still pregnant. The report describes some of the cases and they make sad reading. “Patient A”  was a pregnant woman in her 30s who miscarried. What they refer to as the “products of conception” tested positive for Zika virus. “Patient B” was another woman in her 30s. Her 20 week scan revealed a foetus with severe brain abnormalities. She decided to terminate the pregnancy. What an awful end to a trip overseas.

My previous posts on Zika:

Zika FAQ

Sexual transmission and GM mozzies