Brazilian health officials say the number of cases of microcephaly, a rare brain defect in babies, has risen to 3,893 since authorities began investigating the surge in cases in October.

Fewer than 150 such cases were seen in all of 2014.

The Health Ministry has said the surge is linked to Zika virus, a mosquito-borne disease similar to dengue.

The ministry's emergency response official, Wanderson Oliveira, said most of the cases of microcephaly remain concentrated in Brazil's poor northeastern region. However, the developed southeast where Rio de Janeiro and Sao Paulo are located is the second hardest-hit region.

Oliviera spoke at a press conference Wednesday.

Claudio Maierovitch, who heads the ministry's transmissible disease department, said officials are learning
quickly about microcephaly and Zika, but much still remains unknown.

Canadian and U.S. health officials have issued alerts advising pregnant women and those considering becoming pregnant to discuss their travel plans with their health care provider to assess their risk and to consider postponing travel to areas where the Zika virus is circulating.

Zika virus, first detected in Africa in the 1940's, was unknown in the Americas until last year when it appeared in northeastern Brazil. The virus has quickly spread through Latin America.

Brazil Zika Birth Defects

Brazilian health authorities have linked the Zika outbreak to a surge in the number of babies born with unusually small heads, a damaging neurological condition called microcephaly. (Felipe Dana/Associated Press)

With Brazil's rainy season underway, authorities are scrambling to fight the seasonal surge in mosquito populations.
 
Two weeks ahead of Carnival celebrations, a highlight of Brazil's tourism calendar, officials want to stem international concern about the virus.

One potential strategy to curb cases involves a genetically modified mosquito to help reduce the proliferation of mosquitoes spreading Zika and other dangerous viruses in Brazil, its developers say.

The self-limiting strain of the Aedes aegypti mosquito was developed by Oxitec, the U.K.-subsidiary of U.S. synthetic 
biology company Intrexon.

"If we can control this mosquito properly, then we can control and reduce the viruses that it spreads," said Oxitec CEO Hadyn Parry. 

Modified male mosquitoes are released so their offspring will die before being able to reproduce. The male mosquitoes don't bite or spread disease. 

Populations of Aedes aegypti were reduced by about 90 per cent. 

"You can't get near that level of control with chemicals. The reason it's effective is really we're using biology, we're using a male to go and find a mate and male mosquitoes are very good at finding female mosquitoes," Parry said.

In Brazil's Piracicaba neighbourhood, authorities reported a drop in dengue cases spread by the same mosquito species after 25 million transgenic insects were released between April and November.

Maierovitch cautioned the sterile mosquito solution is not yet ready to be used on a large scale.

For the moment, the best way to prevent transmission is by doing away with stagnant water where the insects breed, using repellent and wearing covering clothing, he said.

With files from Reuters and CBC's Jill English