A surge in underdeveloped brains in Brazilian infants born to women infected with the mosquito-borne Zika virus has public health authorities and doctors taking note.

The Zika virus is found in Latin America, the Caribbean and Mexico. The infections are mild, but emerging evidence of a link to birth defects is a jaw dropper.

Experts at the U.S. Centers for Disease Control and Prevention found evidence of the virus in the placentas from two women who miscarried and the brains of two newborns who died. The newborns had small heads, a rare condition known as microcephaly.

In Brazil, the incidence of microcephaly increased 20-fold in the past few months compared with last year. Brazil's Health Ministry says 3,530 babies have been born with microcephaly in the country since October, compared with less than 150 in 2014.

Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases in Bethseda, Md., and his colleague Dr. David Morens wrote a perspective on Zika virus that appeared in Wednesday's New England Journal of Medicine.

"Despite the lack of definitive proof of any causal relationship, some health authorities in afflicted regions are recommending that pregnant women take meticulous precautions to avoid mosquito bites and even to delay pregnancy. It is critically important to confirm or dispel a causal link between Zika infection of pregnant women and the occurrence of microcephaly by doing intensive investigative research," they wrote.

There's been some evidence that suggests Zika virus may be the cause of microcephaly, said Dr. Kamran Khan, an infectious disease physician and scientist at St. Michael's Hospital in Toronto, who is studying the spread of the virus.

"This is not 100 per cent conclusive, but it is quite concerning and compelling," Khan said.

​To try to prove the link, scientists could infect animals with Zika virus under controlled conditions to see if they give birth to offspring with the birth defect.

Such experiments will take time as health authorities face a rapidly emerging threat.

There are other infectious diseases that can cause problems in pregnant women that are ubiquitous around the world and don't trigger travel notices, said Dr. Michael Gardam, director of infection prevention and control at Toronto's University Health Network.

"To my knowledge, this is the first one specifically related to travel has emerged so quickly that has everybody going from not even thinking about Zika virus, because it seems to be quite a mild illness, to oh my goodness, there could be something significant here," Gardam said.

An estimated 80 per cent of those infected show no symptoms, Khan said.

The rest have mild fever, rash, red eyes and muscle aches for two to seven days after a bite from the Aedes mosquito, which also transmits dengue and chikungya, according to the World Health Organization.

Earlier this week, Canadian health officials said a B.C. resident who recently travelled to El Salvador contracted Zika virus infection.

The Public Health Agency of Canada said the risk to Canadians is low. The agency's travel health notice for Zika virus notes cases appearing in Central and South America as well as Mexico. Previous outbreaks have been reported in Africa, Asia and the Oceanic Pacific region, such as Easter Island.

"An investigation to better understand the relationship between Zika virus infection and increased risk for microcephaly is ongoing," the Public Health Agency of Canada's travel health notice says.

"It is recommended that pregnant women discuss their travel plans with their health-care provider to assess their risk and receive advice on measures to protect themselves against mosquito bites."

Scientists are trying to determine whether anything else could be contributing to the increase in microcephaly, Gardam said.

Precautions travellers can take to avoid mosquitoes include wearing clothing the covers the arms and legs and insect repellent, Khan said.

There is no vaccine or specific treatment for Zika virus.

Anyone travelling to Latin America who develops symptoms on returning home to Canada should let their health-care provider know where they have been, Khan said.

He expects Canadian health-care providers will continue to see individual travel-related cases.

The two species of Aedes moquitoes believed to transmit Zika are not currently found in Canada.

In Thursday's issue of The Lancet, Khan and his co-authors from Toronto, Ottawa, the U.S. and U.K. published a model anticipating the international spread of the mosquito-borne Zika virus from Brazil, based on travellers' flights and climate conditions.

Khan expects it will likely spread locally in Central America, South America, the Caribbean and possibly as far north as southern Florida, Texas and the Gulf states.

The Summer Olympics in Brazil this August heighten the need to be aware of the emerging virus, the researchers said.