Checking whether the Zika virus causes microcephaly, a small head condition in babies that can have devastating effects for families, is a top research priority for the World Health Organization.

Researchers in Brazil reported a disproportionate number of infants with microcephaly, abnormally small heads, a few months after the Zika virus was identified in the country.

"The problem is that the complications of this relatively mild illness do have potentially devastating effects for families. To have a child with microcephaly and to have a 10-fold increase in numbers and the potential for spread not just across Latin America but into Africa, into Asia which have the highest birth rates in the world, we believe is a matter of public health concern and constitutes an international emergency," pediatrician Dr. Anthony Costello, WHO's expert on microcephaly, said at a news conference in Geneva on Tuesday.

Microcephaly simply means a small head. It can be present at birth or develop in the first few years of life, according to the U.S. National Institute of Neurological Disorders and Stroke .

The prognosis varies from no mental deficits to limited ability to communicate, difficulties with co-ordination and seizures, the institute says.

Microcephaly is associated with Down's syndrome and genetic disorders.

The incidence of microcephaly ranges from one in 6,000 to one in 8,000, said Dr. Upton Allen, an infectious disease specialist at Sick Kids Hospital in Toronto.

"This is really not a very common condition at all," Allen said. "The incidence will depend on the specific cause, which is very variable around the globe."

Other infectious causes include German measles and cytomegalovirus.

The many infectious and non-infectious causes of microcephaly are a challenge to rule out, Dr. Albert Ko, a professor of epidemiology and medicine at the Yale School of Public Health in New Haven, Conn., said Tuesday.

Brain abnormalities

Ko and his team have done a preliminary analysis of brain CT scans from about 30 infants in Brazil with microcephaly.

"We were struck by the severity of cerebral atrophy and thinning of the brain matter that we observed in babies with mircocephaly," Ko said.

They also found extensive calcium deposits and an unusual absence of wrinkles on the brain, which he called clues that the brain may not be developed.

"Really it's the severity and the clustering of all these cases at one time that make us think of this as a very different presentation than we see from other common congenital infections," Ko said.

Scientists gained another circumstantial clue when they identified genetic material from the Zika virus in the amniotic fluid of two women whose fetuses had been found to have microcephaly by prenatal ultrasound. They're trying to understand whether the fetus is more vulnerable early in pregnancy.

There's also a cluster of microcephaly that researchers discovered in French Polynesia's Zika outbreak in 2013 after they heard of Brazil's cases and went back to check records.

Adding to concerns, researchers in Brazil recently reported a "dramatic 3,530 cases recorded as of the first week of 2016."

To try to prove the link, scientists are looking for an effective animal model. By infecting animals with Zika virus under controlled conditions, they aim to learn more about how the birth defect may arise.

On Monday, WHO declared clusters of microcephaly and other neurological complications associated with Zika "an extraordinary event and public health threat to other parts of the world."

Without the strongly suspected links to microcephaly and neurologic complication, Zika virus on its own likely wouldn't have warranted the emergency label because it is not a clinically serious disease, said Dr. David Heymann of the London School of Hygiene and Tropical Medicine. He chaired WHO's emergency committee on Zika.

Costello said the association between Zika virus and microcephaly has to be treated as guilty until proven innocent.

Recognizing the scale of the problem could go beyond the Americas, WHO is complementing its regional emergency operation with its global centre, Dr. Bruce Aylward, the organization's assistant director-general, told CBC News Tuesday.

Aylward pointed to three thrusts:

Surveillance for the virus and neurological problems.

Reduce mosquito populations to reduce the chance of contact.

Care for people affected for neurological outcomes that may be linked to the virus.

The research side has two fronts, he said.

"First, to look at association whether it's actually causative between the neurologic problems we're seeing and the virus. That still has not been proven and that's a huge push on the research side," Aylward said in an interview from Geneva.

The second research priority is to improve diagnostics to provide an answer on whether people were historically infected in previous months, he said. Diagnostics to tell if the Zika virus is circulating in the population already exist.

In Canada, the Society of Obstetricians and Gynecologists says that within a month, blood tests will be available out of the National Microbiology Laboratory in Winnipeg for anyone who travelled to the affected zones. Currently, blood is sent to the U.S. for testing.

The window of opportunity to test for Zika virus is small. A positive test result does not necessarily lead to a diagnosis of Zika, as the result could also indicate its viral cousins such as dengue.