Experts in the United States and Europe have released the strongest evidence to date of a link between Zika and microcephaly, after the virus was discovered in the brain and placental tissues of several fetuses and babies born with the rare condition.

The U.S. Centers for Disease Control and Prevention said Wednesday that the Zika virus was found in tissue samples from two babies with microcephaly who died shortly after birth, and two fetuses that were miscarried.

Microcephaly is a congenital condition characterized by an abnormally small head.

All four cases analyzed by the CDC were from the state of Rio Grande do Norte in Brazil.

Another case report from Slovenia, published online Wednesday by the New England Journal of Medicine, describes the discovery of the Zika virus in the brain tissue of a fetus with microcephaly.

A 25-year-old European woman who had been living and volunteering in Natal, the capital of the Rio Grande do Norte state, since December 2013, was ill with a high fever, rash and joint pain during the 13th week of her pregnancy, according to the case report. 

The woman was 28 weeks pregnant when she returned to Europe last year. She had an ultrasound at 29 and 32 weeks of pregnancy in Slovenia, which confirmed that the fetus had microcephaly, with “calcifications in the brain and placenta.” 

The case report says the fetus was “given a poor prognosis for neonatal health” and the woman’s request to terminate her pregnancy was approved by national and hospital ethics committees.

An autopsy performed on the fetus in October, 2015 confirmed the presence of the Zika virus in the brain tissue.

Tatjana Avsic-Zupanc, a microbiology and immunology expert at the University of Ljubljana in Slovenia and one of the authors of the NEJM report, said the team was “quite surprised” to find the virus only in the brain tissue of the fetus.

Since there were no other pathogens present and the complete genome sequence of the Zika virus was recovered from the fetal brain, “we think that this may present the most compelling evidence to date” of a link between Zika and microcephaly, Avsic-Zupanc told in a phone interview Wednesday afternoon, before the CDC released its latest findings. 

The World Health Organization currently says that a “causal relationship” between Zika infection during pregnancy and microcephaly is “strongly suspected,” but not yet scientifically proven.

The suspected link has prompted the WHO to declare an emergency over the virus. A seemingly sudden increase in microcephaly cases in Brazil since April 2015 has also prompted travel warnings for pregnant women.

The warning from the Public Health Agency of Canada now lists 26 countries, and urges travellers to "practice special precautions" when in areas affected by the Zika virus – which now cover most of South America.

However, Colombia’s president saidlast weekend that there’s no evidence Zika has caused any cases of microcephaly in that country, even though more than 3,100 pregnant Colombian women have been diagnosed with the virus.

Zika and eye abnormalities

Another study published this week in the journal JAMA Ophthalmology found that infants infected with the Zika virus may also have “vision-threatening” eye abnormalities. 

Abnormalities of the eye were observed in 10 of the 29 infants with microcephaly at a hospital in Salvador, Brazil.  Of those 10 infants, seven had defects in both eyes. The eye abnormalities included lesions, damage to the retina and damage to blood vessels and tissue below the retina.

Of the 29 mothers involved in the December 2015 study, 23 reported suspected symptoms of a Zika infection during pregnancy, including rash, fever, joint pain, headache and itch.