Toddlers with congenital Zika syndrome have severe developmental delays, researchers report. 

In a study that covered a five-year period, researchers found that children in Brazil with congenital Zika syndrome who had microcephaly at birth suffered severe mental delays. 

Microcephaly is a condition in which the head is smaller than normal. Its severity was the only significant factor linked to developmental delays, according to the study authors.

The study included more than 120 children. At age 2.5 years, nearly all of these children were functioning like 2-month to 4-month-old babies. 

"The research findings reinforce public health concerns during the Zika outbreaks in 2015 and 2016 regarding the severity of disability that children with [congenital Zika syndrome] and their caregivers will be experiencing in the years to come," said public health analyst Anne Wheeler of RTI International, a nonprofit research institute in Research Triangle Park, North Carolina.

"However, results related to the children's variability of developmental delays and their strength to receive and understand communication are opportunities for areas of continued research and intervention to support current — and future — congenital Zika syndrome-impacted families and provide greater understanding of the developmental challenges ahead," Wheeler added in an RTI news release.

The researchers found that most of the children studied were able to respond to speech and their environment, react to sounds and voices, tell the difference between familiar and unfamiliar voices and respond to their names.

They could also interact vocally with their environment, make sounds to express moods and have social interactions.

Zika virus is primarily spread through the bite of an infected mosquito. The Zika infection can be passed from a pregnant woman to her fetus, leading to congenital Zika syndrome.

The report was published online May 5 in JAMA Network Open.