Two weeks shy of his first birthday, doctors began feeding Jose Wesley Campos through a nose tube because swallowing problems had left him dangerously underweight.

Learning how to feed is the baby's latest struggle as medical problems mount for him and many other infants born with small heads to mothers infected with the Zika virus in Brazil.

"It hurts me to see him like this. I didn't want this for him," said Jose's mother, Solange Ferreira, breaking into tears as she cradled her son.

A year after a spike in the number of newborns with the defect known as microcephaly, doctors and researchers have seen many of the babies develop swallowing difficulties, epileptic seizures and vision and hearing problems.

While more study is needed, the conditions appear to be causing more severe problems in these infants than in patients born with small heads because of the other infections known to cause microcephaly, such as German measles and herpes. The problems are so particular that doctors are now calling the condition congenital Zika syndrome.

"We are seeing a lot of seizures. And now they are having many problems eating, so a lot of these children start using feeding tubes," said Dr. Vanessa Van der Linden, a pediatric neurologist in Recife who was one of the first doctors to suspect that Zika caused microcephaly.

Zika, mainly transmitted by mosquito, was not known to cause birth defects until a large outbreak swept through northeastern states in Latin America's largest nation, setting off alarm worldwide. Numerous studies confirmed the link.

Seven percent of the babies with microcephaly that Van der Linden and her team have treated were also born with arm and leg deformities that had not previously been linked to other causes of microcephaly, she said.

To complicate matters, there are babies whose heads were normal at birth but stopped growing proportionally months later. Other infants infected with the virus in the womb did not have microcephaly but developed different problems, such as a patient of Van der Linden's who started having difficulties moving his left hand.

"We may not even know about the ones with slight problems out there," Van der Linden said. "We are writing the history of this disease."

On a recent day, Jose laid on a blue mat wearing just brown moccasins and a diaper, his bony chest pressed by a respiratory therapist helping him clear congested airways.

Jose, who has been visited by The Associated Press three times in the last year, is like a newborn. He is slow to follow objects with his crossed eyes. His head is unsteady when he tries to hold it up, and he weighs less than 13 pounds, far below the 22 pounds that is average for a baby his age.

Breathing problems make his cries sound like gargling, and his legs stiffen when he is picked up. To see, he must wear tiny blue-rimmed glasses, which makes him fussy.

Arthur Conceicao, who recently turned 1, has seizures every day despite taking medication for epilepsy. He also started taking high-calorie formula through a tube after he appeared to choke during meals.

"It's every mom's dream to see their child open his mouth and eat well," said his mother, Rozilene Ferreira, adding that each day seems to bring new problems.

Studies are underway to determine if the timing of the infection during pregnancy affects the severity of the abnormalities, said Ricardo Ximenes, a researcher at the Fiocruz Institute in Recife.

Also, three groups of babies whose mothers were infected with Zika are being followed for a study funded by the U.S. National Institutes of Health. The groups include infants born with microcephaly, some born with normal-sized heads found to have brain damage or other physical problems and babies who have not had any symptoms or developmental delays.

At birth, Bernardo Oliveira's head measured more than 13 inches, well within the average range. His mother, Barbara Ferreira, thought her child was spared from the virus that had infected her during pregnancy and stricken many newborns in maternity wards in her hometown of Caruaru, a small city 80 miles west of Recife.

But Bernardo cried nonstop. The pediatrician told Ferreira that her baby was likely colicky and would get better by the third month. Instead, the crying got worse, so Ferreira took him to a government-funded event where neurologists were seeing patients with suspected brain damage.

"At the end of the second month, beginning of the third, his head stopped growing," Ferreira said. "Bernardo was afflicted by the Zika virus after all. I was in despair."

In Brazil, the government has reported 2,001 cases of microcephaly or other brain malformations in the last year. So far, only 343 have been confirmed by tests to have been caused by Zika, but the Health Ministry argues that the rest are most likely caused by the virus.

Health Minister Ricardo Barros said there was a drop of 85 percent in microcephaly cases in August and September compared to those months last year, when the first births started worrying pediatricians. He credited growing awareness of the virus and government attempts to combat mosquitoes through spraying campaigns.

Despite all the problems, some infants with the syndrome are showing signs of progress.

On a recent evening, 11-month-old Joao Miguel Silva Nunes pulled himself up in his playpen and played peek-a-boo with his mother, Rosileide da Silva.

"He is my source of pride," Silva said. "He makes me feel that things are working out."