The Zika epidemic that has spread from Brazil to the rest of Latin America is now raging in Puerto Rico — and the island’s response is in chaos.

The war against the Aedes aegypti mosquito carrying the virus is sputtering out in failure. Infections are skyrocketing: Many residents fail to protect themselves against bites because they believe the threat is exaggerated.

Federal and local health officials are feuding, and the governor’s special adviser on Zika has quit in disgust.

There are only about 5,500 confirmed infections on the island, including of 672 pregnant women. But experts at the Centers for Disease Control and Prevention say they believe that is a radical undercount.

Just four cases of infection were confirmed last week in Florida. But in Puerto Rico, officials believe thousands of residents — including up to 50 pregnant women — are infected each day.

Most never get tested. Tests on donated blood, the most reliable barometer of the epidemic’s spread, show that almost 2 percent of the donors were infected in the last 10 days.

“That’s a stunning number and reflects an explosion of cases,” Dr. Thomas R. Frieden, the director of the C.D.C., said in an interview.

The proportion of pregnant women testing positive for the virus has risen sevenfold since January, the agency said on Friday. Officials warned that hundreds of infants could be born with microcephaly in the coming year.

But a wave of microcephaly like Brazil’s may yet be averted for two reasons. The pregnancy rate is falling so precipitously that this year, for the first time in history, Puerto Rico will have fewer births than deaths.

Obstetricians, too, are quietly urging their infected pregnant patients to have regular ultrasounds and to consider abortion if brain damage turns up.

And damage is turning up.

In an office at the University of Puerto Rico Hospital, the gentle curves of the graph on Dr. Alberto De la Vega’s computer screen trace a horror story. They are the head circumferences of dozens of fetuses whose mothers have been infected with the Zika virus for at least a month — and almost 75 percent are below the mean; normally, only half should be.
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Only one was clearly microcephalic, with the extreme shrinkage and brain damage that is the worst consequence of the infection, and that curve abruptly ends: The mother chose abortion.

“What worries me is not 100 kids with microcephaly,” said Dr. De la Vega, chief of ultrasound diagnosis at the university. “What worries me is a lot of kids affected in some way we cannot determine yet.”

“We may be facing a generation with learning and behavioral disabilities,” he said.

On the scan of a patient who had fled his office fighting back tears, he pointed out three large white spots — clumps of dead cells.

“Her fetus does not have microcephaly now,” he said, “but this is what leads to it.”
A ‘Very Dire’ Situation

Cases of Zika infection are expected to keep rising through October, and by year’s end, a quarter of the island’s population of 3.5 million will have been exposed, a “very dire” situation, said Dr. Lyle R. Petersen, the C.D.C.’s chief of vector-borne diseases and director of its fight against Zika.

In a normal year, that would mean about 8,000 infected pregnancies, but the birthrate is dropping rapidly, said Jose A. Lopez, the health department’s demographer. In past years, births normally outnumbered deaths by about 400 per month, but since January, that ratio has reversed itself, and there have been, on average, 135 more deaths than births.

The drop is partly a result of rising divorce rates and couples emigrating as the economy deteriorates, but also of “mothers delaying giving birth because of the campaign against Zika,” Mr. Lopez said.