SAN JUAN, April 1 (Reuters) - The United States faces its first real challenge with the Zika virus on the island territory of Puerto Rico, a part of the nation that is perhaps least prepared to cope with what is expected to be its worst outbreak.
Zika is spreading rapidly in Puerto Rico and is expected to peak in late summer and early fall. By year's end, public health officials estimate, hundreds of thousands of people will have been infected.
It is the only part of the country that is experiencing a major local outbreak, but the virus is expected to reach southern U.S. states within weeks with warmer temperatures and rising mosquito populations.
Health officials from across the United States are gathering today at the headquarters of the Centers for Disease Control and Prevention in Atlanta to outline a national strategy for combating Zika. In a measure of the concern surrounding the outbreak in Puerto Rico, CDC director Tom Frieden toured the island, meeting with top health officials and local experts last month to assess the situation first-hand.
Puerto Rico is beset with problems already hampering the response: abundant mosquitoes, high levels of insecticide resistance and economic woes that have left vector control in shambles.
"We don't have good surveillance" here, Frieden said in an interview at the Puerto Rican health department in San Juan during his tour. "We don't have good control measures."
First detected in Brazil last year, the Zika outbreak is spreading through the Americas. The World Health Organization declared a global health emergency last month because of growing evidence that Zika can cause microcephaly, a rare birth defect defined by an unusually small head. In adults, the virus has been linked to the typically rare autoimmune disorder, Guillain-Barre syndrome.
Fighting Zika in Puerto Rico is complicated by the toll of a decade-long recession. Nearly half of its 3.5 million residents live in poverty, and mosquitoes are an accepted nuisance. Puerto Rico has seen repeated outbreaks of dengue and more recently, chikungunya. Both viruses are carried by Aedes aegypti, the same species of mosquito that carries Zika.
"Here in Puerto Rico, we're really starting from square one," said Audrey Lenhart, a CDC vector control expert in an interview at the CDC's Emergency Operations Center in San Juan.
In its latest report, the Puerto Rican health department said there are now 350 confirmed cases of Zika infection, including 40 pregnant women.
"We have a very serious combination of problems," said Dr. Alberto de la Vega, an obstetrician specializing in high-risk pregnancies at San Juan's University Hospital at the Puerto Rico Medical Center.
"If you don't have access to money to buy repellent, to sleep with an air conditioner on so mosquitoes won't bite you, to have mosquito nets around you and you live in areas where there's more stagnant water, obviously you have higher risks," he said.
To mitigate the risk of microcephaly among newborns, the CDC and the Puerto Rican government are distributing Zika protection kits to pregnant women that include condoms to prevent sexual transmission from an infected partner, insect repellent, bed nets and larvicide tablets for standing water that cannot be drained.
De la Vega says many locals are resigned to the idea that everyone in Puerto Rico will be infected. He said he won't accept that people are "surrendering like that."
Government mosquito abatement resources are scarce, with fewer than a dozen trucks equipped with insecticide sprayers. Of the municipalities that do have trucks, most are used to kill nuisance mosquitoes that bite but do not carry disease, said Manuel Lluberas, a Puerto Rico-born entomologist who works at H.D. Hudson Manufacturing, a maker of spraying equipment.
Lluberas, who advises the WHO and the World Bank on vector control programs, said there are a few municipalities that spray insecticide once every seven to 10 days or once every few weeks. Spraying "needs to be done a lot more frequently" to be effective, he said.
Scientists at CDC's Dengue Laboratory in San Juan have been testing insecticides on mosquitoes gathered from 17 sites on the island. Frieden said in one of the experiments, mosquitoes placed in bottles coated with a commonly effective insecticide "were happily flying around."
Eliminating Zika will require spraying insecticide indoors on walls, under beds, behind furniture and inside closets, where Aedes aegypti hide. So far, only two insecticides - deltamethrin and bifenthrin - are approved for indoor residual spraying, and researchers have found high levels of resistance to bifenthrin in Puerto Rico.
Mosquito experts have found similar resistance in parts of Texas and California.
"You find resistance in mosquitoes in one locale, and 20 miles away they are not resistant," said Joseph Conlon, technical advisor for the American Mosquito Control Association, which represents researchers, public health officials and pesticide makers.
Dr. Janet McAllister, a CDC entomologist, said indoor spraying campaigns will be carried out by local contractors, who will target only areas where the mosquitoes hide instead of coating entire walls, as is typically done to control mosquitoes that carry malaria.
"People would not really be coming into direct contact with those surfaces," McAllister said.
She said the CDC does not plan to use experimental methods, including genetically modified mosquitoes, such as those from Intrexon's Oxitec now being tested in Brazil, or those infected with Wolbochia bacteria that prevent Zika transmission.
Given the urgency of the outbreak, health officials need to focus on known methods of curbing mosquitoes "rather than doing research on things that may or may not work," she said. (Reporting by Julie Steenhuysen; Editing by Michele Gershberg and Lisa Girion)