SOURCES: March 18, 2016, Morbidity and Mortality Weekly Report; March 15, 2016, news release, The Lancet; March 10, 2016, media briefing with: Anthony Fauci, M.D., director, U.S. National Institute of Allergy and Infectious Diseases, and Tom Frieden, M.D., M.P.H., director, U.S. Centers for Disease Control and Prevention; March 11, 2016, The New York Times
FRIDAY, March 18, 2016 (HealthDay News) -- During the first two months of this year, 116 U.S. residents have tested positive for infection with the Zika virus, and all but one were linked to travel to regions endemic for the virus.
That's according to a report released Friday by the U.S. Centers for Disease Control and Prevention. Experts there say that of the 116 cases confirmed between Jan. 1 and Feb. 26, 110 involved travel by the patient to a Zika-endemic area, while five involved sexual contact with a person who had recently traveled to such areas.
The remaining case of infection occurred when a mother passed the virus to her child in pregnancy. No details were given on that case.
Zika is a mosquito-borne virus that's been tied to thousands of cases -- mainly in Brazil -- of a severe birth defect called microcephaly. In microcephaly, a newborn's head is smaller than normal, with the potential for long-term neurological damage.
"Among the 115 [U.S.] patients with travel-associated infection, all patients reported clinical illness," the CDC said. In the majority of cases, symptoms included rash (97 percent of cases), fever and joint pain.
"Zika virus disease should be considered in patients with acute onset of fever, rash, arthralgia [joint pain], or conjunctivitis [pink eye] who traveled to areas with ongoing Zika virus transmission or who had unprotected sex with someone who traveled to one of those areas and developed compatible symptoms within 2 weeks of returning," the CDC said.
The findings were reported in the March 18 issue of the agency's Morbidity and Mortality Weekly Report.
Earlier this week, scientists reported more evidence supporting a link between the Zika virus and microcephaly.
Researchers report that one in every 100 pregnant women infected with the virus during the first trimester will give birth to a baby with the birth defect.
The new risk analysis did have one important caveat, however.
"The findings are from the 2013-14 outbreak [of Zika] in French Polynesia, and it remains to be seen whether our findings apply to other countries in the same way," study co-author Dr. Simon Cauchemez said in a news release from The Lancet. The findings were published in the journal on March 15.
"Data from French Polynesia are particularly important since the outbreak is already over," said study co-author Arnaud Fontanet, a colleague of Cauchemez at the Institut Pasteur in France.
"This provides us with a small -- yet much more complete -- dataset than data gathered from an ongoing outbreak," Fontanet added.
His team believes that the findings strengthen the notion that maternal infection during the first trimester of pregnancy may be especially linked to microcephaly in babies.
Last week, U.S. health officials said they were learning much about the virus. However, the more they learn, the more they realize how much they don't know, Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said during a media briefing.
"Unfortunately, the more we learn, the worse things seem to get," Fauci said.
The Zika virus is suspected of causing an epidemic that started last spring in Brazil, where there have been more than 5,600 suspected or confirmed cases of microcephaly.
Zika has also been linked to Guillain-Barre syndrome, an immune system disorder that can occasionally lead to a fatal form of paralysis.
Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said: "We are learning more about Zika every day. The link with microcephaly and other possibly serious birth defects is growing stronger every day. The link to Guillain-Barre syndrome is likely to be proven in the near future, and the documentation that sexual transmission is possible is now proven."
First discovered in Uganda in 1947, the Zika virus wasn't thought to pose major health risks until last year, when it became clear that it posed potentially devastating threats to pregnant women.
But, for most other people the virus offers little threat -- approximately 80 percent of people who become infected never experience symptoms.
Meanwhile, the virus continues to spread in Latin America and the Caribbean.
It is not expected to pose a significant threat to the United States mainland, federal health officials have said in the past.
In Puerto Rico, however, the situation is "of great concern," Frieden said.
"Puerto Rico is on the frontline of the battle against Zika," said Frieden, who had just returned from the island. "And it's an uphill battle."
By next year, Frieden said, there could be hundreds of thousands of cases of Zika in the territory, and "thousands of infected pregnant women."
Last month, President Barack Obama asked Congress for $1.9 billion to fight the Zika virus. To date, Congress has not approved the funding and both Frieden and Fauci expressed concern that efforts to fight Zika are in jeopardy if the funds aren't forthcoming.
One goal is to create a vaccine that can be given to children before they reach puberty to prevent Zika infection, Fauci said. "We cannot do what needs to be done in a sustained way without those resources," he said.
The CDC currently has this advice for pregnant women:
The Zika virus has now spread to over 33 countries and territories in Latin America and the Caribbean. The World Health Organization estimates there could be up to 4 million cases of Zika in the Americas in the next year.
For more on Zika virus, visit the U.S. Centers for Disease Control and Prevention.
To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.