WASHINGTON — The Zika virus continues to surprise. On Monday, the Utah Department of Health reported that a new case had been diagnosed that did not appear to have been contracted through the two known sources of transmission: a mosquito bite or sexual contact.
The patient, who has since fully recovered, was a "family contact" who helped care for an older man who had become infected with the virus after traveling abroad.
The Zika virus is spread mostly by mosquitoes and has caused more than 1,500 cases of birth defects in newborns in Latin America, where it has spread rapidly, and even in the United States, mostly in pregnant women who traveled to countries where it is circulating. In the United States and its territories, there are several hundred pregnant women who have been infected with the Zika virus, with the largest concentration in Puerto Rico.
The new case is something of a medical mystery: Zika is known to be transmitted by the Aedes aegypti mosquito and through sex, but neither seemed to be a plausible explanation for what happened in Utah.
"We have found no evidence that mosquitoes here in Utah are transmitting the Zika virus," said Dr. Angela Dunn, deputy state epidemiologist at the Utah Department of Health.
Dr. Erin Staples of the Centers for Disease Control and Prevention, which has a team on the ground in Utah, said the case was "unique. We're learning something new about Zika every day."
The elderly man died in Salt Lake County in the northern part of the state in June. The man had Zika and his symptoms included a rash and fever, but he also had other ailments and it was unclear whether the virus contributed to his death.
Local health workers had been trapping mosquitoes since last year, including most recently around the house of the man who died, but had found no Aedes aegypti. Joseph Conlon, technical adviser to the American Mosquito Control Association, said neither Aedes aegypti, nor a cousin — Aedes albopictus — are found in Salt Lake County.
If it was not by a mosquito, how else might it have been transmitted?
"This raises some interesting questions," said Dr. William Schaffner, an infectious disease specialist in Tennessee. "Was there a needle stick or injury? Or if not, possible contact with other bodily fluid like urine or saliva?"
Zika has surprised scientists with its ability to be transmitted by sex — both from men to women, and, in a recent discovery in New York City, from women to men. But it is not known to be transmitted in any other way — without the help of a mosquito — so the Utah case is remarkable.
The mosquito that transmits Zika — the Aedes aegypti — does not typically range into northern Utah, but maps of the mosquito's range are unreliable and experts say it is theoretically possible that the bug exists there.
The Centers for Disease Control and Prevention said it has a team on the ground in Utah. Those experts will help determine whether the virus was transmitted by mosquito.