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With more than 2,900 Zika cases identified in the United States, only one has stumped health officials looking for how the virus was transmitted: a Utahn who fell ill after caring for a now-deceased Salt Lake County man diagnosed with the virus.
Officials from the Centers for Disease Control and Prevention traveled to Utah this summer to assist the state with its investigation of this "unusual" case.
In an update released Tuesday, the CDC said it has not discovered how the caretaker was infected. The person has since recovered.
During the investigation, the Utah Department of Health said in a statement, officials tested more than 200 people for the virus, including family contacts and neighbors of both cases, and professionals who cared for the deceased man and the infected patient. No additional cases were found.
Local mosquito abatement districts trapped mosquitoes around the homes of both cases, but found no mosquitoes from the two species known to carry the virus.
"Finding new cases can help lead us to answers during an investigation, and while we're happy nobody else was infected, the lack of additional cases leaves many questions unanswered," said Angela Dunn, deputy state epidemiologist, in the department's statement.
The deceased man, who was infected with Zika while traveling, was infected with an unusually high amount of virus, the state health department said, about 100,000 times higher than an average infection. He was elderly and had an underlying health condition, but officials have said the virus contributed to his death. He was the first confirmed Zika-related death in the continental U.S.
The man's infected caretaker had not traveled to an area with Zika transmission and had no sexual contact with a person with Zika virus.
Dagmar Vitek, medical director from the Salt Lake County Health Department, said the investigation will continue. "People should continue to take the appropriate steps to prevent Zika virus infection especially pregnant women, and health care workers who are caring for severely ill patients with the disease," Vitek said in the statement.
Dunn said that caregivers for individuals infected with Zika should be aware that the virus possibly could be transmitted through bodily fluids.
Cases of Zika are usually mild and rarely result in death.
The most common symptoms of the virus are rash, joint pain, fever and red eyes. Some individuals with Zika never exhibit symptoms, according to the CDC.
Sexual activity can transmit the virus. If a woman is pregnant or trying to get pregnant, she should not have unprotected sex with a man who has been to an area where the virus is spreading. The virus can cause birth defects in children whose mothers were infected during pregnancy, according to the CDC.
There currently is no vaccine or medication to cure Zika.
University of Utah Health Care and the Davis County Health Department also have assisted in the investigation.
In the U.S. states, the CDC reported that there have been 43 locally acquired mosquito-borne cases and 2,920 travel associated cases as of last week.
There have been 13 cases of Zika reported in Utah, according to the department.