The Centers for Disease Control and Prevention is hoping to catch most Ebola patients arriving in the U.S. through a fever, typically one of the earliest symptoms, but recent research shows that not all Ebola patients get the high temperature characteristic of the disease.

CDC airport screening measures are attempting to detect travelers with Ebola primarily through by taking their temperature with a no-contact thermometer, along with enhanced questionnaires. The literature the CDC is currently promoting says that a fever above 101.5 degrees Fahrenheit are typical for those infected, anywhere from two to 21 days after exposure.

But David Willman of the Los Angeles Times reports that not all those infected with Ebola exhibit high temperatures, according to the largest study of this year’s outbreak. Published last month in the New England Journal of Medicine, the study analyzed 3,343 confirmed Ebola cases as well as 667 probable cases in Liberia, Sierra Leone, Guinea and other affected countries and found that 12.9 percent of patients never exhibited a fever above 100.4 degrees Fahrenheit — a more expansive definition than the CDC’s.

The researchers, sponsored by the World Health Organization, noted that some health care workers in Africa don’t have thermometers available, and asked whether body temperature was elevated. The study’s lead author, Dr. Christl Donnelly, told the L.A. Times that he was confident in the prevalence of fever-less patients.

Studies of past outbreaks have found fever in similar proportions of patients, ranging from 85 percent of patients in Uganda in 2000 to 93 percent in the Democratic Republic of the Congo in 1995, according to the L.A. Times.

Because it may take up to 21 days for Ebola symptoms to begin to show in patients who have been exposed, the administration’s enhanced screening measures at select airports will likely let some cases fall through the cracks.

With fewer patients than originally believed exhibiting fever, however, the one symptom Customs and Border Patrol officials would likely be able to detect in incoming travelers from countries with Ebola outbreaks may be less effective than expected.

The CDC is also closely monitoring patients who may have had contact with Ebola patients in the U.S., such as Liberian Thomas Duncan and American nurse Nina Pham, who treated Duncan, based on the assumption that those infected with Ebola will come down with a fever first.

“The only thing we need to ensure is that their temperature is monitored, and if they develop a fever, that they are immediately assessed, isolated, and if found to be positive, then appropriately cared for,” CDC chief Dr. Tom Frieden said last week.

Patients who have Ebola typically become contagious only when their fever symptoms begin, according to National Institute of Allergy and Infectious Diseases head Dr. Anthony Fauci. “Almost invariably, fever is the thing that signals the onset,” Fauci said last week.

Frieden said Monday that 91 passengers had undergone additional Ebola screening at New York’s John F. Kennedy International Airport so far, but none had a fever. Five were referred to the CDC for additional evaluation, but none were found to have been exposed to Ebola.

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