The working assumption in the efforts to manage the ebola epidemic has been that a temperature greater than 101.5 degrees is a necessary condition for a person infected with the ebola virus to become infectious for other people. A study sponsored by the World Health Organization found evidence that a fairly small percentage of ebola victims who never develop a fever in the course of the disease.
Ebola research: Fever not a surefire sign of infection
For public health workers screening more than 1,000 air travelers who arrive each week in the United States from Ebola-stricken West Africa, one symptom above all others is supposed to signal danger: fever.
So long as an individual's temperature does not exceed 101.5 degrees and there are no visible symptoms of Ebola, health authorities say it should be assumed the person is not infectious.
Yet the largest study of the current outbreak found that in nearly 13% of "confirmed and probable" cases in Liberia, Sierra Leone, Guinea and elsewhere, those infected did not have fevers.
The study, sponsored by the World Health Organization and published online late last month by the New England Journal of Medicine, analyzed data on 3,343 confirmed and 667 probable cases of Ebola.
The finding that 87.1% of those infected exhibited fever — but 12.9% did not — illustrates the challenges confronting health authorities as they struggle to contain the epidemic.
U.S. health officials have repeatedly emphasized that fever is a reliable sign of infectiousness. As a defense against the spread of the virus to this country, the Obama administration has ordered that passengers arriving from West Africa at five U.S. airports be checked for fever.
Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, underlined the importance of fever in discussing the case of Thomas E. Duncan, a Liberian who traveled by air to Dallas and was diagnosed with Ebola. He died Wednesday.
Referring to those who had close contact with Duncan, Frieden said a week ago: "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."
The study cites other studies from previous ebola epidemics that produced findings of cases in which fever was never displayed as a symptom. It is clear that most of the people infected with ebola do develop a fever. There are questions as to what the impact of this information should have on the public health measures being taken.
We know that many of the people who have been targeted for further screening because of fever turn out to have diseases different from ebola, so it is at best a preliminary indicator. The assumption that has been used is that the disease course is such that from the point of initial infection to the development of fever the virus load has not reached the critical level for infection of other people. Even if this is not an absolute truth in all cases, it clearly does provide risk probabilities. Nothing is going to guarantee prevention of 100% of all infections.
The question is if fever checking is not a perfect means of screening large numbers of people for travel, what could be done to supplement it. Here is a description of current lab test procedures. They are time consuming and become progressively more reliable as the disease progresses and presents symptoms. They are not practical for things like quick airport screenings. Other test that use disposable materials and give rapid results are in development and testing. While they might be usable for mass screening, they would likely encounter the same problem of being less than perfectly accurate for people in the early stages of infection.
Dr. Frieden of the CDC is citing the figure of 150 air passengers from the three countries where the epidemic is centered entering entering the US a day on average. I'm not sure if this is limited to people arriving on direct flights or if it includes people who make connections in third countries in Europe. My understanding is that present US airport screenings are limited to people arriving on direct flights. The three countries are conducting airport screenings of all departing passengers regardless of their destination or travel route.
Today brings the news that a Dallas hospital worker has been diagnosed with ebola after helping to care for the patient who recently died from the disease. This has also happened with a hospital worker in Spain. Questions are being raised about whether these cases reflect a failure of people to adhere to infection control procedures or whether the procedures are less than 100% reliable.
This epidemic presents many challenges to the people attempting to keep it under control. Fear and panic won't do much to improve the prospects.