It appears a lot of people with the H1N1 flu do not experience a fever, and the absence of a fever could mean they are not taking enough precautions to prevent transmitting the virus to others.
Fever and flu go hand-in-hand - at least that's the case most of the time with seasonal flu. But some doctors say the H1N1 virus doesn't trigger a fever in many patients.
"It's remarkable how many of the clinicians will call me and say, 'But the patient doesn't have a fever,' and then they turn positive," said Dr. Richard Wenzel, who chairs the Department of Internal Medicine at Virginia Commonwealth University.
By "turn positive," Wenzel means that test results later confirm that the patients do have H1N1 even though they don't have classic fever symptoms.
He doesn't have hard numbers yet, just anecdotal stories collected from his own clinical practice and from his travels abroad. In July, Wenzel interviewed infectious disease specialists in Mexico and four South American countries. He was in Columbia, Brazil, Argentina and Chile during height of their H1N1 outbreak.
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Wenzel said those South American doctors told him that the virus was especially hard on pregnant women and obese patients; observations that have since been confirmed by U.S. public health officials. They also mentioned that the fever pattern with the new flu strain was unusual.
"Outpatients often don't have fever," Wenzel said. "About half of the cases that come only to the clinic will not have fever."
"About half of the cases that come only to the clinic will not have fever."
Even among the sicker patients in the hospital, Wenzel was told that about 15 percent never got a fever.
Officials with the Centers for Disease Control and Prevention have acknowledged the absence of fever in some H1N1 cases, though they estimate the situation applies only to a small percentage of patients.
An early study of more than 600 H1N1 patients published in the New England Journal of Medicine found that 6 percent of those patients did not have a fever.
The CDC did not make anyone available to comment for this story, but the agency has issued recommendations telling anyone who is sick with a suspected case of H1N1 to not return to work until they are fever-free for 24 hours without the aid of fever-reducing medication.
Wenzel said that guidance misses a lot of sick people with no fever who potentially could spread the flu without knowing it.
"CDC doesn't recognize the definition of influenza H1N1 without a fever," he said. "So that's a problem, and so they're going to underestimate the extent of this epidemic in my opinion by perhaps as much as 50 percent."
In Minnesota, the no-fever issue has been a hot topic among some health care providers.
"I just actually had a conversation with one of the infectious disease physicians at United Hospital about this very issue, because he had noted that a lot of his colleagues were seeing patients without the fever," said Cindy Larson, an Infection Control Practitioner for Allina Hospitals and Clinics.
Larson said fever symptoms are absent in many of Allina's H1N1 cases, though it's hard to determine the extent of the trend because hospitals and clinics have never seen this many flu patients at one time before.
It's possible that their high patient volume is making the lack of fever seem more common than it really is.
But even so, Larson said it's strange to see so many people complaining of muscles aches, congestion, a cough and yet, no fever.
"This is unique to H1N1," she said. "It's not something that we normally see with seasonal flu."
She said it is harder to diagnose patients if they don't have a fever and it makes providing guidance to patients difficult.
"Our guidance around return-to-work policies and all of that are based on the presence of a fever," Larson said. "And when we don't have one, we don't have enough information to know how infectious that person may be and whether or not they should even return to work or not."
The Minnesota Department of Health has asked doctors to report any unusual patterns that they notice with the H1N1 virus. Epidemiologist Dr. Aaron DeVries said researchers are gathering information on the new flu strain, but they have not been specifically tracking the occurrence of fever.
As for the CDC's fever guidelines on staying home, he said the recommendations are designed to decrease the number of people spreading virus at the height of their illness.
"While it's not preventing 100 percent of the shedding of the virus, it is decreasing the majority of individuals who are infectious or shedding more influenza," DeVries said.
Eventually, researchers might be able to say with more certainty whether fever was less common with the H1N1 virus than it is with regular seasonal flu, but those findings could be months or even years away.
Right now, the medical community is just struggling to keep up with treating patients.