RENEE MONTAGNE, host:
It's MORNING EDITION from NPR News. Good morning. I'm Renee Montagne. In North America, swine flu hasn't spread explosively. And so far, at least, it hasn't caused a lot of serious illness. But in the Southern Hemisphere, winter is coming, which means the flu season is just beginning. So scientists will be watching to see how the new virus behaves there. NPR's Richard Knox reports on why influenza appears so reliably every winter.
RICHARD KNOX: It's autumn in Rio de Janeiro, and Dr. Evelyn Eisenstein, a Brazilian pediatrician, says the first cases of seasonal flu are starting to show up.
Dr. EVELYN EISENSTEIN (Pediatrician): It worries me so much, because children go to the school even when they have a cold, you know? Parents don't stay with them. So we have still to do lots of prevention and health education.
KNOX: Eisenstein has special reason to worry this year. She has no way of knowing whether her patients will get sick with the new flu virus she's been hearing about up north - or if they do, how sick they'll get.
So far, few cases of the new flu have shown up in Central and South America. But Dr. Jon Andrus of the Pan American Health Organization expects many, many more as the southern winter takes hold.
Dr. JON ANDRUS (Pan American Health Organization): The winter months provide the ecological situation where the virus more readily infects people. Transmission is facilitated by the conditions of winter, the grouping of people, the hunkering down due to the weather conditions, which will provide more exposure and contact. We're concerned about that.
KNOX: That makes sense, but Dr. Peter Palese, of the Mount Sinai School of Medicine in New York, says conventional wisdom doesn't really explain why flu comes around every winter.
Dr. PETER PALESE (Mount Sinai School of Medicine): Some of the theories about the seasonality over the years have sort of postulated that it's crowding of schools during the winter, that in the summertime schools are off. But that really turns out to be not the determining factor.
KNOX: Palese and his colleagues have studied how flu viruses spread among guinea pigs.
Dr. PALESE: And what we found was that specifically cold temperature, 40 degrees — exactly conditions which we have in temperate zones in the wintertime — are much more favorable for transmission. At high temperature, like 75, 80 degrees, we don't see any transmission in the system.
KNOX: At low temperatures, the virus-laden droplets that guinea pigs and people sneeze or cough out when they've got the flu shrink in size. These smaller droplets carry much further, and stay suspended in the air longer. Humidity is also a factor. Soggy, warm air is bad news if you're a flu virus.
Dr. PALESE: At higher humidity, these droplets become much bigger and actually sink to the floor, and that also contributes to basically the elimination of the virus in the aerosol.
KNOX: And colder, drier air affects the respiratory tract in a way that gives flu viruses a boost. It thickens mucus. That makes it harder for the body to clear out whatever viruses get inhaled. But it's well-known that schools are fertile breeding grounds for flu. And classrooms aren't kept at 40 degrees. I asked Palese how he squares that with his findings.
Dr. PALESE: It's more likely that actually the virus is transmitted in the hallways, which are a little bit colder; maybe outside of the school when people are waiting; in the subway, where it is colder. So it may not be actually what we think where people aggregate, namely in movie houses or in a lecture hall, that that's actually where the greatest transmission occurs.
KNOX: Other scientists think people's resistance to flu goes down in winter. One controversial hypothesis is that people don't get exposed to enough sunshine in winter. Sun stimulates the production of vitamin D, which has a role in immunity.
Whatever the factors, this winter will be an especially tense time in the Southern Hemisphere. It will reveal if the new flu virus is going to touch off the next serious pandemic, or fade away like the swine flu outbreak of 1976.
Richard Knox, NPR News. Transcript provided by NPR, Copyright NPR.
Since ancient times, the flu has been one of the most predictable seasonal diseases. It strikes regularly from November to March in the Northern Hemisphere, and from May to September in the South. Scientists are still struggling to find out why flu is seasonal and why it spreads faster in colder weather.
It's autumn in Brazil, and the first cases of flu in the Southern Hemisphere's annual flu season are starting to show up.
"It worries me so much," says Dr. Evelyn Eisenstein, a pediatrician in Rio de Janeiro. "Because children go to the school even when they have a cold, you know? Parents don't stay with them. There is not the prevention culture that you have in the United States."
Bracing For Swine Flu
This year, there's special reason to worry. Eisenstein has no way of knowing whether her patients will get sick with the new swine flu virus she's been hearing about up North. Or if they do, how sick they'll get.
So far, only a few cases have shown up in Central and South America — in Costa Rica, El Salvador, Guatemala and Colombia. But Dr. Jon Andrus of the Pan American Health Organization expects many, many more as the southern winter takes hold.
"The winter months provide a situation where transmission is facilitated," Andrus says. "It's very logical when people gather, as they do in the winter months, and hunker down to stay warm. Those kinds of social conditions will contribute to virus transmission."
Why Is Flu So Seasonal?
Indeed, influenza is probably the most reliably seasonal of all diseases. Every year, flu epidemics strike from November to March in the Northern Hemisphere and from May to September in the South.
Despite this ancient pattern, scientists actually know surprisingly little about why flu is so seasonal. Dr. Peter Palese of Mount Sinai School of Medicine says the conventional wisdom — that people spend more time together in confined spaces, making it easier for the flu virus to transmit — is not the whole story, or perhaps even the main explanation.
"Some people over the years have postulated that it's the crowding of schools," Palese says. "But that really turns out to be not the determining factor."
Flu Virus Spreads Faster In Colder, Drier Conditions
Palese and his colleagues have conducted what flu experts consider the most telling studies of flu's seasonality. Since 2007, they have studied the transmission of flu between guinea pigs, which can get infected with human flu viruses.
"What we found specifically was that cold temperatures, around 40 degrees, and low humidity — exactly the conditions in temperate zones in winter — are much more favorable for transmission of flu," Palese says. "At 75 to 80 degrees, we don't see any transmission."
That's partly because of how temperature and humidity affect flu viruses, and partly because of how the environment affects the respiratory tracts of humans and other mammals.
Palese says flu viruses are more stable in colder, drier conditions.
Equally important, the droplets of fluid that people (and guinea pigs) spray with each sneeze and cough are much smaller at lower temperatures and humidity levels. So these droplets carry much farther and stay suspended in the air longer.
The droplets also penetrate deeper into the respiratory passages and lungs when breathed in, Palese says.
"At higher humidity, the droplets become much bigger and sink to the floor," he adds.
Colder, drier air also affects mammals' respiratory tracts in a way that gives flu viruses a boost.
"The mucous is much more viscous," Palese explains. That clogs up the tiny hairs, called cilia, that continually beat in waves to clear virus-laden particles out of the breathing passages.
But how do these findings square with the well-documented fact that schools are fertile breeding grounds for flu outbreaks? After all, classrooms aren't kept at 40 degrees.
"It's more likely that the virus is transmitted in hallways, maybe outside of school where people are waiting," Palese says. "It may occur on the way to school, or under conditions where it's a little bit colder."
Whatever the factors, this winter will be an especially tense time for the Southern Hemisphere. It will reveal whether the new flu virus is going to touch off the next serious flu pandemic, or fade away like the swine flu outbreak of 1976.