 Researchers in New York think they have finally found the answer to the flu’s greatest mystery. Why does it spread more during the winter? The answer is found in the virus itself. Apparently, the virus requires colder temperatures to really thrive. It tends to be more stable and will stay in the air longer when the air is cold and dry.
Dr. Peter Palese, a flu researcher who is professor and chairman of the microbiology department at Mount Sinai School of Medicine in New York and the lead author of the flu study, published his findings in the Oct. 19 issue of PLoS Pathogens. The crucial piece of the puzzle came in a newspaper published during the aftermath of the 1918 flu pandemic. Doctors were puzzling as to why the virus had spread so quickly and been so deadly. Approximately one third of the world’s population was infected and anywhere from 50 million to 100 million people died from it. In other words, more people died from the flu in one year than from all four years of the Bubonic Plague. As long as the flu has been recognized, people have asked, “Why winter?” Even the name “influenza” is said to come from an Italian phrase meaning “influence of the cold.” Flu season in northern latitudes is from November to March and southern latitudes it is from May until September, meaning that flu season is during the coldest months for both hemispheres. However, in the tropics, there is not much flu at all and no real flu season. There has never been a shortage of hypotheses. The most popular is of course that children going to school during the winter months, all crowded together, is the reason for the spreading. But most scientists have never been convinced. "We know one of the largest factors is kids in school— most of the major epidemics are traced to children,” said Dr. Jonathan McCullers, a flu researcher at St. Jude Children’s Research Hospital in Memphis. “But that still does not explain wintertime. We don’t see flu in September and October.” But getting data to finally crack the case was surprisingly difficult, according to Dr. Palese. The ideal study would expose people to the virus under different conditions and ask how likely they were to become infected. Such a study, Dr. Palese said, would not be permitted because there would be no benefit to the individuals. Finding suitable test animals was also difficult. Mice can be infected with the influenza virus but do not transmit it. Ferrets can be infected and transmit the virus, but they are somewhat large, they bite and they are expensive, so researchers would rather not work with them. This is where the old newspaper came in. Reading a paper published in 1919 in the Journal of the American Medical Association on the flu epidemic at Camp Cody in New Mexico, he stumbled upon a key passage: “It is interesting to note that very soon after the epidemic of influenza reached this camp, our laboratory guinea pigs began to die.” At first, the study’s authors wrote, they thought the animals had died from food poisoning. But, they continued, “a necropsy on a dead pig revealed unmistakable signs of pneumonia. So, Dr Palese bought some guinea pigs and exposed them to the flu. Just as the paper indicated, they contracted the virus and spread it amongst themselves. Dr. Palese and his team then began conducting their tests. By varying air temperature and humidity in the guinea pigs’ quarters, they discovered that transmission was excellent at 41 degrees. It declined as the temperature rose until, by 86 degrees, the virus was not transmitted at all. The virus was transmitted best at a low humidity, 20 percent, and not transmitted at all when the humidity reached 80 percent. Flu viruses spread through the air, unlike cold viruses, Dr. Palese said, which primarily spread by direct contact when people touch surfaces that had been touched by someone with a cold or shake hands with someone who is infected, for example. Flu viruses are more stable in cold air, and low humidity also helps the virus particles remain in the air. That is because the viruses float in the air in little respiratory droplets, Dr. Palese said. When the air is humid, those droplets pick up water, grow larger and fall to the ground. “It was great work, and work that needed to be done,” said Dr. Terrence Tumpe, a senior microbiologist at the Centers for Disease Control and Prevention. Dr. McCullers said he was pleased to see something convincing on the flu season question.
“It was a really interesting paper, the first really scientific approach, to answer a classic question that we’ve been debating for years and years,” he said. As for Dr. Palese, he was glad he spotted the journal article that mentioned guinea pigs.
“Sometimes it pays to read the old literature,” he said.
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