By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Autopsies on people who have died from the new pandemic H1N1 flu show this virus is different from seasonal influenza, even if it has not yet caused more deaths, experts told a meeting on Tuesday.

Americans who died from swine flu had infections deep in their lungs, Dr. Sherif Zaki of the U.S. Centers for Disease Control and Prevention told a meeting of flu experts, including damage to the alveoli -- the structures in the lung that deliver oxygen to the blood.

This in turn caused what is known as acute respiratory distress syndrome -- an often fatal development that leaves patients gasping for breath.

The World Health Organization has confirmed 3,205 deaths globally from swine flu but experts agree all estimates of the extent of the pandemic are grossly understated because so few patients are ever actually tested.

Seasonal flu kills, too -- about 250,000 to 500,000 cases a year globally, according to the WHO. But not in the same way as swine flu, which unlike seasonal flu frequently causes severe disease in young adults and children.

"It is very rarely you see what we call diffuse alveolar damage in fatal seasonal influenza," Zaki told a meeting sponsored by the U.S. Institute of Medicine, which advises government on health matters.

Seasonal flu causes bronchitis and other upper respiratory disease. But Zaki, the chief infectious disease pathologist at CDC, said the new virus had burrowed into the lungs of the 90 or so people he examined after they died, and they had huge amounts of the virus in their blood.

"This is almost exactly what we see with avian flu," Zaki said. "This looks like avian flu on steroids."

EXPERIMENTAL DRUGS

Dr. Yoshi Kawaoka of the University of Wisconsin said tests in monkeys showed the virus lives and replicates 1,000-fold better in the lungs than does seasonal flu.

He said the No. 1 drug of choice against H1N1 -- Roche AG's and Gilead Sciences Inc's Tamiflu -- lowered the so-called viral load of virus in the lungs just enough to help the body fight back.

Experimental flu drugs lower it even more, notably Daiichi Sankyo Co Ltd's CS 8958 and another drug called T-705 or favipiravir, made by Fujifilm Holdings Corp unit Toyama Chemical Co, Kawaoka said.

Zaki said 90 percent of the fatalities he looked at had some condition that would predispose them to serious disease. They had a median age of 38 and one victim was a two-month-old infant who died within a day of getting sick.

Nearly half -- 46 percent -- were obese, many had fatty liver disease, 27 percent had heart disease and 22 percent had asthma, he said.

Dr. Guillermo Ruiz-Palacios of Mexico's National Institute of Medical Sciences and Nutrition said many Mexican patients with severe disease were also obese. In addition, patients came in late for treatment and many were infected with a second common virus, called parainfluenza virus.

Fewer than a third of the U.S. deaths, 29 percent, had a so-called secondary bacterial infection, usually Streptococcus pneumoniae, Zaki said.

Ruiz-Palacios also said the new virus can be found in the urine and feces of patients, something that may affect how it spreads.

(Editing by Cynthia Osterman)

• More News on Flu
• More News on H1N1 Flu (Swine Flu)

Related MedlinePlus Pages:

• Flu
• H1N1 Flu (Swine Flu)