Jul 2, 2009 (CIDRAP News) – The novel H1N1 (swine) influenza now circling the globe causes more serious lung disease than seasonal flu strains and sheds from the lung and throat tissue where it reproduces at higher rates, according to two animal studies published today—findings that could explain autopsies and case reports of severe pneumonia as well as the virus's rapid spread.
And while the studies, conducted in ferrets and mice, agree that the new flu passes fairly well between individuals, they disagree over the effectiveness of different modes of transmission.
A team from Erasmus Medical Center in the Netherlands found that the virus transmits easily between ferrets housed in cages whose walls are 4 inches apart. But a team from the Massachusetts Institute of Technology (MIT) and the Centers for Disease Control and Prevention (CDC) found that the novel virus only transmitted well when the ferrets shared direct contact—a sign, that team said, that the new virus has not yet fully adapted to mammals.
The European team, though, warned that the new H1N1 is adapted enough to compete with seasonal flu strains for turf in humans. It "has the ability to persist in the human population, potentially with more severe clinical consequences," they wrote.
Both studies were published online today by the journal Science.
CDC/MIT: Deeper infections with novel H1N1
In the study from the CDC and MIT group, investigators tested the ability of three different novel H1N1 isolates to cause disease in ferrets and mice and assessed the transmissibility of the new virus in ferrets by comparing it to a seasonal H1N1 virus.
Ferrets groups were inoculated intranasally with one of the four viruses, then 24 hours later were either placed in the same cage with flu-naive ferrets to gauge contact transmission or were placed in a cage adjacent to naive animals to test droplet transmission. Researchers monitored ferrets' clinical signs for 14 days.
Investigators measured transmission by assessing viral titers in nasal washes and by detecting viral antibodies in the blood of recovering animals.
The animals that received the novel flu viruses lost more weight than the ones that were infected with the seasonal flu strain. Viral shedding patterns were similar for the novel flu groups and the seasonal flu group.
Investigators, however, detected high titers of viruses in the lower respiratory tracts of animals infected with two of the three novel H1N1 viruses but not the seasonal flu virus. The group also found the novel flu virus in the intestinal tracts of the novel-H1N1–infected animals, consistent with gastrointestinal symptoms that have been reported in humans infected with the new virus.
Direct contact vs droplet spread
In the transmission comparison arm of the study, the researchers found that the seasonal H1N1 virus transmitted quickly and completely by droplet transmission and direct contact among the ferrets. However, the novel H1N1 viruses did not spread by droplets to all ferrets, and transmission was delayed by 5 or more days in two of six infected pairs.
The group wrote that their findings suggest the morbidity and lung viral titers are higher in ferrets infected with the novel H1N1 virus and that, while the virus spread completely among contact group, it did not always spread to the ferrets in the droplet transmission group.
"This lack of efficient respiratory droplet transmission suggests that additional virus adaptation in mammals may be required to reach the high-transmissible phenotypes observed with seasonal H1N1 or the 1918 pandemic virus," the group wrote.
In just one of the puzzles presented by the new virus, the CDC-MIT team found that the novel H1N1 lacks two genetic characteristics that normally encourage flu viruses to bind to the respiratory tract and replicate: an affinity for alpha-2,6 receptors in the lungs, and an amino acid substitution in a gene known as PB2 (for polymerase basic protein 2) that allows the virus to replicate at airway temperatures.
The lack of those two features suggests that the new flu has not completed its adaptation to humans, and "should be closely monitored as markers for enhanced virus transmission," the authors wrote.
Terrence Tumpey, PhD, the study's corresponding author and senior microbiologist in the CDC's influenza division, told CIDRAP News that the study shows that the new virus is transmitting, but not quite as well as seasonal strains. "Seasonal strains transmit like clockwork as soon as we push the cages together," he added.
Researchers who have worked with ferrets during influenza studies can readily tell the difference when they stand outside the cages and listen for sneezes, he said. Ferrets infected with seasonal strains sneeze frequently, those inoculated with avian influenza strains sneeze hardly at all, and those who are sick with the new H1N1 virus sneeze somewhere in between those two levels, Tumpey said.
"We think it [the new virus] could still make additional changes to become more transmissible," Tumpey said. "We need to keep a close eye on the virus and monitor for changes."
Dutch study: High transmission with droplets and aerosol
In the Dutch study, ferrets were inoculated with a novel H1N1 from the first known case in the Netherlands, a 3-year-old child who had developed a fever and respiratory symptoms and recovered. (Genomic analysis showed that the virus from the Dutch child was only minimally different from the California isolate used in the CDC study.)
The virus was administered to ferrets, and their course was compared to that of ferrets given a seasonal flu virus isolated in the Netherlands in the 2006-2007 season.
Across the board, the ferrets that received the novel H1N1 shed 1.5 times more virus from their respiratory tracts than ferrets infected with seasonal flu. Virus was present in their lungs and trachea, rather than just in their noses, and they also experienced more tissue damage that went deeper into their lungs.
In the most striking difference from the CDC group, the ferrets in the Dutch study caught the H1N1 flu from one another "via aerosol or respiratory droplets." In another difference, the Dutch study did not find any viral replication outside the animals' respiratory tracts; livers, spleens, kidneys and brains were free of H1N1 on necropsy. (There was no indication the Dutch team did pathology on their ferrets' intestinal tracts.)
Though the ferrets in the Dutch study were moderately ill at most, the authors warned that the novel H1N1's ability to transmit easily and replicate throughout the respiratory tract could make it a serious health threat in humans. Because seasonal flu strains replicate high in the nose and throat, and avian flu replicates deep in the airways, co-infection with either could product a reassorted virus that causes more serious human disease or picks up drug-resistance factors from currently circulating seasonal strains, they said.
Transmission around globe not slowing
Lending anecdotal credence to the transmissibility of novel H1N1 has been its continued spread among people worldwide. As community spread of the virus takes hold in more parts of the world, the virus is showing up anywhere that groups of people gather, such as:
- Summer camps. Officials from the Muscular Dystrophy Association shuttered the group's summer camps after the virus was detected at several locations, prompting the CDC to issue new guidance.
- Prisons. The virus struck a women's prison near Miami, sickening at least six women, with more than a dozen other suspected cases, and the virus is suspected at a facility in Queensland, Australia
- Athletic events. Members of a women's basketball team returning to Iraq were confirmed as the country's first cases, an Australian athlete was infected with the virus at the World University Games in Belgrade, and flu fears surfaced this week as some Wimbledon players became ill.
- Cruise ships. Three crew members on a Seattle-Alaska cruise were isolated in their rooms after testing positive for the virus, and a Dutch boat based in Aruba was turned away from different ports in the Caribbean after some crew and passengers began showing symptoms.
In addition, earlier this week, health authorities from Saudi Arabia warned high-risk groups to avoid the hajj pilgrimage that starts in late November because the crowded setting could post a flu-transmission risk. And a study that appeared this week in the New England Journal of Medicine showed that of 20 countries that had the highest travel volume to Mexico in 2007 and 2008, 16 had experienced recent novel H1N1 importations from the country.
Commenting on this community spread during the summer months, influenza expert William C. Schaffner, MD, told CIDRAP News, "This transmission in summer camps reminds us that not all summer camps are rural, with kids living in open cabins. There are many summer camps that resemble school—my grandchildren just went to theater summer camp; other kids go to computer or music summer camps.
"That plus the fact that this is a new virus, in a completely susceptible population, namely children and young adults, is clearly contributing to this ongoing transmission. I don't mind saying the degree of transmission has surprised me and frankly most of my colleagues also."
Schaffner, professor and chair in the department of preventive medicine at Vanderbilt University School of Medicine, added "I think the ongoing transmission this summer and what we know is happening in the southern hemisphere—in Argentina and in Australia and New Zealand—is going to be a harbinger of what is going to happen this next influenza season in the United States.
"We all anticipate that H1N1 will be a dominant if not the dominant influenza strain."
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Munster VJ, de Wit E, van den Brand JM, et al. Pathogenesis and transmission of swine-origin 2009 A(H1N1) influenza virus in ferrets Science 2009 Jul 2; early online edition: [Abstract]
Khan K, Arino J, Hu W, et al. Spread of a novel influenza A (H1N1) virus via global airline transportation. (Letter) N Engl J Med 2009 Jul 9;361:212-4 [Full text]