H5n1 Avian Influenza (Bird Flu) may Transmit from Person to Person
We do know a few important things about H5N1. First, it's particularly virulent. Second, the virus appears to be spreading among birds. Third, it seems to be affecting more species, including cats, which usually aren't susceptible to bird flu. But whether this virus will ever make the genetic changes needed to infect humans on a mass scale, or how long that might take — there's just no way of knowing.
Of the hundreds of strains of avian influenza A viruses, only four are known to have caused human infections: H5N1, H7N3, H7N7, and H9N2. In general, human infection with these viruses has resulted in mild symptoms and very little severe illness, with one notable exception: the highly pathogenic H5N1 virus.
The virus the world flu experts are watching right now is called H5N1 influenza, known informally as avian flu or bird flu. If the flu bugs we see in a regular year are handguns, H5N1 is an AK-47.A bird-adapted strain of H5N1, called HPAI A(H5N1) for "highly pathogenic avian influenza virus of type A of subtype H5N1", is the causative agent of H5N1 flu, commonly known as "avian influenza" or "bird flu".H5N1 has killed about 60 percent to 70 percent of those who've been infected in Asia, and birds carrying the virus have recently been found in Romania and Turkey, showing it's moving west.
Human influenza is transmitted by inhalation of infectious droplets and droplet nuclei, by direct contact, and perhaps, by indirect (fomite) contact, with self-inoculation onto the upper respiratory tract or conjunctival mucosa.4,5 The relative efficiency of the different routes of transmission has not been defined. For human influenza A (H5N1) infections, evidence is consistent with bird-to-human, possibly environment-to-human, and limited, nonsustained human-to-human transmission to date.
It was originally thought that these avian viruses could not directly infect humans, because humans have receptors for human viruses, and birds for avian viruses. Therefore, it was hypothesized that an intermediate host was required in which co infection and reassortment could take place and that this host was the pig, which possessed receptors for both kinds of viruses.
When a new strain of flu infects people, the infection can spread around the world quickly. This is what could potentially happen with some new human flu viruses that come from bird flu viruses.” Recently, some strains of bird flu viruses have infected people in Asia,” said Robert Belshe, M.D., director of the Center for Vaccine Development at Saint Louis.
As of January 12, 2007, 265 people had been infected with the bird flu in 10 countries. More than half of these people have died, according to the World Health Organization. Although the direct transmission of the virus from birds to humans is itself cause for concern, the greatest worry is that human-to-human transmission may begin to occur if there is a change in the viral genome. Even without genetic change, some inefficient spread among humans may now be expected.
A 19-year-old man has contracted bird flu in China; the country's Ministry of Health said .China has reported 25 human cases of bird flu since 2003 and 15 of those people have died. The seasonality of H5N1 influenza seems similar to that of human influenza: the virus has apparently been more transmissible among chickens, and consequently to humans, during the cooler months. The cases in humans in Turkey, Iraq, and Egypt occurred during the cooler months and coincided with explosive outbreaks of the disease in wild and domestic poultry.
Indonesian health officials say avian influenza has been confirmed in a 29-year-old woman and a 5-year-old daughter died after showing some symptoms of the disease. A 2-year-old who lived nearby is now being treated in the hospital. The researchers note the Vietnam is the country hardest hit by the ongoing H5N1 epidemic, with 87 confirmed human cases of HPAI (with 38 deaths) and 1,838 verified outbreaks in poultry in July 2005.
That study suggests that there is an association between direct contact with dead or sick poultry and flu-like illness in humans and that the transmission is probably more common than expected. The symptoms most often are relatively mild and that close contact is needed for transmission to humans.
The virus can improve its transmissibility among humans via two principal mechanisms. The first is a “reassortment” event, in which genetic material is exchanged between human and avian viruses during co-infection of a human or pig. Reassortment could result in a fully transmissible pandemic virus, announced by a sudden surge of cases with explosive spread.
The second mechanism is a more gradual process of adaptive mutation, whereby the capability of the virus to bind to human cells increases during subsequent infections of humans. Adaptive mutation, expressed initially as small clusters of human cases with some evidence of human-to-human transmission, would probably give the world some time to take defensive action, if detected sufficiently early.
Infection clusters in human households may arise from transmission from:
- Humans who were infected by animals (primary human-to-human transmission), or
- Humans who were infected by humans (secondary human-to-human transmission).
It is efficient secondary human-to-human transmission that is a prerequisite for pandemic spread.
A mathematical analysis has confirmed that H5N1 avian influenza spread from person to person in Indonesia earlier, U.S. researchers reported .They said they had developed a tool to run quick tests on disease outbreaks to see if dangerous epidemics or pandemics may be developing.
Health officials around the world agree that a pandemic of influenza is overdue, and they are most worried by the H5N1 strain of avian influenza that has been spreading through flocks from Asia to Africa. It rarely passes to humans, but since 2003 it has infected 322 people and killed 195 of them.
Most have been infected directly by birds. But a few clusters of cases have been seen and officials worry most about the possibility that the virus has acquired the ability to pass easily and directly from one person to another. That would spark a pandemic.
Ira Longini and colleagues at the Fred Hutchinson Cancer Research Center in Seattle looked at two clusters -- one in which eight family members died in Sumatra in 2006, and another in Turkey in which eight people were infected and four died. Experts were almost certain the Sumatra case was human-to-human transmission, but were eager to see more proof.
"We find statistical evidence of human-to-human transmission in Sumatra, but not in Turkey," they wrote in a report published in the journal Emerging Infectious Diseases.” This does not mean that no low-level human-to-human spread occurred in this outbreak, only that we lack statistical evidence of such spread."
In Sumatra, one of Indonesia's islands, a 37-year-old woman appears to have infected her 10-year-old nephew, who infected his father. DNA tests confirmed that the strain the father died of was very similar to the virus found in the boy's body.
"It went two generations and then just stopped, but it could have gotten out of control," Longini said in a statement.
"The world really may have dodged a bullet with that one, and the next time, we might not be so lucky," he added. The researchers estimated the secondary-attack rate, which is the risk that one person will infect another, was 20 percent. This is similar to what is seen for regular, seasonal influenza A in the United States.
The researchers developed a software product called TranStat and said they would provide it free of charge on the National Institutes of Health's Models of Infectious Disease Agent Study, or MIDAS, Web site.
"We know the key to preventing a pandemic is early detection, containment and mitigation with antiviral therapy and this tool will enable those on the front lines, such as physicians, epidemiologists and other public-health officials, to carry that out efficiently," said Elizabeth Halloran, who worked on the study.
Scientists have found key features that distinguish influenza viruses found in birds from those that infect humans. Specific mutations linked to immune suppression and viral replications differ between bird and human flu viruses could be used to monitor emerging pandemics. The aim of that project was to enable researchers to gain insights into H5N1 and to provide the first fundamental insight into the evolution of influenza viruses in nature--the source of all influenza viruses that affect humans, domestic animals and birds.
Today, public health experts and infectious-disease scientists do not know whether H5N1 avian influenza virus threatens an imminent pandemic. Most indications, however, suggest that it is just a matter of time: witness the increasing number of
H5N1 infections in humans and animals, the documentation of additional small clusters of cases suggestive of near misses with respect to sustained human-to-human transmission, the ongoing genetic changes in the H5N1 Z genotype that have increased
its pathogenicity, and the existence in Asia of a genetic-reassortment laboratory — the mix of an unprecedented number of people, pigs, and poultry.
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