A strain of the virus that causes bird influenza has been implicated in the death of a three-year-old boy in Hong Kong. The case, though isolated, has sent alarm bells ringing
isolation of the influenza virus strain -- previously only known to infect birds -- from a three-year-old boy who died in Hong Kong in May has put health authorities on the alert. The child, who had pneumonia and the Reye syndrome (a rare disorder that causes inflammation of the brain), fell ill on May 11 and died on May 21. On August 18, the Centers for Disease Control (cdc) in Atlanta, usa , confirmed the presence of the influenza virus strain a h5 n1, which is found in poultry, from autopsies.
The outbreak of this virus was reported in April 1997 at the Yeun Long farm in Hong Kong. Chicken that were infected began ignoring their food, slumped listlessly, and died within a few days. The infection spread rapidly, and killed the entire poultry population in some farms. The virus responsible for the child's death could not be identified for about three months, although tests were carried out. Margaret Chan Fung, director of health, Hong Kong, referred it to three international laboratories. After exhausting all possibilities of infection by human virus strains, the viral culture from the throat of the child was found to match samples of the influenza virus strain known to infect birds, pigs, and horses. The agriculture and the fisheries department of Hong Kong had also sent the infected poultry samples to a us laboratory, where they found the viral strain to be the same as that detected at cdc .
The influenza a h5 n1 virus has now officially been named "Hong Kong 1997". This is the only case so far to have been detected in a human being. "There is no indication at present that this strain has spread from person to person. There is consequently no need for special measures to be taken, as of today," said Daniel Lavanchy of the who division of emerging and other communicable diseases surveillance and control (emc), Geneva.
Efforts are being made to determine whether other persons in Hong Kong or other parts of southern China have been infected with this strain. The source of transmission has not yet been traced and it cannot be established whether it has been transmitted directly from birds or through an intermediary host.It is also not known whether the strain is a result of mutations affecting the virus's ability to invade animal cells.
"If it is a minor change, the general population will have immunity from previous infections, but if it is a shift, you are almost dealing with a new virus. It can cause a major disaster," says Paul Saw Thian, deputy director of health, Hong Kong. Shifts can take place by reassortmentm of genes that takes place when a human virus and an animal virus infect the same cell, facilitating exchange of genetic material.
But how do the viruses that afflict animals cross over to humans? Animal viruses that develop mutations can infect humans. This phenomenon, known as antigenic shift, can lead to an epidemic in human populations if not contained. "Viruses circulate in the ecosystem. Changes in the environment, pollution of the air and water, chronic parasitic infections and bacterial infections affect our immunological system. So, we become easy targets for many infections. Improper use of antibiotics can also lead to viruses developing drug resistance. If proper treatment is not given on time, a new monster is created," said A G Andjaparidz, regional advisor for communicable diseases at the who regional office for Southeast Asia in New Delhi.
Genetic changes in the 'flu virus have been observed a number of times. The virus is notorious for antigenic shifts and drifts (development along a particular line in isolated strains). The influenza type a virus that prevailed before 1947 showed such changes and caused many epidemics. Its viral antigenicity was termed h1 n1 -- to describe changes in antigens (protiens) h and n . A major drift took place in 1957, causing pandemics in various parts of the world. This antigenicity was termed h 2 n 2, where both h and n antigens mutated.
Prevalence of a strain known as h 3 n 2 was recorded in 1968, and since then, a constant drift in the genes affecting antigens has been observed. Before 1977, whenever a new strain of influenza type a virus appeared, the earlier strain used to disappear. But then a strange thing occurred: an older strain -- h1 n1 -- appeared along with h 3 n 2. Both strains are now infecting human populations. How and why the influenza virus a keeps changing is not well understood, but it is known that both the animal and human strains of the virus constantly undergo antigenic drift.
There is uncertainty regarding the influenza a h5 n1 strain: its virulence is not known and only one case has so far been reported so in humans. In the three months since the death in Hong Kong, about 4,000 tests have been conducted on unrelated patients, but the virus did not show up in any of the cases. "This is important because it gave us an early indication that this is a very weak virus in terms of transmissibility," says Chan. "But from past pandemics, you may have a lee-time of a few months or a few years before the pandemic hits." Adds Saw: "In the virus outbreak in 1968, the first case was actually reported in 1965." So the chances are that it is an oddity or a strain that can lead to a pandemic. The outbreak in poultry ended with the death of all infected birds, but the risk to humans is yet to be assessed.
"The virus could be a potential dan ger to India," says Prasad. "If a pandemic -like situation arises it would be difficult to control since we do not have many centres dealing with viruses -- in particular, viruses that affect the respiratory system," he adds. "There is an urgent need to establish an institute to study respiratory viruses on the lines of the National Institute of Virology (niv), Pune. niv basically studies arbo viruses (those transmitted among invertebrates). There are over 300 different antigenic viruses which affect humans. There is no monitoring of these viruses. There is no centre in the country to keep track of virus strains."
Surprisingly, the National Institute of Communicable Diseases, New Delhi, is not aware of the new strain of bird 'flu virus. However, Andjaparidz told Down To Earth that the government is taking steps to establish surveillance of the influenza virus in India. The project is being directed by V Ramalingaswami, former director-general of the Indian Council of Medical Research.
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