Credit: AFP
The influenza virus is a tiny, redoubtable foe that survives by stealth and sheer numbers.
Seen through an electron microscope, it resembles a spiky ball, comprising a protective shell studded with rods. It measures around 100 nanometres (100 billionths of a metre) across, which is about a thousand times smaller than a bacterium.
Because it is so minute, the virus is unable to carry around the enzyme toolkit that it needs to reproduce. Instead, it hijacks the machinery of cells in the throat, nose and lungs to do this.
It first enters the nose or mouth, inhaled in droplets expelled by a cough or a sneeze by an infected person. A virus can also survive on surfaces for up to 72 hours, depending on the type of surface, temperature and humidity. This means the virus can picked up on the fingers and transferred to the mucous membranes if the person touches their nose, eyes or mouth.
The virus uses its spike to bind to, and then invade, an epithelial cell in the respiratory tract. Once inside, it releases a package of genetic instructions, called RNA, that use the cell's machinery to make parts for new virus particles.
The parts are knitted together to form hundreds of new virus particles that then burst out of the now wrecked cell and go on to infect other cells.
Cells that are attacked in the throat, lungs and muscles give rise to the classic symptoms of a sore throat, respiratory wheeze and muscle ache. The high fever that is also a hallmark of flu is a response of the immune system against the invader. This defensive reaction continues until the viruses are eliminated.
Most people recover without complications after a week or two, but the disease can be dangerous for people with a chronic condition such as asthma or heart disease, or for the elderly, very young and others with a weaker immune system. It can also lead to bacterial infection, such as bronchitis or life-threatening pneumonia.
FLU VIRUSES FALL INTO THREE MAIN FAMILIES:
Type A, the commonest, not only circulates among humans but also among birds and pigs, providing a unique opportunity to acquire new genetic variants that can leap the species barrier and spark a pandemic.
Type B can also cause epidemics, but usually produces a milder disease than Type A.
Type C viruses, like Type B, are humans-only pathogens but have never been associated with a large epidemic.
Virus families are further sub-divided according to their two surface proteins, haemagglutinin (H) and neuraminidase (N).
Mexico's swine flu virus has sparked alarm because it presents a new genetic mix for which there may be no immunity and for which it will take months to devise and produce a vaccine.
It is a substrain of H1N1, presenting a new pattern of genes that resulted from a mix of existing viruses of human, pig and avian flu. Still unclear, though, is how contagious and virulent it is.
WHAT ARE THE ORIGINS OF SWINE FLU?
Swine influenza is a common and sometimes fatal respiratory disease among pigs, first identified in 1930, that is caused by a Type A influenza virus. Normally the disease is specific only to pigs. But sometimes pigs can harbour more than one flu virus at one time, which enables the pathogens to mix genes. As a result, a new viral strain emerges that can cross the species barrier to humans, starting with people in contact with infected pigs. The latest threat is a strain of the H1N1 type of flu virus.
WHY THE ALARM?
The World Health Organisation (WHO) says the virus can be contagious among humans in close contact and the outbreak has "pandemic potential," meaning there is risk of a spread across regions or continents. In the past century, novel flu viruses have killed tens of millions of people and cost billions of dollars in economic costs. Worries about a new pandemic have focused in recent years on the H5N1 strain of bird flu, which has killed around 250 people since 2003, mainly in Southeast Asia. But H5N1 is hard to transmit among humans, and its threat has been contained by culling infected poultry.
WHAT ABOUT THE UNKNOWNS?
Experts insist there is no certainty that a pandemic will happen or if so that it will be a mass killer. There are many unknowns about the new strain, especially how easily it spreads between people, how virulent it is or could become. Figuring this out will be the work of gene scientists and epidemiologists.
WHICH COUNTRIES ARE MOST AFFECTED?
Mexico is the epicentre of the outbreak, with 84 probable deaths from swine flu, eight of them confirmed and many more hospitalised. In the U.S. 91 are confirmed infected and one has died. Canada, Spain, Britain, New Zealand, Peru, Germany, Austria, Israel and Switzerland all now have confirmed cases too.
HOW DOES THE VIRUS SPREAD?
Swine flu is thought to spread like typical flu, i.e. in viral particles expelled in coughs and sneezes that are then breathed in by someone nearby, or deposited on surfaces that are then touched by the hand and transmitted to the mouth, nose or eyes. People with the virus may be able to infect others beginning a day before symptoms develop, and up to seven days or more after becoming sick. Young children may be contagious for somewhat longer.
WHAT ARE THE SYMPTOMS?
Sudden fever above 38ºC, cough, headache, aching joints, nasal congestion, general fatigue and lack of appetite. Some people who have contracted the virus report runny nose, sore throat, nausea, vomiting and diarrhoea. In past cases, swine flu has caused pneumonia and respiratory failure and worsened chronic medical conditions.
HAVE THERE BEEN OUTBREAKS IN THE PAST?
From December 2005 through February 2009, only 12 cases of swine flu were reported in the United States. In 1988 a pregnant woman died after contact with sick pigs. In 1976, swine flu at an U.S. military base at Fort Dix, New Jersey killed one soldier. Four were hospitalised with pneumonia. At first, experts feared the strain was related to the Spanish Flu of 1918, which killed millions, but the strain never spread beyond the base.
WHAT TREATMENTS ARE AVAILABLE?
The U.S. Centres for Disease Control and Prevention (CDC) recommends oseltamivir (marketed as Tamiflu) and zanamivir (marketed as Relenza) for treating or preventing infection. These drugs work by preventing the virus from reproducing. Most of the previously reported swine flu cases have recovered fully without requiring medical attention and without antiviral medicines.
IS THERE A SWINE FLU VACCINE?
There is a vaccine for pigs, but not for humans. It is unclear whether current "seasonal" vaccines, designed to combat smaller genetic shifts in major strains of flu virus that are in circulation, may provide a shield.
WHAT ARE THE PREVENTATIVE MEASURES?
Public health authorities in many countries have installed classic control measures, screening points of entry and isolating people suspected to have fallen ill. Mexico has ordered the closure of schools and cancelled public gatherings. Individuals can wear a face mask, avoid greeting someone with a kiss or a handshake, wash their hands frequently and clean commonly-touched surfaces such as telephones, door handles, tables and lift buttons.
CAN SWINE FLU BE CAUGHT FROM EATING PORK?
No. The virus is respiratory, and not transmitted by food. Cooking pork to an internal temperature of 160ºC kills viruses and bacteria.
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SOURCES: WHO, the US CDC, European Centre for Disease Prevention and Control (ECDC), French ministry of health
