Swine fever: how flu becomes a killer

How often do flu pandemics occur?

Historical evidence suggests they've occurred at 10- to 40-year intervals since the 15th century. Three have occurred in the past century: the Spanish influenza pandemic in 1918-19 that claimed at least 50 million lives; and the Asian (1957) and Hong Kong (1968-69) pandemics, each resulting in one to two million deaths. The current strain is of the same subtype that caused the Spanish flu pandemic – described as "the greatest medical holocaust in history".

Could swine flu become that lethal?

Spanish flu was highly atypical: it killed 2.5-5.0 per cent of those infected, whereas the usual flu epidemic mortality rate is 0.1 per cent. It also targeted young adults, while most flus are more deadly to the very young and very old. At the moment – despite the reported 29 deaths in Mexico – scientists say the swine flu strain lacks characteristics associated with the most lethal viruses. But then again, no one knows how it might develop. Spanish flu started off as a mild illness at this time of year, died off in the summer, then came back with deadly virulence.

So what kind of disease is influenza?

It's an acute viral infection that falls into one of three broad types, – A, B or C – according to its protein compo-sition. Type B viruses are common in humans; type C, found in pigs, dogs and humans, causes mild respiratory infections. It's the type A virus that causes epidemics. Type A has a range of subtypes with cryptic names like H3N2 or H1N1, the 'H'  being for hemagglutinin, a protein on the flu mole-cule surface that connects with matching receptors on the outside of healthy respiratory system cells. The virus then melds with the healthy cell and begins replicating. Another protein, neuraminidase (the 'N' in the flu name), then uncouples the virus from its host, tears the cell membrane to let the virus escape and in so doing kills the cell. Once loose, the virus starts repeating the process deeper and deeper into the respiratory tract.

And is swine flu a type A virus?

Yes, a subtype H1N1, that combines a strain of human flu virus, a strain of avian, and two separate strains of swine flu virus. Its origins are unknown. It hasn't yet been detected in pigs, so the decision by the WHO to call it swine flu has dismayed pig farmers, who have been hit with a global slump in pork sales and bans by several countries on meat imports from Mexico and the US.

Then why pick on pigs?

Because pigs can be infected by both bird and human flu. If hens, humans and pigs live cheek by jowl, pigs can act as a sort of influenza virus mixing bowl, setting in train a process called 'antigenic shift' or 'reassortment' – a far deadlier way for the flu virus to mutate than the more usual process of 'antigenic drift'. A big worry is that if swine flu gets to places like Egypt, where the H5N1 strain (bird flu) is endemic, it could 'reassort' into a very powerful H5N1 far more easily transmittable among humans.

How does 'shift' differ from 'drift'?

All flu viruses mutate at a high rate, but usually the genetic changes involved are minor (a "drift") - enough to cause the seasonal flu outbreaks that occur every winter (and that every year kill more than 250,000 people globally), but not enough to cause a pandemic. That's because with antigenic drift, the new viruses differ only slightly from their predecessors, so most people exposed to them retain some immunity. But when Type A viruses undergo a "shift" or "reassortment" - for example, of avian, swine and human strains – humans lack the antibodies to fight them off and the new viruses spread uncontrollably.

How can one prevent infection?

You can't really. There's no vaccine (see box) and face masks are no help. The Department of Health may have ordered millions for its health-care workers, but these have a special filter to ensure virus particles can’t pass through, and health staff are trained to change them frequently. With most face masks, including those issued in Mexico, the virus gets through the pores and the mask gets moist, thereby enhancing the risk of the virus being transmitted. All you can do is keep your distance from people who look unwell (the virus is spread in droplets of coughs or sneezes), cover your nose/mouth with a tissue if sneezing or coughing yourself, and wash your hands regularly.

So how will I know if I’ve got swine flu?

Symptoms – similar to those of standard, seasonal flu – include fever, cough, sore throat, body aches, chills and fatigue. Quite a few victims also report diarrhoea and vomiting. It takes one to four days after infection for symptoms to appear. Patients are being asked not to go into GP surgeries in order to minimise the risk of spreading the disease to others. Instead, they should stay at home and call their GP surgery or NHS Direct for advice.

What about drugs to take after you've got swine flu?

Two anti-viral medicines, Tamiflu and Relenza, appear to be effective if started within 48 hours of first symptoms. They work by targeting not the virus itself but an enzyme that helps the virus spread from cell to cell. A stockpile of 33 million courses has been bought under the Government’s pandemic plan (enough for half the population) and Gordon Brown has announced a fresh order that will bring supplies up to 50 million doses. Patients who fall ill will be assessed by phone, and those thought to have swine flu should get antivirals within 12 to 24 hours of diagnosis.

And will they cure you?

Don’t expect miracles. They help relieve symptoms: studies show they shorten the illness by about one day. But little is known about whether they cut the chances of serious flu complications, like pneumonia. In the longer term, resistance is likely to be a problem. Other flu viruses with the N1 protein have developed significant resistance to Tamiflu, which is why the Government has ordered supplies of Relenza as a backup. ·