A flu epidemic has caused the death of at least 20 people in Mexico over the past month. More than a hundred deaths are still being investigated for a connection to the disease. Hundreds more are ill with flu symptoms. The virus, a swine flu variant, has the potential to cause a global pandemic; outbreaks have been confirmed in the US and Canada and it is feared that people have been infected by the flu virus in five other countries.

The street scene in Mexico City has changed drastically since last weekend. Public buildings such as schools, libraries, bars and churches are closed. People in the street are wearing face masks. These measures are reminiscent of the scenes in Asia during the bird flu in 2006.

Uncharacteristically, the outbreak has occured at the end of the flu season on the northern hemisphere, which has been relatively trouble-free. Every year around 500,000 people die as a result of flu, usually because they get pneumonia. Generally, it is the elderly, chronically ill patients and young people who fall victim to the flu. What is remarkable about the Mexican strain of flu is that it affects healthy people between 25 and 45 years old. The influenza A and B virus are the main causes of human flu.

The subtypes are named on the basis of a combination of the two main proteins the virus contains: haemagglutinin (HA) and neuraminidase (NA). These are the only proteins the human immune system recognises. There are 16 types of haemagglutinin and nine different neuraminidase enzymes.
There are also mutations within these above-mentioned types. For this reason an effective vaccine can only be produced if the exact strain of the flu virus is known. 

Infection
The Mexican flu - an influenza A virus, H1N1 subtype - contains genetic material that is typically found in strains of the virus that affect humans, birds and pigs. This new strain has never been seen before. The Mexican swine flu has the potential of becoming a greater threat than the bird flu, as the virus appears to be transmitted from human to human. Up to now this has not been the case with the bird flu virus. The possibility of a global influenza epidemic depends on how contagious the virus turns out to be. As yet, it doesn't seem too bad.

Not only in Mexico, but also in the American states of Texas, California, Kansas and possibly in New York people fell ill, but they have recovered. According to the American Centre for Disease Control the same virus is responsible, but it is unclear whether there is a connection with the outbreak in Mexico. Ten children in New Zealand, who returned from a trip to Mexico recently, are thought to have contracted the virus.

Mutations
H1N1 is known as a troublemaker because it mutates quickly. It easily exchanges genetic material with pigs, birds and people. 71 different strains have been recorded. The virus was the cause of the Spanish flu that killed tens of millions of people between 1918 and 1919. In the 1920s and 1950s the virus also caused major epidemics.

During the last major flu epidemic in 1977, the virus turned up again as the so-called Russian flu, which was a lot less devastating. The Russian flu mainly affected schoolchildren, juveniles who hadn't yet built up resistance against the virus. Adults coped better, having been exposed to the virus before. This would explain why in Mexico young people in particular are falling ill. The new strain also appears to be resistant to the flu vaccinations that are given every winter.

Treatment
Patients in Mexico are reacting well to antiviral drugs such as Oseltamivir (Tamiflu). That's a blessing in disguise, as last March the World Health Organisation of the UN (WHO) announced that 1291 out of the 1362 H1N1 viruses that were tested turned out to be resistant against this drug. According to the WHO there is not enough information on the virus to determine the size of the threat it poses. More research is needed.

The WHO has called on all nations to "intensify surveillance", but at the moment there is no negative travel advice, nor have additional health and safety measures been taken, and there is no export embargo on pigs. However, a number of countries including China and Russia have banned meat imports from Mexico and the US states affected by the virus.

* RNW translation (hs)