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Influenza A, Mexico, US
25 April 2009
The American authorities, Centers for Diseases Control and Prevention – CDC , write in their latest update that a new H1N1 influenza has been laboratory confirmed in California (6 cases), Texas (2 cases) and Mexico (7 cases). All confirmed cases had mild Influenza-Like Illness (ILI), with only one requiring brief hospitalization. No deaths have been reported. 


The Government of Mexico has reported three separate events. In the Federal District of Mexico, surveillance began picking up cases of ILI starting 18 March. The number of cases has risen steadily through April and as of 23 April there are now more than 854 cases of pneumonia from the capital. Of those, 59 have died. In San Luis Potosi, in central Mexico, 24 cases of ILI, with three deaths, have been reported. And from Mexicali, near the border with the United States, four cases of ILI, with no deaths, have been reported. Of the Mexican cases, 18 have been laboratory confirmed in Canada as Swine Influenza A/H1N1, while 12 of those are genetically identical to the Swine Influenza A/H1N1 viruses from California. The majority of these cases have occurred in otherwise healthy young adults. Influenza normally affects the very young and the very old, but these age groups have not been heavily affected in Mexico. The Swine Influenza A/H1N1 viruses characterized in this outbreak have not been previously detected in pigs or humans. The viruses so far characterized have been sensitive to oseltamivir, but resistant to both amantadine and rimantadine. 


ProMED also reported on the outbreak, and conclude that “It is unclear at present to what extent the current outbreak of acute respiratory disease in Mexico is a consequence of influenza virus infection, and whether the outbreak virus is in any way related to the atypical strain of swine influenza virus associated with mild illness in a small number of people in southern California and Texas” 


Comment on flu in Mexico
The key information is the statement by the W.H.O. that “The Influenza A/H1N1 viruses characterized in this outbreak have not been previously detected in pigs or humans.” This mean that this is a new virus, which apparently is able to spread from humans to humans and the situation is therefore very much different from the avian flu (bird flu), which so far has not been able to spread from humans to humans. 

The statement in the W.H.O. news release also means that existing influenza vaccines do not protect against the new virus. 

The virus is resistant the amantadine and rimantadine, but sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza), and travelers to Mexico could carry either of these two drugs with them for self treatment. 

Influenza is highly contagious and it is expected that there will be a number of cases in the community which is not diagnosed, and the true extent of the outbreak is therefore not know. 

It is too early to advise restriction on travels to Mexico, but visitors to Mexico should follow the news and decide whether the visit is necessary right now. Cases have been reported in San Luis Potosi, in central Mexico, and Mexicali at the border to the US and it is highly probable that other cases exist in other parts of Mexico.