Influenza-like illness in the United States and Mexico

24 April 2009 - The United States Government has reported seven confirmed human cases of Swine Influenza A/H1N1 in the USA (five in California and two in Texas) and nine suspect cases. All seven 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.

Because there are human cases associated with an animal influenza virus, and because of the geographical spread of multiple community outbreaks, plus the somewhat unusual age groups affected, these events are of high concern.

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.

The World Health Organization has been in constant contact with the health authorities in the United States, Mexico and Canada in order to better understand the risk which these ILI events pose. WHO (and PAHO) is sending missions of experts to Mexico to work with health authorities there. It is helping its Member States to increase field epidemiology activities, laboratory diagnosis and clinical management. Moreover, WHO's partners in the Global Alert and Response Network have been alerted and are ready to assist as requested by the Member States.

WHO acknowledges the United States and Mexico for their proactive reporting and their collaboration with WHO and will continue to work with Member States to further characterize the outbreak.