Mexico's first swine flu case surfaced in mid-March

May 1, 2009 (CIDRAP News) – The first person in Mexico to have a confirmed case of swine influenza H1N1 fell ill on Mar 17, only 11 days before the first case on the American side of the border, the US Centers for Disease Control and Prevention (CDC) reported yesterday in a profile of Mexico's flu outbreak.

The report also suggests that the relatively high proportion of severe cases reported in Mexico mainly reflects the reality that sicker patients are more likely to receive medical care and testing.

In an MMWR Dispatch, the CDC said Mexico had a total of 1,918 suspected swine flu cases between Mar 1 and Apr 30, including 286 probable cases and 97 confirmed. A total of 84 deaths were reported in that period, the report said, but only 7 deaths were in confirmed cases.

The latest World Health Organization update on the global swine flu situation, issued this afternoon, says Mexico has had 156 cases with 9 deaths.

The first Mexican patient who had a confirmed case said symptoms began Mar 17. The first US case was in a 9-year-old girl in Imperial County, Calif., who got sick with a fever on Mar 28, the CDC reported earlier. The second US case involved a 10-year-old boy in neighboring San Diego County who fell ill on Mar 30.

Mexico first notified international health officials of a potentially worrisome outbreak on Apr 12, when it told the Pan American Health Organization (PAHO) about cases in a small community in Veracruz state, the CDC report says.

"On Apr 17, a case of atypical pneumonia in Oaxaca state prompted enhanced surveillance throughout Mexico," the CDC said. Six days after that, the government informed the PAHO that it had several severe respiratory illness cases known to involve the new virus.

Detailed information was available on only 24 of the confirmed case-patients at the time of the CDC report. They ranged in age from less than 1 year to 59 years; 4 patients were between 5 and 19 years, 9 were between 19 and 39, and 6 were between 40 and 59.

Suspected and probable cases were reported in all 31 Mexican states and the Federal District, the report says. "Several findings indicate that transmission in Mexico involved person-to-person spread with multiple generations of transmission."

Since most cases elsewhere have been mild, the severe cases in Mexico have caused intense speculation and concern. The CDC report does not explain the mystery, but it says it appears that the new virus is widespread and that less severe illness is common.

"To date, case-finding in Mexico has focused on patients seeking care in hospitals, and the selection of cases for laboratory testing has focused on patients with severe disease," the article states. "Therefore, a large number of undetected cases of illness might exist in persons seeking care in primary care settings or not seeking care at all."

At the same time, the CDC says the clinical spectrum of the new flu in Mexico "is not well characterized" and further investigation is urgently needed.

CDC. Outbreak of swine-origin influenza A (H1N1) virus infection—Mexico, March-April 2009. MMWR Dispatch 2009 Apr 30;58 [Full text]

See also:

Apr 21 CIDRAP News story "Human swine flu cases with unique strain raise concern"

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