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Texas Tamiflu Resistance Cluster Confirmed
Recombinomics Commentary 21:19
August 31, 2009


Influenza A/H1N1 (novel) 5 2 (40%) 0 (0) 5 5 (100%)

The above description is from the week 32 MMWR surveillance report from the Texas Department of State Health Services. The number of H1N1 isolates tested for oseltamivir resistance rose for 3 to 5, and both new isolates were resistant (presumably H274Y). It is likely that these two cases are those on the Mexican border which were disclosed and denied earlier this month.


Although both cases were on the Mexican border (see map), they were at opposite ends of the Texas border, raising concerns that resistance in Texas was widespread. Indeed, as noted above, the rate to date in Texas is 40% (2/5). Initial reports speculated that the resistance was linked to cross-border access to Tamiflu in Mexico. However, the US border with Mexico extends well beyond Texas, so cross border traffic is not limited to Texas, or the two locations described.

The clustering of these cases in Texas, but at widely separated locations is similar to the furst two pandemic H1N1 cases in southern California. Both were reported at the same time, but were not linked, raising concern that the H1N1 Tamiflu resistance was widespread.


Two cases in Texas in the same report raise similar concerns. Texas has only tested five pandemic H1N1 samples, so the two positives clearly dictate more aggressive testing is warranted.


There has been a recent resurgence of cases in Mexico, and these two resistant cases raise concerns that the resurgence is linked to resistance, which may be circulating as a minor species or may be evolutionarily fit, as suggested for the two isolates from northern California.

Release of the sequences from the Texas isolates would be useful, as would more testing of cases pre and post oseltamivir treatment.


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Widespread Tamiflu Resistance in Texas?
Recombinomics Commentary 11:28
September 1, 2009


"We have found resistance to Tamiflu on the border. We have observed some cases, few to be sure, in El Paso and close to McAllen, Texas," said Maria Teresa Cerqueira, head of the local PAHO office.

Cerquiera said one patient diagnosed with a Tamiflu-resistant strain had been treated with Zanamivir -- an anti-viral made by GlaxoSmithKline -- and another was given no alternative medication. Both survived.

The above comments are from the widely reported AFP report on oseltamivir resistance in Texas. The distance between the two border locations (see map) raised concerns that H274Y was widespread. The events that followed did not decrease those concerns. The original report was followed by a claim that the source was misquoted, and the original source was a physician in Columbia. However, the existence of the two patients described above was not questioned. Moreover, comments were made on limited testing, which seemed to deny confirmations, rather than the existence of the resistance. In addition, the resistance was said to be linked to cross border traffic.

However, these claims still left open the strong possibility that resistance was widespread. Testing in Texas was limited. Prior to the week 32 report, Texas had reported antiviral results on three pandemic H1N1 isolates. Consequently, the two new cases raised the resistance rate to 40%. However, those numbers were based on the week 32 report, which was delayed. Last week the CDC released their week 33 report, which included 3 new cases of resistance, and many assumed the three cases were the hospitalized patient in northern California, as well as the two cases described previously along the Texas / Mexican border.

The delay in the report from Texas raises concerns that there are more cases. The above quote, which was made almost a month ago, suggested that the number of examples of resistance may have been higher than the two cases that were detailed, and the only two new cases reported last month were both resistant.

The clustering in Texas was similar to the cluster in Seattle, which involved two immuno-compromised patients who were infected by closely related H1N1, and the two cases in the summer camp in North Carolina, which were also reported simultaneously. These cases that are clustered in time and space suggest that the resistance is not due to independent random mutations which are selected by Tamiflu treatment or prophylaxis. Instead, the clustering suggests that the H274Y has already been acquired, and the treatment simply increases the H274Y to the detection level.

The increased detection rate raises concerns that the H274Y will become dominant. Thus, further testing is warranted, yet Texas has only tested five pandemic H1N1 isolates, even though H1N1 is widespread in Texas (see map), and will dramatically spread in upcoming weeks due to the start of a new school year.

The level of testing of school outbreaks has declined dramatically, even as the number of cases and Tamiflu resistance increased. Students provide a vehicle for rapid spread, and the lack of resistance testing of Texas isolates, where 40% of tested H1N1 has H274Y, remains a cause for concern.


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