Summer Camp Mates Develop Drug-Resistant Pandemic FluDownload...

Summer Camp Mates Develop Drug-Resistant Pandemic Flu
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By Michael Smith, North American Correspondent, MedPage Today
Published: September 10, 2009
Reviewed by Zalman S. Agus, MD; Emeritus Professor
University of Pennsylvania School of Medicine. Earn CME/CE credit
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Two cabin-mates at a North Carolina summer camp developed drug-resistant pandemic H1N1 influenza this summer despite antiviral prophylaxis, the CDC said.

The cases are the first reports of oseltamivir (Tamiflu) resistance in symptomatic patients who had close contacts with a confirmed case, the agency said in the Sept. 11 issue of Morbidity and Mortality Weekly Report.

And, the agency said, the two cases highlight one of the dangers of mass use of antiviral drugs to prevent flu: the possibility of generating resistance.

The pandemic H1N1 strain is susceptible to the neuraminidase inhibitors oseltamivir and zanamivir (Relenza), but resistant to the other class of antivirals, the adamantanes.

Despite that susceptibility, there have been a handful of oseltamivir-resistant cases around the world, including nine in the U.S., the CDC said.

In this report, the first adolescent girl to become ill had been given oseltamivir prophylaxis at 75 milligrams daily from June 26 through July 5, although she had not been exposed to an ill person.

The prophylaxis was part of a mass chemoprophylaxis at the summer camp that saw 418 campers and 189 staff members get 10 days of either oseltamivir or zanamivir.

After finishing her initial course of oseltamivir, the girl was exposed to an ill cabin-mate (dubbed patient C) and started a second 10-day course. She began experiencing symptoms the next day -- cough and headache without fever, which progressed to chills, worsening headache, and loose stools.

Despite the symptoms, the oseltamivir dose was not increased to a therapeutic level.

The camp physician -- noting that the girl had become ill while taking the drug -- took a nasopharyngeal swab specimen which was confirmed to be the pandemic strain of flu.

The isolate had two mutations in its neuraminidase gene, including one known to cause oseltamivir resistance.

The second adolescent girl started oseltamivir prophylaxis at 75 milligrams daily after she was exposed to patient C. Four days later, during a two-day home visit, she developed a fever or 101.9ºF (or 38.8ºC), sore throat, and cough.

She returned to camp with fever, headache, cough, malaise, and myalgias. A rapid influenza detection test the same day was positive for influenza A. The camp doctors stopped oseltamivir and started zanamivir treatment, at two five-milligram inhalations, twice daily.

Testing of an isolate, obtained by nasopharyngeal swab, confirmed the pandemic flu and found the same two mutations, the CDC said.

The agency said there are several possible explanations for the events:

•The first girl might have transmitted oseltamivir-resistant virus to her cabin-mate, since the onset of illness in the second case was four days later than the first.
•Both girls might have contracted the resistant strain from patient C or another ill person.
•Both might have developed resistance independently.

The agency said its updated recommendations on the use of antivirals during the H1N1 pandemic now suggest that post-exposure chemoprophylaxis be reserved for people at higher risk for flu-related complications who have had contact with someone likely to have been infected.

The agency is also urging an "emphasis on early treatment once a patient has developed symptoms, rather than chemoprophylaxis" in order to reduce opportunities for resistance to develop.

"Chemoprophylaxis should not be used for prevention of illness among healthy persons after exposures in community settings," the CDC said.

The analysis was supported by the CDC. There were no disclosures.