MUMBAI, India — A surge in swine flu infections has killed more than 800 people in India and is challenging health workers, who say the virus is harder to treat than the version that caused a global pandemic in 2009.

High-risk patients with the infection now need an average 10 days of treatment with the anti-viral medicine oseltamivir, according to Om Shrivastav, director of the infectious diseases department at Jaslok Hospital in Mumbai. That’s double the length of treatment in previous outbreaks, he estimates.

The H1N1 type of the influenza virus that is spreading in India is similar to the virus that caused the 2009 outbreak that killed more than 18,000 people worldwide. India, whose public spending on health as a percentage of gross domestic product is among the lowest in the world, is racing to procure more antiviral drugs for its emergency stocks. The government is also launching an education campaign for prevention that includes promoting hand hygiene.

Compared to the 2009 outbreak, “this time, we see that it’s become more aggressive,” said Ritesh Gupta, a senior consultant at Fortis Hospital’s diabetes center in New Delhi. “The virus is responding less to therapy. Even the so-called uncomplicated cases are taking a long time for symptomatic improvement.”

The virus is transmitted by infected droplets from coughs and sneezes that are inhaled or picked up from contaminated hands or surfaces. Symptoms include high fevers, sore throat, fatigue and chills. People in the affected pockets of India may lack immunity against the virus and have no vaccination history, said Masato Tashiro, a Tokyo-based director at Japan’s National Institute of Infectious Diseases.

It is also possible that the virus circulating in India has developed resistance to oseltamivir, he said.

“As far as we recognize, significant antigenic changes have not occurred with H1N1 viruses circulating in the world, but my concern is that such changes have occurred in India,” said Tashiro.