H5N1 cases dropped in 2012, but fatality rate rose

Apr 2, 2013 (CIDRAP News) – The world registered fewer human infections with H5N1 avian influenza in 2012, but the fatality rate was a little higher than in previous years, the World Health Organization (WHO) noted in a recent review of the year's data.

Otherwise, the epidemiologic pattern looked generally similar to that of past years, the WHO said in the Mar 29 issue of its Weekly Epidemiological Record.

Thirty-two human H5N1 cases were confirmed last year, as compared with 62 in 2011, 48 in 2010, and 73 in 2009, the WHO reported. There were 11 cases in Egypt in 2012, 9 in Indonesia, 4 in Vietnam, 3 in Cambodia, 3 in Bangladesh, and 2 in China. The virus is considered endemic in poultry in five of those six countries, and a recent study suggested it is endemic in the sixth, Cambodia, as well.

Egypt accounted for much of the decline in cases in 2012, since its 11 cases compared with 39 in 2011.

The case-fatality rate for confirmed cases in 2012 was 62.5% (20 of 32 cases), the report says. That topped the 55% for 2011, 50% in 2010, and 44% in 2009, but it was still close to the overall CFR of 59% (360 of 610) for cases reported to the WHO since 2003.

As in past years, the fatality rate was higher in females than males: 69% (11 of 16) versus 56% (9 of 16), the WHO reported. Since 2003, the CFR has been 64% for females and 53% for males.

The disease hit children and young adults hardest, much as in previous years: 90% of cases (29 of 32) were in people under age 40, and 34% (11 of 32) were in children. With an age range of 6 months to 45 years, the median age was 18 years. The median has varied considerably in recent years: 5 years in 2009, 25 in 2010, and 13 in 2011.

The median age for case-patients in Egypt was very young a few years ago, but it has increased in recent years, reaching 31 in 2012, the report notes.

Eleven of the 32 case-patients in 2012 had been exposed to sick or dead poultry, making that the most common exposure route. Other types of exposures included backyard poultry and visits to live-bird markets. Only one case cluster was reported in 2012, as an Indonesian case early in the year was linked to a cluster from 2011, according to the report.

The three cases in Bangladesh—all of them mild—were found through routine surveillance in bird markets. The report says the finding of mild cases "reinforces concerns that many milder cases of infection occur undetected."

Although recent reviews of serologic studies found little evidence that many H5N1 cases have gone undetected, the frequency of such cases remains uncertain, given the variations in study protocols and uncertainties about serologic responses, the authors say.

The report also notes that 22 candidate vaccine viruses for H5N1 are available, with new ones in development. There is no evidence of increasing resistance to oseltamivir (Tamiflu).

The WHO summary also briefly reviews reports in 2012 of human infections with other flu viruses from animals, all of which were mild. Those included:

  • 309 cases of swine-origin variant H3N2 (H3N2v) in the United States
  • 2 cases of a swine-origin H1N1 virus, one in Canada and one in the United States
  • 3 cases of a swine-origin H1N2 in the United States
  • 2 H7N3 cases in Mexico linked to a highly pathogenic H7N3 outbreak in poultry

All of the H3N2v viruses and the swine-origin H1N1 and H1N2 isolates included the M gene from the 2009 pandemic H1N1 virus, probably because of reassortment of swine viruses with the pandemic strain, the report notes. The role of the M gene remains uncertain, but some evidence suggests it may increase the ability of a swine virus to jump to humans.

See also:

WHO Weekly Epidemiological Record for Mar 29

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