WHO: Influenza B gaining foothold in more countries

Mar 12, 2010 (CIDRAP News) – Though pandemic flu is circulating at low levels in many parts of the world, Thailand and some West African nations are reporting increased activity, and the virus is being edged out by influenza B in China and other Asian regions, with signs of westward spread, the World Health Organization (WHO) said today.

Alhough surveillance information is limited in the West Africa area, the WHO said community transmission is occurring without any sign of a peak. Senegal, Cote D'Ivoire, and Rwanda are among the countries reporting increased flu activity, but so far little is known about the clinical patterns of the illnesses there, the group said in its weekly update.

Pandemic flu activity persists throughout South and Southeast Asia, with Thailand reporting the region's highest level of activity since the middle of January. Though about 25% of Thai patients with flu-like illness are testing positive for influenza, the WHO said the increase is well below the peak the country experienced last summer.

Mongolia reported a sharp increase in flu activity, attributed mostly to an increase in influenza B activity. Though pandemic flu activity declined in neighboring China, influenza B activity continued to increase.

Influenza B virus circulation seems to be moving westward, with the Russian Federation and Sweden now reporting that the virus is cocirculating with or dominating the pandemic virus. Iran also reported that, although flu activity is low, all recent detections have been influenza B.

Some countries reported increases in respiratory disease activity, though not all of it is thought to be pandemic flu, the WHO report said. For example, Afghanistan reported increased respiratory disease with a moderate impact on the country's health system. Bangladesh, Nicaragua, Honduras, and Brazil also reported similar increases.

The United States hasn't seen a spike in influenza B detections, though the US Centers for Disease Control and Prevention (CDC) said today that it is circulating at low levels. The nation saw a surge of influenza B cases near the end of the 2008-2009 flu season, which was problematic, because the B strain that was circulating didn't match the one contained in the seasonal flu vaccine. So far the influenza B strains identified at US labs this flu season are a good match for the current seasonal flu vaccine.

Vincent Racaniello, PhD, a virologist at Columbia University who writes Virology Blog, told CIDRAP News that the influenza B pattern the WHO is reporting is typical. He said the two main lineages, B/Victoria-like and B/Lee-like, have been cocirculating for 25 years, with changing patterns of prevalence and geographic distribution.

"There are frequent bottleneck years during which prevalence of B strains is low; this usually corresponds to high prevalence of influenza A strains," he said. "Once the bottleneck is relieved, there are usually changes in the prevalence of the two B lineages."

The current spread of influenza B may be a reflection of waning pandemic H1N1 activity that has occurred later in the Northern Hemisphere's flu season, Racaniello said.

Experts have noted that influenza B viruses in general cause less severe disease than influenza A and are associated with smaller disease clusters and illnesses in younger people, though hospitalizations and deaths from influenza B infections are seen in all age-groups.

In an accompanying update on oseltamivir-resistant pandemic H1N1 viruses, the WHO said it received no new reports of cases last week and said all have the H275Y substitution and are sensitive to zanamivir. The organization characterized the cases are sporadic, with rare onward transmission.

See also:

Mar 12 WHO global flu update

Mar 23, 2009, CIDRAP News story "TypeB viruses rise as overall flu activity declines"

Mar 12 WHO antiviral resistance update

CDC weekly Fluviewreport

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