Texas health investigators have now linked two infections in a severe influenza-like illness (ILI) cluster to the 2009 H1N1 virus, and the a dramatic flu activity jump in the state has vaulted Texas and three other southern states into the "widespread" category for the first time this season.
Earlier this week, the Montgomery County Health District (MCHD), located near Houston, reported a severe ILI cluster that put eight people in a local hospital, killing four of them. Initial tests were negative for flu, which raised speculation about what pathogen was fueling the outbreak.
Local media outlets noted that patients in the cluster were middle-aged adults who were reportedly previously healthy, which also ramped up the level of concern.
Yesterday, however, the MCHD said further tests now show that two of the survivors were infected with the 2009 H1N1 virus and that further tests were under way to pinpoint the cause of the other infections.
Hospital cluster reflects intense flu in Texas
The 2009 H1N1 cases in Montgomery County come at the time of a steep rise in flu in Texas, along with similar spikes in Louisiana, Mississippi, and Alabama, according to the latest update today from the US Centers for Disease Control and Prevention (CDC).
Most of the flu activity in those states, and the nation as a whole, is linked to the former pandemic virus, which is now considered a seasonal flu strain and has been included in all formulations of flu vaccines since the 2009-10 flu season.
Dominant activity from the H1N1 strain stands in stark contrast from last year, when the H3N2 virus was most common and was marked by an early, moderately severe season.
The Texas Department of State Health Services (TDSHS) said today in its latest flu surveillance update that flu activity is increasing steadily in the state, with all of its regions reporting cases, with widespread, high-intensity activity. Of respiratory samples that tested positive for flu, nearly all were influenza A, and of the subtyped influenza A samples, 96.1% were the 2009 H1N1 virus.
Testing for other respiratory viruses in Texas last week also found high levels of rhinovirus and respiratory syncytial virus (RSV).
The TDSHS said two clusters of ILI have been detected in hospitalized patients from the state's Houston region. Tests revealed the 2009 H1N1 virus from one cluster, and the cause in the second one is still undetermined.
In a separate statement, the TDSHS said though recent spikes in flu activity aren't unusual, it issued flu testing and treatment today to doctors and encouraged the public to get vaccinated against influenza. It urged health providers to consider antiviral treatment, even if initial rapid tests are negative for flu. It added that a negative test does not rule out flu in a patient with suspected illness.
Subtle differences in H1N1 impact
Michael Jhung, MD, MPH, a medical epidemiologist in the CDC's influenza division, told CIDRAP News that the epidemiology of 2009 H1N1 infections is a little different than other seasonal strains, and that for a variety of reasons it has a bigger impact on younger adults and older children.
If the 2009 H1N1 virus continues as the dominant strain, it's possible that health providers will see more impact on those groups, but the situation wouldn't be similar to the pandemic months, because the virus is already included in the seasonal flu vaccine, he said. "It's not the start of another pandemic."
The very young and elderly people, as well as those with underlying health conditions, are always at increased risk no matter what the flu strain, Jhung said.
Last flu season the H3N2 strain dominated most of the season and exacted a heavy toll on older people. Jhung emphasized that it's too early in the season to predict flu's pattern and impact.
Overall, flu activity in the United States is low, but there are pockets of high activity, such as Texas, and the CDC expects activity to increase in other parts of the country in the coming weeks and months, he said.
Jhung said the overall message remains the same, that influenza can be very serious, and there is still time to be vaccinated.
Steep rise in national indicators
Nationally, all flu markers rose last week, especially the percentage of respiratory specimens that tested positive for flu, which rose from 13.3% to 17.8% from the week before, according to the CDC.
Like Texas, nearly all of the flu viruses tested so far are influenza A, and subtyping shows that 2009 H1N1 virus is the dominant strain.
Two pediatric flu deaths were reported, one of which occurred during the 2012-13 season. The most recent death involved the 2009 H1N1 strain and raises the total so far this season to four.
Texas, Louisiana, Mississippi, and Alabama became the first states to report widespread geographic flu activity this season, and 20 states reported regional spread
The CDC's marker for visits to clinics for ILI also showed high activity for Texas, Louisiana, Mississippi, and Alabama.
Canada reports rising flu and RSV activity
Canada's flu activity indicators also increased sharply last week, according to an update today from the Public Health Agency of Canada (PHAC). The areas of heaviest activity are in the Alberta region that includes Calgary, an area of Ontario that includes Ottawa and Toronto, and part of Quebec.
As in the United States, the 2009 H1N1 virus is also the dominant strain in Canada, accounting for 88% of the subtyped influenza A samples. The PHAC also reported a sharp rise in positive samples for RSV.
Flu levels show little EU increase
Elsewhere, flu activity in Europe remained low, though there are signs that activity is picking up in some countries, according to the latest report from the European Centre for Disease Prevention and Control (ECDC).
Based on reports from 29 countries, Bulgaria is reporting medium-intensity flu activity, with the rest reporting the lowest level. Countries reporting increasing flu trends include Bulgaria, Greece, Ireland, Malta, Poland, and Spain.
Testing over the past several weeks shows that most of the flu viruses detected in European countries are influenza A, with about 56% of the subtyped samples the H3N2 strain and about 44% the 2009 H1N1 strain.
Dec 20 CDC FluView report
Dec 20 TDSHS flu surveillance report
Dec 20 TDSHS news release
Montgomery County Health District Web page
Dec 20 PHAC FluWatch report
Dec 20 ECDC weekly flu surveillance report