December 28, 2011 — The number of reported cases of a novel swine influenza virus has risen to 12 since July, encompassing 5 states, according to the US Centers for Disease Control and Prevention (CDC). The virus includes a gene from the human pandemic strain and affects mostly children.
The agency is taking the influenza newcomer seriously, urging public health departments, hospitals, and clinicians engaged in influenza surveillance to consider the possibility of the virus in patients presenting with influenza-like illness (ILI).
The infections in question involve a variant of the A(H3N2) virus that circulates among pigs. It contains a gene from the pandemic 2009 influenza A(H1N1) virus that codes for matrix proteins found in the viral shell.
The novel virus is worrisome enough that the CDC, the World Health Organization, and the World Organisation for Animal Health have dubbed it A(H3N2)v. The "v," which stands for "variant," distinguishes the novel virus from the seasonal A(H3N2) virus. The 2011-2012 seasonal influenza vaccine is designed to protect against this strain, the 2009 pandemic strain, and an influenza B strain.
In 3 of the 5 states where the A(H3N2)v virus has surfaced — Indiana, Pennsylvania, and Maine — patients became infected after direct or indirect contact with pigs. In Iowa and West Virginia, the other 2 states, the evidence suggests that the virus spread from human to human on a limited basis.
The CDC has not found evidence of sustained human-to-human transmission of the virus, but "all influenza viruses have the capacity to change, and it's possible this virus may become widespread," the agency states on its Web site.
One seeming bit of good news is that, so far, the A(H3N2)v virus causes illnesses that are generally no worse than those triggered by the seasonal influenza virus. Three of the 12 patients were hospitalized, but they and all the others recovered.
However, there is a bad-news chaser: The seasonal influenza vaccine for 2011-2012 is unlikely to protect people from the A(H3N2)v virus, the CDC reports. In addition, limited serologic studies indicate that young children have little preexisting immunity, although older children and adults may have limited immunity.
New Flu Question: "Have You Been Around Pigs?"
The CDC reported the first 2 cases of the A(H3N2)v influenza — both involving children under 5 years of age — on September 2 in its Morbidity and Mortality Weekly Report (MMWR). In 1 case, a girl in Pennsylvania came down with the infection after exposure to pigs at an agricultural fair on August 16. Direct exposure to pigs was not a factor in the other case, involving a boy in Indiana, but his caretaker had come into direct contact with asymptomatic pigs before the onset of the boy's illness in July.
The first likely instance of limited human-to-human transmission of the A(H3N2)v virus was described in an MMWR report published December 2. The virus was discovered in 3 children in 2 counties in Iowa who had been in contact with each other (2 were brothers). None of them had had any recent exposure to pigs.
On December 23, the CDC published another MMWR report that brought the infection count up to 12 and described the 3 latest cases: another in Indiana and 2 cases in West Virginia. In Indiana, a man who had worked with pigs was hospitalized for 4 days in October. In contrast, 2 children under 5 years of age in West Virginia were infected with the A(H3N2)v virus without having come into contact with pigs or traveling recently. Both attended the same daycare center.
The authors of the MMWR report write that it is unlikely that the first child to be infected transmitted the virus to the second one, given a gap of more than 10 days between their onset of symptoms. Nevertheless, the authors attribute those 2 cases to "apparent limited human-to-human transmission" of the virus, which conceivably could have spread from others at the daycare center.
These different epidemiologic scenarios figure into CDC advice to clinicians and public health agencies about the A(H3N2)v virus. Clinicians should consider the possibility of the novel virus when patients presenting with febrile respiratory illness have recently been near pigs. They also should keep A(H3N2) as a diagnostic option even when there has been no human–pig encounter, particularly for young children in states where A(H3N2)v infections have been reported. If they suspect an A(H3N2)v infection, they should obtain a nasopharyngeal swab, place it in a viral transport medium, and contact their local or state health department to have it tested.
Likewise, the CDC recommends that state and local health departments should consider collecting more specimens from patients presenting with ILI in 3 high-priority areas:
•ILI outbreaks, especially among children in school and childcare settings;
•Unusual or severe presentations of ILI, especially among children; and
•Medically attended ILI and acute respiratory illness in children under 18 years of age.
A Second Novel Influenza Virus Is on the CDC's Radar
In addition to the A(H3N2)v virus, another novel version of swine influenza is on the CDC's radar. In a recent post on its Web site, the agency reported the discovery of an A(H1N1) virus with genes from avian, swine, and human influenza genes. Like A(H3N2)v, it features the matrix gene from the pandemic 2009 influenza A(H1N1) virus, which probably was transmitted from humans to swine after the outbreak of the 2009-2010 pandemic. The novel virus, called A(H1N1)v, was detected in a Wisconsin man who had worked with pigs before getting sick. The CDC did not identify any human-to-human transmission of this virus.
As with A(H3N2)v, the current formulation of seasonal flu vaccine probably will not protect people from A(H1N1)v. Laboratory tests show that A(H1N1)v is susceptible to the antiviral medications oseltamivir (Tamiflu, Roche) and zanamivir (Relenza, GlaxoSmithKline). Those 2 medications also should prove effective against A(H3N2)v, according to the CDC.
More information on the A(H1N1)v virus is available at the CDC Web site.http://www.medscape.com/viewarticle/756148