CDC updates novel H3N2 info for clinicians

Aug 10, 2012 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) today reported that it has received 153 reports of novel H3N2 infections (H3N2v) since illnesses started surfacing in the middle of July, nearly all of them in people who had contact with pigs or were around pigs at fairs.

In another development, the CDC issued new information on H3N2v for clinicians, including an evaluation of rapid influenza tests that found their sensitivity varies greatly, and the agency urged clinicians not to use negative results as the basis for treatment.

The CDC's latest total includes seven new cases that the Indiana State Department of Health reported yesterday, plus one new case from Ohio. Meanwhile, the Ohio Department of Health (ODH) today announced six more cases, which would push its total to 36 and the national total to 158.

In a joint statement with the Ohio Department of Agriculture (ODA), the ODH said the two departments are launching an outreach plan to boost H3N2v awareness at county fairs, including visits at each fair and in-person contact with swine exhibitors. The ODA has established guidance for fair boards, and the agencies have produced a YouTube video for fairgoers and exhibitors.

At a press briefing yesterday, the CDC said the surge in reports involving the swine-origin H3N2 virus that contains the matrix gene from the 2009 H1N1 virus reflects a real rise in the number of cases, as well as a change in how the cases are reported.

In an early-release edition of Morbidity and Mortality Weekly Report (MMWR) today, the CDC said state laboratories have been using the CDC Flu rT-PCR Dx Panel to detect the novel virus, and their positive findings were initially considered inconclusive until confirmed as H3N2v by the CDC. However, on Aug 7 the CDC updated the results interpretation of the test for public health labs, which allows them to report the presumptive H3N2v-positives as confirmed cases.

The CDC said 128 (93%) of the patients are younger than age 18 and the median age is 7 years. Though the infections have predominantly occurred in kids, the fact that 10 of the illnesses involved adults underscores the point that people of any age can be infected, the CDC said. It urged people with underlying health conditions that pose a high risk of complications from flu to avoid swine barns this summer, especially if sick pigs have been identified.

In initial tests on seven rapid flu diagnostic tests, four detected all of the H3N2v viruses, the CDC reported. It said the findings reinforce that negative test results shouldn't be considered conclusive for ruling out H3N2v infection. CDC added that more data are needed on the performance of the rapid tests but that both positive and negative findings should be interpreted in the broader context of circulating strains in the community, level of clinical suspicion, disease severity, and patients' underlying risk factors.

The CDC said that although human-to-human transmission hasn't been seen in this summer's H3N2v infections, limited spread was suspected in a few cases that were detected in households and in a child-care setting in 2011. It urged state health officials to enhance surveillance, especially in areas where infections have already been confirmed.

"The initial goal of enhanced surveillance is to detect the source and geographic spread of these viruses, but once cases are detected, particular emphasis should be placed on detection of ongoing transmission within the community through investigation of close contacts of patients with confirmed cases," the CDC wrote.

In a separate information release for clinicians, the CDC said the H3N2v infections can't be distinguished clinically from seasonal flu infections and other flulike illnesses. The key to identifying a suspected H3N2v infection is to tease out a link to swine exposure in the week before illness onset, including direct contact with pigs, indirect exposure (such as walking through a swine barn at a fair), and close contact with a sick person who had recent swine exposure.

The virus has been circulating in US pigs since 2011, has been detected in many states, and appears to be circulating widely in the nation's pig populations, the CDC said.

Healthcare workers have noted some instances of asthma exacerbation in patients infected with H3N2v, and the CDC warned that people with underlying medical conditions are at greater risk for complications, as with seasonal flu.

Limited serologic data show that children younger than 10 lack immunity to the virus and are most susceptible, and that while other age-groups may have cross-protective antibodies, some persons are still susceptible to the new virus, according to the CDC.

Patients with flu-like illness and an epidemiologic link to swine exposure should be considered probable H3N2v cases and local health departments should be notified as soon as possible for additional testing, the agency said.

Meanwhile, overall US flu markers remained at low summer levels, according to the CDC's weekly flu report, which made note of the new H3N2v infections.

See also:

Aug 10 ODH/ODA statement

Aug 10 MMWR report

Aug 10 CDC interim information for clinicians on H3N2v infections

Aug 10 CDC weekly influenza update

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