ID Consult

Summer flu? Think variant swine influenza virus infection


 

Two children presented with influenza, and both recovered without the need for hospitalization. This scenario would fail to pique the interest of any pediatrician in January. But what about when it happens in July?

In early August, public health authorities in Ohio announced that two children had tested positive for the variant swine influenza virus H3N2v. Both children had direct contact with pigs at the Clark County Fair in late July. Along with a handful of cases diagnosed in Michigan, these represent the first H3N2v cases in the United States in 2016.

Dr. Kristina K. Bryant

Dr. Kristina A. Bryant

Influenza viruses that normally circulate in swine are designated as “variant” when they infect humans. According to the Centers for Disease Control and Prevention (CDC), human infections with H1N1v, H1N2v, and H3N2v have been identified in the United States. Influenza A H3N2v viruses carrying the matrix gene from the 2009 H1N1 pandemic virus were first detected in pigs in 2010, and in people in the summer of 2011. Since that time, 357 human cases have been reported from 14 states, with nearly 75% occurring in Indiana and Ohio. Most infections occurred after prolonged exposure to pigs at agricultural fairs.

Fortunately, most H3N2v infections have been mild: Since July 2012, only 21 individuals have required hospitalization and a single case resulted in death. Notably, though, many of the hospitalizations involved children.

On Aug. 15, the Centers for Disease Control and Prevention released Interim Guidance for Clinicians on Human Infections with Variant Influenza Viruses.

Because variant virus infection is indistinguishable from seasonal influenza or any other virus that cause influenzalike illness (think fever, cough, sore throat), physicians and other frontline providers need to maintain an index of suspicion. The key is eliciting a history of swine exposure in the week before illness onset. Practically, this means asking about direct contact with pigs, indirect contact with pigs, or close contact with an ill person who has had contact with pigs. Kudos to the astute clinicians in Ohio who thought to send the appropriate influenza testing in July.

When variant influenza virus is suspected, a nasopharyngeal swab or aspirate should be obtained for testing at a state public health lab or the CDC. Rapid antigen tests for influenza may be falsely negative in the setting of H3N2v infection, just as they may be with seasonal influenza infection. Molecular tests such as reverse transcription polymerase chain reaction (RT-PCR) are likely more sensitive, but cannot distinguish variant influenza A viruses from seasonal influenza A viruses.

The Kentucky State Fair opened on Aug. 18, making the CDC guidance especially timely for health care providers in my area. I called a friend who is a pediatric emergency medicine physician to ask if she and her colleagues were routinely screening patients for encounters of the porcine kind.

“For example, are you asking, ‘Have you been showing, raising or feeding swine? Have you been to the pig barn at the fair?’ ”

When my friend quit laughing, I confessed to her that I had not been doing that routinely either. The exposure history is often the most interesting part of the infectious disease evaluation and in the last month, I’ve asked about exposure to sheep (a risk factor for Q fever), exposure to chickens (a risk factor for Salmonella infections), and exposure to beaver dams (a risk factor for blastomycosis). But I’ve not asked about exposure to pigs.

“The emergency medicine approach is to avoid a lot of viral diagnostic testing unless it is going to impact management,” she said. “Tell me how this changes management of my patient.”

From the patient perspective, making a presumptive diagnosis of H3N2v infection would open the door to empiric treatment with antivirals, at least for individuals who are hospitalized, have severe or progressive disease, or who at high risk for complications of influenza. Neuraminidase inhibitors, including oral oseltamivir, inhaled zanamivir, and intravenous peramivir, can be used for treatment of H3N2v infections.

From a societal perspective, making the diagnosis gives public health experts the opportunity to investigate and potentially prevent further infections by isolating sick pigs. Human to human transmission of H3N2v is rare, but has occasionally occurred in households and in one instance, a child care setting. Careful surveillance of each swine flu case in a human is important to exclude the possibility that the virus has developed the ability to spread efficiently from person to person, creating the risk for an epidemic.

Seasonal influenza vaccine does not prevent infection with variant viruses, so avoidance is key. Those at high risk for complications from influenza infection, including children younger than 5 years of age and those with asthma, diabetes, heart disease, immunocompromised conditions, and neurologic or neurodevelopmental disorders, are urged to avoid pigs and swine barns when visiting fairs where the animals are present. Everyone else needs to follow common sense measures to prevent the spread of infection.

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