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Increase Seen in H3N2v Influenza, Infections Linked to Contact with Swine

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September 10, 2012

A recent jump in the number of H3N2v influenza (commonly called swine flu) cases in humans, particularly in the Midwest, has prompted the Centers for Disease Control and Prevention (CDC) to begin updating the number of cases each week and alerting health care providers to the growing number. While just twelve human cases of the current circulating strain known as influenza A H3N2 variant virus (H3N2v) were reported between August and December 2011, the number has increased since then. From July 12 through September 7, 2012, a total of 297 infections have been reported along with 16 hospitalizations and 1 H3N2v-associated death. The twelve states where the virus has been detected include Hawaii, Illinois, Indiana, Iowa, Maine, Maryland, Michigan, Minnesota, Ohio, Pennsylvania, West Virginia, and Wisconsin.

(For the most recent numbers, see the CDC's Case Count: Detected U.S. Human Infections with H3N2v by State since August 2011.)

H3N2v influenza is a common respiratory infection found in pigs caused by a type of influenza virus. The current strain that is circulating within communities has a gene called the matrix (M) gene from the H1N1virus that caused the 2009 pandemic in humans. It's the presence of this M gene in the genetic material of the influenza A H3N2v virus that has epidemiologists a bit concerned, according to the CDC, because it may increase transmissibility of the virus from pigs to people and among people.

In order to try to reduce the risk of a severe outbreak, the CDC is asking health professionals who see patients with flu-like illness and recent contact with pigs to get a lab specimen from a patient's nose and/or throat for testing. The CDC urges caution when using the commercially available rapid influenza tests as they may not be sensitive enough to detect the current circulating influenza and may miss some cases. A negative rapid test could be a false negative. If suspicion of H3N2v influenza remains high in a patient with a negative result, particularly if the individual is at high risk of serious illness, then the health care provider should contact their local or state public health department to arrange for follow-up testing.

The CDC requires that all positive results be confirmed at its labs. The test needed for confirmation is a molecular-based assay called real-time polymerase chain reaction or RT-PCR. Many state labs have this technology, which is why the CDC is allowing states to report their own case numbers, but all samples must still be confirmed by the CDC lab as well. Early reporting from states will help in the effort to have more real-time data on the number and location of cases.

So far, the virus has been passed mainly among pigs, and most of the cases among humans have been in children with direct contact with pigs on farms or at county fairs, now in full swing in many parts of the country. The virus cannot be transmitted by eating pork, according to the CDC. Symptoms of the virus are largely mild and similar to seasonal flu, including sore throat, fever and runny nose, according to Thomas Friedan, MD, MPH, and director of the CDC, who spoke to reporters in a telebriefing to announce the decision to release weekly case reports.

"We expect the number of H3N2v cases to rise since this virus has been found in pigs in a number of U.S. states per the USDA and there is so much interaction between people and pigs in fair settings at this time of year, " says Joseph Bresee, PHD, chief of the Epidemiology and Prevention Branch in CDC's Influenza Division. "The good news," said Bresee, "is that the main risk factor for H3N2v virus infection continues to be exposure to pigs. This H3N2v virus is not spreading readily from person-to-person and illness so far has been similar to seasonal flu."

As with other viral infections, influenza poses a greater potential risk of severe illness, including ear infections, sinus infections, pneumonia, bronchitis, and even death for those who are very young, very old or who have weakened immune systems. Influenza viruses are thought to spread from infected pigs to humans in the same way that seasonal influenza viruses spread between people: through infected droplets created when an infected pig coughs or sneezes and the droplets land on surfaces then touched by humans, or in a human's nose or mouth, or by inhaling dust that has influenza virus.

Many precautions used to prevent the spread of seasonal flu pertain to H3N2v influenza:

  • Cover your mouth and nose when you sneeze or cough, using a tissue and throw the tissue away.
  • Wash your hands with soap and water often and especially after sneezing or coughing; alcohol-based hand sanitizers are an acceptable alternative.
  • Avoid close contact with people who are ill.
  • Stay home if you are sick.

Additional precautions apply to H3N2v influenza:

  • People who may be more susceptible to serious illness, such as young children, the elderly or people with weakened immune systems, should consider avoiding fairs that include pigs or pens or barns that house pigs.
  • Wash hands with soap and water after coming into contact with animals, especially pigs.
  • Don't eat, drink or touch your face or mouth while in animal areas.
  • If you have pigs, call a veterinarian for those who appear to be ill and avoid close contact with the animals.

As for treatment, antiviral drugs used for seasonal flu are also effective for the circulating H3N2v influenza if used within 48 hours of becoming ill, says the CDC. Means of prevention are on the horizon as vaccine companies are set to begin clinical trials on a vaccine against the circulating H3N2v virus, according to Tom Skinner, a CDC spokesman.

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Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used. To access online sources, copy and paste the URL into your browser.

(August 3, 2012) CDC Telebriefing: Influenza A (H3N2) Variant Virus. Available online at through Accessed August 2012.

(August 17, 2012) CDC MMWR. Evaluation of Rapid Influenza Diagnostic Tests for Influenza A (H3N2)v Virus and Updated Case Count — United States, 2012. Available online at through Accessed August 2012.

(May 6, 2011) CDC MMWR. Compendium of Measures to Prevent Disease Associated with Animals in Public Settings, 2011. Available online at through Accessed August 2012.

(August 9, 2012) Mike Stobbe. CDC: 158 cases of new swine flu strain from pigs. Associated Press. Available online at through Accessed August 2012.

(August 10, 2012) Todd Neale. Rapid Tests May Miss New Swine Flu. Medpage Today. Available online at through Accessed August 2012.

(January 5, 2012) Centers for Disease Control and Prevention. Key Facts about Swine Influenza (Swine Flu) in Pigs. Available online at through Accessed August 2012.

(August 20, 2012) Centers for Disease Control and Prevention. Information on Influenza A (H3N2) Variant Viruses ("H3N2v"). Available online at through Accessed August 2012.

(December 23, 20122) Centers for Disease Control and Prevention. Interim Guidance on Specimen Collection, Processing, and Testing for Patients with Suspect Influenza A(H3N2)v Virus Infection. Available online at through Accessed August 2012.

(August 17, 2012) Centers for Disease Control and Prevention. More H3N2v Cases Reported, Still Linked to Pig Exposure. Available online at through Accessed August 2012.

(June 29, 2011) Centers for Disease Control and Prevention. People at High Risk of Developing Flu–Related Complications. Available onlien at through Accessed August 2012.

(August 7, 2012) Centers for Disease Control and Prevention. Fact Sheet: Protect Yourself Against H3N2v. Available online at through Accessed August 2012.