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Novel and Variant Influenza Viruses: Information for Healthcare Professionals


Background

Seasonal influenza viruses that infect humans each year and are passed from person-to-person include influenza B viruses (which infect only humans and not other animals) and two influenza A subtypes - H3N2 and H1N1 viruses.

A novel influenza virus has a subtype that is different from currently circulating human influenza H1 and H3 viruses. Novel subtypes include, but are not limited to H2, H5, H7, and H9 subtypes. Influenza H1 and H3 subtypes originating from a non-human species or from genetic reassortment between animal and human viruses are also novel subtypes.

Occasionally, influenza A viruses that typically infect other animals (such as birds and pigs) are transmitted to humans. Sometimes, human infections with novel or variant influenza viruses occur because of the close contact between humans and animals. Other times, the infections occur because of changes in the influenza virus.

Human infections from novel viruses can range in severity from mild respiratory illness to severe illness with increased mortality.

Swine Flu Viruses

Influenza A viruses that regularly circulate in pigs are called swine flu viruses. While swine flu viruses do not typically infect humans; occasional human infections have occurred. When the infection happens in humans, the viruses are called variant viruses. Variant viruses are a subset of novel influenza viruses. The influenza A (H3N2) variant virus, or H3N2v, was first identified in pigs in the United States in 2010. In 2011, twelve cases of human infection with H3N2v occurred in five different states (not including Virginia). In 2012 and 2013, additional cases occurred, including one in 2013 in an out-of-state resident with contact to swine in Virginia.

H3N2v Key Points:

  • The majority of H3N2v cases in the United States have reported exposure to swine at agricultural fairs.
  • Illnesses have been compatible in severity with seasonal influenza virus infection.
  • Commercially available rapid influenza diagnostic tests (RIDTs) may not detect H3N2v. A positive RIDT cannot confirm H3N2v virus infection, because these tests cannot distinguish between influenza A virus subtypes.
  • If you have a patient who has an influenza-like illness (fever plus cough or sore throat) AND reports recent contact* (within 7 days of illness onset) with swine or recent attendance at an event where swine were present, please contact your local health department to coordinate testing at the state public health laboratory.

*Contact with swine may be direct contact (touching/handling a pig) or indirect contact (coming within about 6 feet of a pig without known direct contact).

Avian Flu Viruses

Influenza A viruses that regularly circulate in birds are called “avian flu viruses”. While avian flu viruses do not typically infect humans; occasional human infections have occurred. Human infections usually occur after close contact with infected birds (both live and dead) or areas contaminated with a bird flu virus. In recent years, two strains of avian influenza, H5N1 and H7N9 have been reported to cause severe illness in the human cases that were reported. Information about the avian influenza strains can be found below.

H5N1 Key Points:

  • Human infection with highly pathogenic avian influenza (HPAI) A (H5N1) virus is rare; however, sporadic cases have been reported. In the majority of cases, the person got HPAI H5N1 virus infection after direct or close contact with sick or dead infected poultry.
  • More than 600 human HPAI H5N1 cases have been reported to the World Health Organization from 15 countries in Asia, Africa, the Pacific, Europe and the Near East since November 2003. Approximately 60% of the cases have died.
  • Most cases of HPAI H5N1 cases have occurred among children and adults younger than 40 years old. Most cases have presented late in their illness for medical care and have been hospitalized with severe respiratory disease. However, some clinically mild HPAI H5N1 cases have been reported, especially in children.
  • Commercially available rapid influenza diagnostic tests (RIDTs) may not detect H5N1. A positive RIDT cannot confirm H5N1 virus infection, because these tests cannot distinguish between influenza A virus subtypes.
  • CDC recommends that healthcare providers consider HPAI H5N1 testing for persons who have both a clinical syndrome consistent with HPAI H5N1* disease and relevant possible exposure to HPAI H5N1** virus during the 7 days prior to the person’s illness onset. If you have a patient who meets these criteria, please contact your local health department.

*The most common clinical syndrome is fever and cough which in some patients progresses rapidly to several viral pneumonia with shortness of breath, difficulty breathing, and hypoxia. Frequent complications include respiratory failure, acute respiratory distress syndrome (ARDS), multi-organ failure, and death.

**Relevant exposures include: direct contact or very close contact with sick or dead poultry known or highly suspected to be infected with HPAI H5N1 virus, or visiting a live poultry market where poultry are known to be infected with HPAI H5N1 virus, or very close contact with a severely ill confirmed H5N1 patient.

H7N9 Key Points:

  • Beginning in April 2013, over 130 human infections with a new strain of H7N9 were identified from across 10 provinces and municipalities in mainland China and Taiwan. Most patients were hospitalized with severe respiratory illness and reported poultry contact prior to illness onset.
  • Preliminary surveillance suggests that H7N9 transmission has been limited.
  • No sustained person-to-person spread of the virus has been found at this time.
  • No cases of human infection with H7N9 have been detected in the U.S.
  • Commercially available rapid influenza diagnostic tests (RIDTs) may not detect H7N9. A positive RIDT cannot confirm H7N9 virus infection because these tests cannot distinguish between influenza A virus subtypes.
  • If you have a patient with new-onset severe respiratory infection requiring hospitalization AND recent travel (within 10 days of illness onset) to China, please contact your local health department to coordinate testing at the state public health laboratory.
  • For confirmed or suspected cases, antiviral treatment with a neuraminidase inhibitor (i.e., oseltamivir or zanamivir) is recommended as early as possible. Treatment should be initiated even if it is more than 48 hours after onset of illness.
  • Interim Guidance for Infection Control Within Healthcare Settings When Caring for Patients with Confirmed, Probable, or Cases Under Investigation of Avian Influenza A(H7N9) Virus Infection is available on the CDC website. This interim guidance recommends a higher level of infection control measures than for seasonal influenza. Standard Precautions, plus Contact and Airborne Precautions are recommended for patients with confirmed or probable H7N9 virus infection and for cases under investigation of H7N9 virus infection.
  • For additional information about H7N9, refer to:

Last Updated: 10-09-2013

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