What CDC Does About Novel Flu: Outbreak Investigations

CDC works with state and local health departments to investigate human outbreaks with novel flu viruses (i.e., flu viruses that do not normally circulate in people) and animal outbreaks caused by unusual or concerning flu viruses in order to:

  1. identify the flu virus causing the outbreak,
  2. assess the public health risk posed by this virus outbreak,
  3. and to help prevent people from getting infected.

CDC takes novel flu outbreaks seriously because of their potential to harm large numbers of people. CDC scientists become concerned when:

When any of these scenarios happens, CDC works with state public health officials on a public health investigation and also coordinates with the United States Department of Agriculture (USDA) and state animal health officials as appropriate.

In the case of novel flu, the goals of an outbreak investigation are to identify the source of the infection in people, to determine if person-to-person spread of the virus is happening, and to slow or stop the spread of the virus in people and sometimes animals.

Epidemiologists, sometimes called “disease detectives”, do this type of investigative work. Disease detectives use various methods to figure out the “anatomy of an outbreak.”  For influenza, this might include:

  • Interviewing people who report illness.
  • Asking that all people reporting to a healthcare provider (physicians’ offices, health clinics, hospitals, and other health care facilities) with influenza-like illness (ILI) be evaluated for influenza and that they be reported to the local public health department.
  • Interviewing people who have tested positive for flu.
  • Evaluating the capacity for person-to-person spread of the virus in question through contact tracing, which is the identification and diagnosis of people who may have come into contact with an infected person.
  • Investigating reports of exposure to animals known to carry influenza viruses, such as pigs and birds.
  • Monitoring responders to animal influenza outbreaks, such as people working in outbreaks among pigs and poultry.

Following are a few examples of this investigative process showing how CDC takes quick action to respond to novel flu outbreaks in order to help stop or slow the spread of disease.

Swine/Variant Flu Outbreaks:

When a flu virus that normally spreads in pigs but not people is found in people, it is called a “variant influenza virus.” In the past, CDC received reports of approximately one human infected with influenza viruses that usually are found in swine every one to two years, but beginning in 2012, there was a jump in the number of these cases being reported to CDC. The majority of variant flu virus infections in the United States have occurred among people attending an agricultural fair where they were exposed to infected pigs or their contaminated environments. Agricultural fairs are large community events where people have the opportunity to show and sell animals, such as pigs, that they have raised. Large numbers of people and animals attend agricultural fairs. These settings provide an opportunity for people to come in close contact with pigs, some of which may be infected with influenza viruses, and their environment, which may become contaminated with influenza viruses when ill pigs cough or sneeze. Below are examples of variant flu outbreaks:

  • 2012 Multi-State Outbreak:  From July to September 2012, an outbreak of influenza A(H3N2) variant virus (or H3N2v) infection involving 10 states resulted in 309 infections reported among attendees of agricultural fairs where swine were present. CDC worked closely with state and territorial health departments to investigate each human infection so that the risk of infection could be more fully understood and appropriate public health measures could be taken to minimize the chance that other people might get sick. The Ohio Department of Health requested CDC’s short-term assistance during this outbreak (called an Epi-Aid). CDC staff joined the state health department staff and others to support their efforts to:
    • Determine the extent of the outbreak and the severity of illness among people infected;
    • Identify any instances of person-to-person spread; and
    • Examine possible risk factors for infection.

    At a single agricultural fair in Ohio, CDC and state public health officials identified 20 confirmed and 94 probable cases of (H3N2)v virus infection. No evidence of human-to-human spread was found.

    CDC’s risk analysis contributed to the development of a new recommendation that people at high risk for developing serious flu complications, including children younger than 5 years old, people 65 years and older, pregnant women, and people with certain chronic medical conditions (for example asthma, other lung diseases, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions) avoid pigs and swine barns at fairs. Learn more.

  • 2016 Michigan Outbreak:  In August 2016, an influenza H3N2v virus infection identified in a child who exhibited pigs during an agricultural fair in Michigan also resulted in a CDC Epi-Aid investigation. A team of CDC staff, including Epidemic Intelligence Service (EIS) officers, traveled to Michigan to look for additional variant flu infections among people exhibiting pigs at county fairs. They tested the use of a “text-illness monitoring” system (called “TIM”) to look for potential variant virus infections among swine exhibitors and other members of their households. TIM is a text-based illness monitoring system originally developed as a way to more easily remind outbreak responders and other potentially exposed people to report any signs and symptoms of illness to public health personnel. The CDC team used this opportunity to evaluate the feasibility and acceptability of using TIM for future novel flu virus outbreaks. The CDC team piloted the use of TIM at nine agricultural fairs involving an estimated 1,052 swine exhibitors. They used TIM to conduct active surveillance of 392 people in 87 households for signs and symptoms of illness and potential variant virus infections. Of the 392 people who signed up for active monitoring, 22 reported an illness to their health department through the TIM system. Nine of those people sought medical care and five of them were tested. Of those tested, two tested positive for (H3N2)v. Using a text-based system for active surveillance helped the team identify people with infections that may not have been identified otherwise.

Avian Flu Outbreaks:

Every so often, avian influenza (i.e., bird flu) viruses that naturally occur in wild aquatic birds or domestic poultry can infect other birds, domestic poultry, animals or people causing bird flu outbreaks. In some instances, avian influenza outbreaks in poultry have been associated with illness and death in people globally, although this is rare in North America. Avian flu viruses spread mainly from animals to people through direct contact with infected birds or their contaminated environments. The spread of avian flu viruses from one ill person to another is rarely reported. However, finding an avian influenza virus in an unexpected host (like a domestic cat) and any human infection with an avian influenza virus is always concerning because there is little or no immunity to these viruses in the human population. Below are examples of avian flu outbreaks:

  • 2014-2015 Multi-State Outbreak: From December 2014 through mid-June 2015, high pathogenic avian influenza (HPAI) H5 infections were reported in U.S. domestic poultry (backyard and commercial flocks), captive wild birds, and wild birds. Throughout that time, HPAI H5 bird flu virus outbreaks were reported in 21 states across the U.S. At the time, no human infections with these viruses were detected, however similar viruses were infecting people in other countries, causing serious illness and death in some cases. While the risk of human infection among the public was low during this bird flu outbreak, it is still possible for these viruses to infect people. CDC provided recommendations and guidance to clinicians and public health professionals in the United States on appropriate follow-up, preventive treatment, testing, specimen collection and processing of samples from patients who may be infected with H5 viruses.
  • 2016 New York City Animal Shelter Outbreak: In December 2016, CDC was notified about an outbreak of avian influenza A (H7N2) virus in cats at a New York City (NYC) animal shelter. Avian Influenza viruses have occasionally been detected in cats previously, although not in an outbreak situation. CDC and New York state and local public health authorities conducted an investigation to identify any human infections associated with exposure to the ill cats and to ensure the virus wasn’t spreading from person to person. During this investigation, nearly 400 people who were exposed to the infected cats during the outbreak were screened or tested for H7N2 virus infection. This work took on renewed urgency when a veterinarian who worked closely with the sick cats tested positive for H7N2 virus infection on Dec. 20, 2016, becoming the first known instance of cat-to-human transmission of flu and only the third human case of H7N2 bird flu in the United States. The vet experienced flu symptoms, including sore throat, cough, and fatigue. The course of illness was brief, mild and was resolved completely without requiring hospitalization. No further human infections were identified at the time of the outbreak.
  • 2017 Multi-State Commercial Poultry Outbreak: In early March 2017, health officials reported high pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI) H7N9 viruses in Tennessee and subsequently in Alabama, Kentucky and Georgia. No human infections associated with the outbreaks were detected. CDC considered the risk to the public’s health from this North American H7N9 virus outbreak to be low. However, in the past there have been a small number of reported human infections with North American bird flu A H7 viruses. Most infections were associated with poultry exposure and resulted in mild respiratory illness and/or conjunctivitis. CDC worked closely with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (USDA APHIS) and state and local agriculture and public health partners to minimize any human health risk posed by the bird flu outbreak, including steps they could take to reduce possible risk. Continuous monitoring of flu viruses that spread between animals and people is critical so public health and animal health researchers can quickly identify emerging flu viruses and assess their pandemic potential.

For more information on past outbreaks, see: