Public Health

Not The Flu, But Feels Like It! What Is Influenza-like Illness (ILI)

Khang T. Vuong, MHA
Khang T. Vuong, MHA2 Jun 2023

Influenza-like illnesses (ILI) are a group of respiratory infections with similar symptoms to the flu but are caused by different viruses or bacteria. Common misconceptions include ILI being the same as the flu and antibiotics being an effective treatment. It's crucial for individuals and healthcare professionals to differentiate between ILI and influenza to prevent misdiagnosis and provide appropriate treatment.

I. What is Influenza-like Illnesses (ILI)?

Definition and Symptoms of ILI

Influenza-like illnesses (ILI) encompass a group of respiratory infections exhibiting symptoms similar to the flu. While ILIs can be caused by the influenza virus, they can also result from other viruses and bacteria. Based on a study published in the journal PLoS One, the distribution of symptoms experienced by patients diagnosed with ILI is as follows:

  1. Fever: 95.0%
  2. Cough: 92.0%
  3. Sore throat: 51.0%
  4. Muscle aches (myalgia): 42.0%
  5. Fatigue: 42.0%
  6. Headache: 40.0%
  7. Chills: 36.0%
  8. Shortness of breath: 28.0%
  9. Nasal congestion: 23.0%

What Causes Influenza-like Illnesses?

The most common viruses that cause influenza-like illness (ILI) are:

  • Rhinoviruses are the most common cause of ILI, accounting for up to 80% of cases. They are a large family of viruses that can spread through respiratory droplets produced when an infected person coughs or sneezes. Rhinoviruses can infect people of all ages but are most common in children.
  • Coronaviruses are another common cause of ILI, accounting for up to 15% of cases. They are a group of viruses that can cause various respiratory illnesses, including the common cold, SARS, and MERS. Coronaviruses can spread through respiratory droplets produced when an infected person coughs or sneezes.
  • Human respiratory syncytial virus (RSV) is a respiratory virus that can cause a range of illnesses, from mild colds to severe pneumonia. RSV is most common in young children but can infect adults. RSV can spread through respiratory droplets produced when an infected person coughs or sneezes.
  • Adenoviruses are a group of viruses that can cause various illnesses, including respiratory infections, conjunctivitis, and diarrhea. Adenoviruses can spread through respiratory droplets produced when an infected person coughs or sneezes or through contact with contaminated surfaces.
  • Human parainfluenza viruses are a group of viruses that can cause a range of illnesses, from mild colds to severe pneumonia. Human parainfluenza viruses are most common in young children, but they can also infect adults. Human parainfluenza viruses can spread through respiratory droplets produced when an infected person coughs or sneezes.

Transmission and Prevention

Much like the flu, ILI can be transmitted through respiratory droplets when an infected person talks, coughs, or sneezes. They can also spread via contact with contaminated objects or surfaces. To prevent the spread of ILI, practicing proper hand hygiene, getting vaccinated for known flu strains, and adhering to local public health guidelines (Source: CDC).

One study, published in the journal "Influenza and Other Respiratory Viruses" in 2016, found that the prevalence of ILI was highest among Janitors and Cleaners (PR = 2.5, 95% CI = 1.3, 4.7) and Secretaries (PR = 2.4, 95% CI = 1.1, 5.4). These occupations may be more likely to be exposed to the virus that causes ILI, either through contact with contaminated surfaces or close contact with sick people.

Influenza-like illness (ILI) is more common during the winter months than during the summer months. This is because the influenza virus that causes ILI is more stable in cold, dry air. The virus can also survive longer on surfaces in cold, dry air.

In the United States, the peak season for ILI is typically between December and March. However, the exact timing of the peak season can vary from year to year. For example, the 2021-2022 season had a late peak, with the highest number of ILI cases reported in April.

There are many things that can contribute to the seasonality of ILI. These include:

  • Human behavior: People tend to spend more time indoors during the winter months, which increases the chances of coming into contact with the virus.
  • Weather conditions: Cold, dry air can help to preserve the virus, making it more likely to spread.
  • Seasonal changes in the immune system: The immune system may not be as effective at fighting the virus during winter.

II. Common Misconceptions and Facts about Influenza-like Illnesses (ILI)

Misconception 1: ILI and Flu are the Same

One common misconception about ILI is that it refers exclusively to the flu. In reality, ILI encompasses a variety of infections that present with flu-like symptoms but are caused by different viruses and bacteria (Source: US National Library of Medicine). These infections include respiratory syncytial virus (RSV), human metapneumovirus (HMPV), and parainfluenza viruses.

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Misconception 2: Antibiotics are Effective Against ILI

As viruses cause most ILIs, antibiotics—specifically against bacterial infections—are often ineffective in treating ILIs (Source: CDC). The misuse of antibiotics in treating viral infections contributes to antibiotic resistance, a significant global public health concern (Source: WHO).

The best way to prevent ILI is to vaccinate against influenza and RSV. You can also help to prevent the spread of ILI by washing your hands frequently, avoiding close contact with people who are sick, and covering your mouth and nose when you cough or sneeze.

In addition to the above, there are a number of things that can be done to help relieve the symptoms of ILI:

  • Drink plenty of fluids: Staying hydrated is important for helping to loosen mucus and prevent dehydration.
  • Get plenty of rest: Resting gives your body the time it needs to fight off the infection.
  • Take over-the-counter medications: Over-the-counter medications such as ibuprofen, acetaminophen, and naproxen can help to relieve pain, fever, and inflammation.
  • Use a humidifier: A humidifier can help to add moisture to the air, which can help to loosen mucus and make it easier to breathe.
  • Gargle with salt water: Gargling with salt water can help to soothe a sore throat.
  • Elevate your head when you sleep: Elevating your head can help drain mucus from your sinuses and make breathing easier.

III. Importance of Differentiating ILI from Influenza (the Flu)

Diagnostic Accuracy and Treatment

Diagnosing the specific cause of an ILI is critical, as different viruses and bacteria require different treatments (Source: NIH). Misdiagnosing an ILI can lead to unnecessary antibiotic prescriptions or the use of inappropriate treatments.

There are no lab tests or medications that are specifically effective against the viruses that cause influenza-like illness (ILI). However, there are a number of things that can be done to help diagnose and treat ILI.

  • Rapid influenza diagnostic test (RIDT): A RIDT is a rapid test that can be used to detect influenza virus in a patient's nasal swab. RIDTs are not always accurate, but they can be helpful in diagnosing influenza early on.
  • Viral culture: A viral culture is a laboratory test that can be used to grow and identify the virus that is causing an infection. Viral cultures are more accurate than RIDTs but can take several days to produce results.
  • Serology: Serology is a blood test that can be used to detect antibodies to a particular virus. Serology can be used to confirm a diagnosis of influenza, but it cannot be used to diagnose ILI early on.

Public Health Implications

Being able to differentiate between ILI and influenza accurately has several public health benefits. Early detection and treatment of specific infections can help prevent outbreaks and the spread of infectious diseases. Proper management of ILI cases reduces the burden on healthcare systems, freeing up resources for severe cases and other medical needs (Source: European Centre for Disease Prevention and Control).

IV. Current Research on ILI

Advances in ILI Diagnostics and Treatment

In recent years, there have been advancements in diagnostic tests for ILIs. Rapid diagnostic tests have been developed to help identify specific infections, enabling healthcare providers to make informed decisions about patient care and treatment (Source: USFDA). Additionally, researchers are studying the potential benefits of antiviral medications for non-influenza ILIs (Source: The Lancet Infectious Diseases].

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Epidemiological Studies on ILI Prevalence and Causes

Understanding the distribution of ILIs and their causes can help public health efforts in addressing diagnostic and treatment challenges. Research has examined ILI prevalence, seasonality, and distribution to inform public health strategies aimed at preventing and controlling such infections (Source: BMC Infectious Diseases). Further studies are working to identify risk factors and populations particularly vulnerable to ILIs (Source: The Journal of Infectious Diseases).

V. Frequently Asked Questions

How can I tell if I have an ILI or the flu?

Determining the cause of ILI symptoms can be difficult without medical consultation. Healthcare professionals can accurately diagnose whether your symptoms are due to the flu or another virus or bacterium using diagnostic tests (Source: USFDA). To ensure proper care, it's crucial to consult a healthcare provider if you're experiencing flu-like symptoms.

How can I prevent ILIs?

Preventing ILIs involves following proper hygiene practices such as handwashing, receiving applicable vaccinations for known flu strains, and adhering to local public health guidelines (Source: CDC). It's also essential to stay informed about circulating viruses and bacteria in your area to take appropriate precautions (Source: WHO).

When should I see a doctor for ILI symptoms?

You should visit a healthcare professional if your ILI symptoms are severe, last longer than usual, or if you have underlying health conditions that could make you more susceptible to complications (Source: CDC). Also, consult a doctor if your symptoms are not improved with over-the-counter medications and adequate rest.

Khang T. Vuong, MHA

Khang T. Vuong received his Master of Healthcare Administration from the Milken Institute School of Public Health at the George Washington University. He was named Forbes Healthcare 2021 30 under 30. Vuong spoke at Stanford Medicine X, HIMSS conference, and served as a Fellow at the Bon Secours Health System.