Influenza-like illness

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Flu-like syndrome
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Influenza-like illness
Proportion of positive influenza tests during a recent flu season.

Influenza-like illness (ILI), also known as flu-like syndrome or flu-like symptoms, is a

sneezing typically in connection with a sudden onset of illness.[1] In most cases, the symptoms are caused by cytokines released by immune system activation,[citation needed
] and are thus relatively non-specific.

Common causes of ILI include the common cold and influenza, which tends to be less common but more severe than the common cold. Less common causes include side effects of many drugs and manifestations of many other diseases.[citation needed]

Definition

The term ILI can be used casually, but when used in the surveillance of influenza cases, can have a strict definition. The World Health Organization defines an illness as an ILI if the patient has a fever greater than or equal to 38 °C and a cough, which began in the last 10 days. If the patient requires hospitalisation, the illness is classified instead as a severe acute respiratory infection (SARI).[2] Other organisations may have different definitions. For instance, the CDC defines it as a 100 °F (38 °C) fever or greater, and a cough or sore throat.[3]

Causes

The causes of influenza-like illness range from benign self-limited illnesses such as gastroenteritis, rhinoviral disease, and influenza, to severe, sometimes life-threatening, diseases such as meningitis, sepsis, and leukemia.[citation needed]

Influenza

Technically, any clinical diagnosis of influenza is a diagnosis of ILI, not of influenza. This distinction usually is of no great concern because, regardless of cause, most cases of ILI are mild and self-limiting. Furthermore, except perhaps during the peak of a major outbreak of influenza, most cases of ILI are not due to influenza.[4][5] ILI is very common: in the United States each adult can average 1–3 episodes per year and each child can average 3–6 episodes per year.[4]

clinical surveillance by a global network of more than 110 National Influenza Centers. These centers receive samples obtained from patients diagnosed with ILI, and test the samples for the presence of an influenza virus. Not all patients diagnosed with ILI are tested, and not all test results are reported. Samples are selected for testing based on severity of ILI, and as part of routine sampling, and at participating surveillance clinics and laboratories.[6] The United States has a general surveillance program, a border surveillance program, and a hospital surveillance program, all devoted to finding new outbreaks of influenza.[citation needed
]

In most years, in the majority of samples tested, the influenza virus is not present (see figure above). In the United States during the 2008–9 influenza season through 18 April, out of 183,839 samples tested and reported to the CDC, only 25,925 (14.1%) were positive for influenza. The percent positive reached a maximum of about 25%.[5] The percent positive increases with the incidence of infection, peaking with the peak incidence of influenza (see figure). During an epidemic, 60–70% of patients with a clear influenza-like illness actually have influenza.[7]

Samples are respiratory samples, usually collected by a physician, nurse, or assistant, and sent to a hospital laboratory for preliminary testing. There are several methods of collecting a respiratory sample, depending on requirements of the laboratory that will test the sample. A sample may be obtained from around the nose simply by wiping with a dry cotton swab.[8]

Other causes

poliomyelitis, pneumonia, measles, SARS, COVID-19
, and many others.

Chemotherapeutic agents also commonly cause flu-like symptoms. Other drugs associated with a flu-like syndrome include bisphosphonates, caspofungin, and levamisole.[14][15] A flu-like syndrome can also be caused by an influenza vaccine or other vaccines, and by opioid withdrawal in physically dependent individuals.[citation needed
]

Diagnosis

Influenza-like illness is a nonspecific respiratory illness characterized by fever,

adenoviruses, and human parainfluenza viruses). Less common causes of ILI include bacteria such as Legionella, Chlamydia pneumoniae, Mycoplasma pneumoniae, and Streptococcus pneumoniae. Influenza, RSV, and certain bacterial infections are particularly important causes of ILI because these infections can lead to serious complications requiring hospitalization. Physicians who examine persons with ILI can use a combination of epidemiologic and clinical data (information about recent other patients and the individual patient) and, if necessary, laboratory and radiographic tests to determine the cause of the ILI.[4] The use of multiplexed point-of-care testing such as CRP (C-reactive protein) along with an examination by a doctor may help to identify a bacterial and avoid an unnecessary antibiotic prescription.[16]

During the

2009 flu pandemic in the United States, the CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five U.S. states that have reported swine flu cases or in Mexico during the 7 days preceding their illness onset."[17] A diagnosis of confirmed swine flu required laboratory testing of a respiratory sample (a simple nose and throat swab).[17]

In rare cases

If a person with ILI also has either a history of exposure or an occupational or environmental risk of exposure to Bacillus anthracis (anthrax), then a differential diagnosis requires distinguishing between ILI and anthrax.[18][19][20] Other rare causes of ILI include leukemia and metal fume fever.

In horses

ILI occurs in some horses after

Non-steroidal anti-inflammatory drugs (NSAIDs) may be given with the vaccine.[21]

See also

References

  1. ^ "Case definitions". European Influenza Surveillance Scheme. 12 December 2005. Retrieved 15 July 2009.
  2. ^ "WHO surveillance case definitions for ILI and SARI". WHO. Archived from the original on 19 October 2014. Retrieved 11 March 2020.
  3. ^ "Surveillance Manual". cdc.gov. 29 March 2019. Retrieved 11 March 2020.
  4. ^
    PMID 11724153
    .
  5. ^ a b "2008–2009 Influenza Season Week 15 ending April 18, 2009". FluView: A Weekly Influenza Surveillance Report Prepared by the Influenza Division. Centers for Disease Control and Prevention. 24 April 2009. Retrieved 26 April 2009.
  6. ^ "Flu Activity & Surveillance". Centers for Disease Control and Prevention. 17 April 2009. Retrieved 26 April 2009.
  7. ^ "Influenza – Frequently asked questions". European Influenza Surveillance Scheme. 21 November 2005. Archived from the original on 10 February 2009. Retrieved 28 April 2009.
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  17. ^ a b Centers for Disease Control and Prevention (26 April 2009). "CDC Health Update: Swine Influenza A (H1N1) Update: New Interim Recommendations and Guidance for Health Directors about Strategic National Stockpile Materiel". Health Alert Network. Retrieved 27 April 2009.
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