Influenza

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Influenza, commonly called “the ‘Flu”, is a common viral infectious disease. That means that It is contagious (easily passed from one person to another) and is caused by viruses of the Orthomyxoviridae family.

Hosts

Influenza in humans

Symptoms of Influenza include headache, coughing and other cold-like symptoms. These are accompanied by a high fever. The flu is easily treated with modern medicines, but many strains are nevertheless quite serious, particularly for children and older persons.

There have been several worldwide Flu epidemics, resulting in millions of deaths.

Equine Influenza

This is a species-specific strain that infects horses, ponies, donkeys and mules. Humans do not catch this disease but are carriers of it. It is airborne and highly contagious. Equine influenza generally does not kill its victims, and affected horses display similar symptoms to those of humans.

In 2007, a horse flu epidemic began in New South Wales, Australia. The outbreak is believed to have begun as a result of infractions of proper quarantine procedure and is expected to cost millions of dollars in lost revenue. As of September, 2007, it had spread to the Victorian border, causing the cancellation of events at the Melbourne Show, and the possibility that the Spring Carnival, including the running of the Melbourne Cup horse race, would be canceled.

Avian flu

Avian flu, also known as Bird flu or H5N1, has led to recent outbreaks.[1][2]

Diagnosis

History and physical examination

A systematic review by the Rational Clinical Examination concluded that the best findings for excluding the diagnosis of influenza are:[3]

Most sensitive individual findings for diagnosing influenza[3]
Finding: sensitivity specificity
Fever 86% 25%
Cough 98% 23%
Nasal congestion 70–90% 20–40%

Notes to table:

  • All three findings, especially fever, were less sensitive in patients over 60 years of age.

Since anti-viral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of findings below can improve diagnostic accuracy.[4] Unfortunately, even combinations of findings are imperfect. However, Bayes Theorem can combine pretest probability with clinical findings to adequately diagnose or exclude influenza in some patients. The pretest probability has a strong seasonal variation; the current prevalence of influenza among patients in the United States receiving sentinel testing is available at the CDC.[5] Using the CDC data, the following table shows how the likelihood of influenza varies with prevalence:

Combinations of findings for diagnosing influenza[3]
Combinations of findings Sensitivity Specificity

As reported in study[3]
and projected during local outbreaks
(prevalence= 66%)

 Projected during influenza season
(prevalence=25%)		 Projected in off-season
(prevalence=2%)
PPV NPV PPV NPV PPV NPV
Fever and cough 64% 67% 79% 49% 39% 15% 4% 1%
Fever and cough and sore throat 56 71 79 45 39 17 4 2
Fever and cough and nasal congestion 59 74 81 48 43 16 4 1

Two decision analysis studies[6][7] suggest that during local outbreaks of influenza, the prevalence will be over 70%[7] and thus patients with any of the above combinations of symptoms may be treated with neuramidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.[7]

Differential diagnosis

Treatment

The two classes of anti-virals are neuraminidase inhibitors and M2 inhibitors (adamantane derivatives). Neuraminidase inhibitors are currently preferred for flu virus infections.

Different strains of influenza virus have differing degrees of resistance against these antivirals and it is impossible to predict what degree of resistance a future pandemic strain might have.[8]

Neuraminidase inhibitors

These drugs are often effective against both influenza A and B.[9] Examples are oseltamivir (trade name Tamiflu) and zanamivir (trade name Relenza) are neuraminidase inhibitors.[10] The Cochrane Collaboration concluded that these drugs reduce symptoms and complications.[11] Zanamivir may cause bronchospasm in patients with asthma.

M2 inhibitors (adamantanes)

These drugs are sometimes effective against influenza A if given early in the infection, but are always ineffective against influenza B. Examples include the antiviral drugs amantadine and rimantadine which block a viral ion channel and prevent the virus from infecting cells. [9]

Prevention

Vaccination

Neuraminidase inhibitors

The Cochrane Collaboration has concluded that neuraminidase inhibitors can prevent symptomatic influenza.[11] Zanamivir may cause bronchospasm in patients with asthma.

History

Spanish influenza pandemic of 1918

A pandemic of Spanish Influenza swept across Europe in the wake of the Great War (1918-19). More people are believed to have died of the flu in the year after the war then in the Great War itself. Estimates vary but it is estimated that one third of the world's population were infected and up to 50 million people perished in this pandemic (it is possibly that fatalities world wide were as high as 100 million).[12] Some comparisons are drawn between Spanish Flu and the Black Death (Bubonic Plague) of 1347 to 1351. Estimates are that the flu killed more people in one year than plague did in four.

In the United States, the epidemic led to closing of schools and banning of public meetings in order to successfully reduce mortality.[13]

External links

References

  1. ↑ Oner AF, Bay A, Arslan S, et al (2006). "Avian influenza A (H5N1) infection in eastern Turkey in 2006". N. Engl. J. Med. 355 (21): 2179–85. DOI:10.1056/NEJMoa060601. PMID 17124015. Research Blogging.
  2. ↑ Kandun IN, Wibisono H, Sedyaningsih ER, et al (2006). "Three Indonesian clusters of H5N1 virus infection in 2005". N. Engl. J. Med. 355 (21): 2186–94. DOI:10.1056/NEJMoa060930. PMID 17124016. Research Blogging.
  3. ↑ 3.0 3.1 3.2 3.3 Call S, Vollenweider M, Hornung C, Simel D, McKinney W (2005). "Does this patient have influenza?". JAMA 293 (8): 987-97. DOI:10.1001/jama.293.8.987. PMID 15728170. Research Blogging. Cite error: Invalid <ref> tag; name "pmid15728170" defined multiple times with different content
  4. ↑ Monto A, Gravenstein S, Elliott M, Colopy M, Schweinle J (2000). "Clinical signs and symptoms predicting influenza infection.". Arch Intern Med 160 (21): 3243–7. PMID 11088084.
  5. ↑ Centers for Disease Control and Prevention. Weekly Report: Influenza Summary Update. Accessed January 1, 2007.
  6. ↑ Smith K, Roberts M (2002). "Cost-effectiveness of newer treatment strategies for influenza.". Am J Med 113 (4): 300-7. DOI:10.1016/S0002-9343(02)01222-6. PMID 12361816. Research Blogging.
  7. ↑ 7.0 7.1 7.2 Rothberg M, Bellantonio S, Rose D (2003). "Management of influenza in adults older than 65 years of age: cost-effectiveness of rapid testing and antiviral therapy.". Ann Intern Med 139 (5 Pt 1): 321-9. PMID 12965940.
  8. ↑ Webster, Robert G. (2006). "H5N1 Influenza — Continuing Evolution and Spread". N Engl J Med 355 (21): 2174–77. PMID 16192481.
  9. ↑ 9.0 9.1 Stephenson, I; Nicholson K (1999). "Chemotherapeutic control of influenza". J Antimicrob Chemother 44 (1): 6–10. PMID 10459804.
  10. ↑ Moscona, A (2005). "Neuraminidase inhibitors for influenza". N Engl J Med 353 (13): 1363–73. PMID 16192481.
  11. ↑ 11.0 11.1 Jefferson, T; Demicheli V, Di Pietrantonj C, Jones M, Rivetti D. "Neuraminidase inhibitors for preventing and treating influenza in healthy adults". Cochrane Database Syst Rev 3: CD001265. DOI:10.1002/14651858.CD001265.pub2. PMID 16855962. Research Blogging.
  12. ↑ http://www.cdc.gov/ncidod/EID/vol12no01/05-0979.htm
  13. ↑ Markel H, Lipman HB, Navarro JA, et al (2007). "Nonpharmaceutical interventions implemented by US cities during the 1918-1919 influenza pandemic". JAMA 298 (6): 644–54. DOI:10.1001/jama.298.6.644. PMID 17684187. Research Blogging.
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