Influenza

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 a virus.

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.

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:

Most sensitive individual findings for diagnosing influenza

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. 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. Using the CDC data, the following table shows how the likelihood of influenza varies with prevalence:

Combinations of findings for diagnosing influenza

Combinations of findings	Sensitivity	Specificity	As reported in study 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 suggest that during local outbreaks of influenza, the prevalence will be over 70% 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%.

Differential diagnosis

• Parainfluenza
• Adenovirus
• Parvovirus B19 - arthropathy is prominent
• Respiratory syncytial virus
• Flaviviruses
  • Dengue fever - bonebreak fever with retro-orbital pain
  • West Nile virus - tremor is common

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.

Neuraminidase inhibitors

These drugs are often effective against both influenza A and B. Examples are oseltamivir (trade name Tamiflu) and zanamivir (trade name Relenza) are neuraminidase inhibitors. The Cochrane Collaboration concluded that these drugs reduce symptoms and complications. 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.

Prevention

Neuraminidase inhibitors

The Cochrane Collaboration has concluded that neuraminidase inhibitors can prevent symptomatic influenza. 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). 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.

External links

• Influenza at CDC
• Influenza at World Health Organization
• Health encyclopedia entry at NHS Direct
• BioHealthBase Bioinformatics Resource Center Database of influenza sequences and related information.
• Overview of influenza at MedicineNet
• Orthomyxoviridae The Universal Virus Database of the International Committee on Taxonomy of Viruses
• Influenza Virus Resource at the NCBI