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Acute infection caused by the influenza viruses type A or B (RNA orthomyxovirus).
Influenza A causes worldwide annual epidemics and is infamous for the much rarer pandemics, the most serious of which occurred in 1918 when ∼40 million people died worldwide. Influenza B is also associated with annual outbreaks that are usually milder in nature than those caused by influenza A. Influenza C is of doubtful pathogenicity in humans. Spread is by respiratory droplets.
Influenza viruses develop new antigenic variants at regular intervals through random mutations and antigenic drift. Human immunity depends largely on the haemag-glutinin (H) antigen and the neuraminidase (N) antigen on the viral surface. Major shifts in these antigenic regions in influenza viruses occur through acquisition of a new H or N from animals such as birds, horses and pigs. These can cause a pandemic, whereas antigenic drift causes the milder annual epidemics. Influenza B only occurs in humans and only undergoes antigenic drift.
Rapid onset of fever usually >38◦ C, cough, headache, shivering and myalgia start after an incubation period of 1–3 days. Other upper and lower respiratory symptoms may develop. Individuals are infective for 1 day prior to and for around 1 week after symptoms commence.
Otitis, sinusitis and viral pneumonia are common. Influenza may exacerbate underlying respiratory disease including asthma and chronic obstructive pulmonary disease.
Secondary bacterial infection particularly with Strep. pneumoniae and H. influenzae is common following influenza pneumonia. Less commonly, secondary Staph. aureus infection which has a mortality rate of 20%.
Postviral syndrome: Debility and depression may develop after the acute illness, and take weeks to months to resolve.
Postinfectious encephalomyelitis is rare but does occur.
Diagnosis is confirmed by detection of virus in nasal or throat swabs by culture, antigen detection or PCR. Retrospective diagnosis can be made by a rise in specific complement-fixing antibody or haemagglutinin antibody measured 2 weeks apart, but this is usually unnecessary.
Bed rest, antipyretics such as paracetamol for symptoms. Fluids may be needed.
The neuraminidase inhibitors zanamivir and oseltamivir are effective in treating acute influenza occurring during annual influenza epidemics. They work against Influenza A and B if given within 48 hours of symptoms and especially in the first 24 hours. They are particularly indicated in the elderly, those with underlying respiratory disease such as chronic obstructive pulmonary disease, and those with other underlying disorders such as cardiovascular disease, diabetes or renal failure. These drugs are also highly effective for prophylaxis in family or institutional settings.
Prophylaxis by vaccination is effective in up to 70% and in elderly people reduces hospital admission and mortality by about 50%. Some are manufactured in chick embryos and these should not be given to anyone allergic to eggs. Routine vaccination is reserved for susceptible people with chronic heart, lung or renal disease, diabetes, immunosuppression and the elderly. It needs to be given yearly.
The vaccine needs to be prepared each year based on predictions of which antigenic variants are present. These predications depend on global surveillance organised by the World Health Organisation (WHO). This surveillance depends on viruses being cultured and therefore on nose/throat swabs being taken and sent to local labs.
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