Influenza is an acute respiratory illness caused by RNA viruses of the…
Influenza is an acute respiratory illness caused by RNA viruses of the family Orthomyxoviridae (influenza viruses). There are three types of influenza virus:
Influenza A occurs more frequently and is more virulent. It is responsible for local outbreaks, larger epidemics or pandemics.
Influenza B often co-circulates with influenza A during the yearly outbreaks. Generally, influenza B causes less severe clinical illness, although it can still be responsible for outbreaks. In children, the severity of the illness may be similar to that associated with influenza A.
Influenza C usually causes a mild or asymptomatic infection similar to the common cold.
Seasonal influenza virus types A and B are also divided into a number of subtypes. These subtypes are defined by the H and N antigens present on the virus.
Influenza-like illness presents as a similar symptom complex to true influenza, but it is caused by a virus other than influenza A, B, or C (for example the respiratory syncytial virus).
Uncomplicated influenza is an acute respiratory infection caused by influenza A or B viruses that is usually self-limiting in the general population.
Complicated influenza is more severe and is associated more often with influenza A infection rather than influenza B infection. It is defined by signs and symptoms that: require hospital admission, involve the lower respiratory tract, central nervous system, or cause significant exacerbation of an underlying medical condition. Treatment may require more aggressive supportive care or hospitalisation, including treatment with antibiotics and/or antivirals.
Most complications of influenza in adults are respiratory in nature and include:
Exacerbations of asthma and chronic obstructive pulmonary disease.
Pneumonia — this can be caused by a secondary bacterial infection (particularly with Staphylococcus aureus) or primary viral infection.
Cardiac complications — myocarditis, pericarditis, exacerbation of underlying cardiac disease.
Myalgia, myositis and rhabdomyolysis.
Neurological complications — Reyes syndrome, encephalomyelitis, transverse myelitis, Guillain-Barré syndrome, aseptic meningitis, and encephalitis.
Toxic shock syndrome
In pregnancy, women and their unborn children are at high risk of morbidity and mortality from influenza infection, and it can cause preterm labour and low birth weight.
The highest risk of morbidity is in the third trimester.
The diagnosis of influenza is generally made using clinical features alone when it is known to be circulating in the community (for weekly influenza surveillance reports, visit www.gov.uk).
Diagnosis of influenza can only be confirmed by laboratory testing, although the probability that an influenza-like illness is caused by influenza is higher if influenza is known to be circulating and if a person has a high fever.
Rapid testing for influenza should be undertaken in all people with complicated influenza (although often this takes place in hospital).
Initiation of antiviral treatment should not be delayed while awaiting laboratory confirmation.
Laboratory testing to confirm or exclude influenza is particularly important if an individual develops symptoms despite antiviral prophylaxis, or has a persistent infection despite antiviral treatment, in order to identify the potential development of antiviral resistance.
Coryza, nasal discharge.
Generalised symptoms (headache, malaise, myalgia, arthralgia).
Ocular symptoms (such as photophobia, conjunctivitis, lacrimation and pain upon eye movement).
No clinical features of complicated influenza.
Complicated influenza clinical features include:
Signs and symptoms that require hospital admission.
Presence of a lower respiratory tract infection (hypoxaemia, dyspnoea, lung infiltrate).
Central nervous system involvement.
Significant exacerbation of an underlying medical condition.
Symptoms of influenza-like illness can be different in infants and children and may include:
Diarrhoea and vomiting — gastrointestinal symptoms are more common in children than in adults.
Fever (typically 38–40°C) — children can have a higher maximum temperature. They may also present with undifferentiated fever or febrile seizures.
Hyperaemia of oropharynx.
Laryngotracheobronchitis (croup), bronchiolitis, or bronchitis.
Myalgia, or myositis — this can affect the calf muscles and be severe. It is a more common complication in children compared to adults.
The common cold (coryza) — causes more nasal problems, and fever, fatigue, and myalgia are less common and/or less severe. For more information, see the CKS topic on Common cold.
Respiratory syncytial virus (RSV) infection — upper and lower respiratory symptoms peak in 3–5 days and resolve within 7–10 days. The most common cause of lower respiratory tract infections in children aged under 1 year. Also a significant and often unrecognised cause of lower respiratory tract infection in older people and people who are immunosuppressed.
Parainfluenza virus infection (PIV) — generally causes mild upper respiratory tract infections, but can induce life-threatening lower respiratory tract infections in people who are immunocompromised. The second most common cause, after RSV, of acute lower respiratory tract infections in infants and young children.
Streptococcal pharyngitis — is characterized by an acutely sore throat; usually, pain is more severe if infection with Streptococcus pyogenes is responsible. Cough, sneeze, and nasal congestion are absent. For more information, see the CKS topic on Sore throat - acute.
Meningitis — should be ruled out. Signs and symptoms that make a diagnosis of meningitis more likely include:
In infants and babies: a high fever; loss of consciousness; blank, staring expression; vomiting and loss of appetite; high-pitched screaming or whimpering; floppiness, with a dislike of being held; and/or a tense or bulging fontanelle.
In older children and adults: fever, vomiting, stiff neck, photophobia, severe headache, muscular pains, fits, stomach pain/cramps (caused by sepsis), and/or confusion.
A late of sign of meningococcal sepsis is a purpuric rash.
For more information, see the CKS topic on Meningitis - bacterial meningitis and meningococcal disease.
Bacterial pneumonia or acute bronchitis — may be mistaken for influenza or may develop following a viral upper respiratory tract infection (including influenza). Factors that increase the risk of lower respiratory tract infections include elderly age, being immunocompromised, having asthma or chronic obstructive pulmonary disease, or being a current smoker. For more information, see the CKS topics on Chest infections - adult and Cough - acute with chest signs in children.
Infectious mononucleosis (glandular fever) — is prevalent in young adults and adolescents. It is a chronic infection characterized by prolonged fever, severe sore throat, fatigue, and swollen lymph nodes. For more information, see the CKS topic on Glandular fever (infectious mononucleosis).
Pertussis (whooping cough) — may cause prodromal symptoms similar to those of influenza, but it should be easily diagnosed once the characteristic cough develops. For more information, see the CKS topic on Whooping cough.
Malaria — should be suspected in people presenting with fever who have recently travelled to an area where malaria is endemic. For more information, see the CKS topics on Malaria and Malaria prophylaxis.
Prescribe an antiviral drug (oral oseltamivir or inhaled zanamivir) for people with influenza if all of the following apply:
The national surveillance scheme indicates that influenza is circulating — the Department of Health and Social Care notifies GPs directly if influenza is circulating in the community.
The person is in an 'at risk' group of people who are likely to suffer a worse prognosis from influenza than otherwise healthy people.
The person can start treatment within 48 hours of the onset of symptoms (36 hours in the case of zanamivir in children).
Starting treatment after 48 hours of the onset of symptoms (or 36 hours in the case of zanamivir in children) is an off-label use and clinical judgement should be exercised.
Consider prescribing oral oseltamivir for previously healthy people with influenza if all of the following apply:
The person is not in an 'at risk' group, but it is felt that they are at serious risk of developing complications.
The national surveillance scheme indicates that influenza is circulating.
The person is able to start treatment within 48 hours of the onset of symptoms.
At risk' groups
'At risk group' includes people aged over 65 years, children aged under 6 months, pregnant women (at any stage of pregnancy, or women up to two weeks post partum), and people with any of the following conditions:
Asplenia or dysfunction of the spleen — including conditions such as homozygous sickle cell disease and coeliac syndrome that may lead to splenic dysfunction.
Chronic respiratory disease, including:
Chronic obstructive pulmonary disease, chronic bronchitis and emphysema, bronchiectasis, cystic fibrosis, interstitial lung fibrosis, pneumoconiosis, and bronchopulmonary dysplasia.
Asthma that requires continuous or repeated use of inhaled or systemic corticosteroids or with previous exacerbations requiring hospital admission.
Children who have previously been admitted to hospital for lower respiratory tract disease.
Chronic heart disease — including congenital heart disease, hypertension with cardiac complications, chronic heart failure, and individuals requiring regular medication or follow up for ischaemic heart disease.
Chronic kidney disease — including chronic kidney disease stage 3, 4 or 5, chronic kidney failure, nephrotic syndrome, and kidney transplantation.
Chronic liver disease, including cirrhosis, biliary atresia, and chronic hepatitis.
Chronic neurological conditions, including stroke and transient ischaemic attack. Conditions in which respiratory function may be compromised (for example, polio syndrome). Individual assessment should also be considered in clinically vulnerable individuals including those with cerebral palsy, learning disabilities, multiple sclerosis and related or similar conditions; or hereditary and degenerative disease of the nervous system or muscles; or severe neurological disability.
Diabetes mellitus, including type 1 diabetes and type 2 diabetes (requiring oral hypoglycaemic drugs or a controlled diet).
Immunosuppression due to disease or treatment, including:
People undergoing chemotherapy (or radiotherapy) leading to immunosuppression.
People undergoing bone marrow transplant.
HIV infection (all stages).
Individuals treated with, or likely to be treated with, systemic steroids for more than 1 month at dosages equivalent to prednisolone 20 mg or more daily (at any age) or, for children weighing less than 20 kg, a dose of 1 mg or more per kg body weight per day.
Genetic disorders affecting the immune system (for example, IRAK-4, NEMO, complement disorder).
Morbid obesity (body mass index of 40 or more).
Contraindications and cautions
Do not prescribe oseltamivir in premature infants.
Prescribe oseltamivir with caution to people with renal impairment — dosage adjustments may be necessary.
Common adverse effects include:
Uncommon adverse effects include:
Rare adverse effects include:
Abnormal behaviour abnormal.
Anxiety, confusion, delirium, delusions, hallucination, self-injurious behaviour.