Influenza

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.

Influenza usually occurs in the UK during the winter months, typically between December and March.

Symptoms of influenza appear abruptly, around 2 days after exposure.

Uncomplicated influenza signs and symptoms include:

Coryza, nasal discharge
Cough
Fever
Gastrointestinal symptoms
Generalised symptoms (headache, malaise, myalgia, arthralgia)
Ocular symptoms (such as photophobia, conjunctivitis, lacrimation and pain upon eye movement)
Sore throat
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

Other illnesses that present with features of influenza-like illness are more likely at times when seasonal influenza is not known to be circulating in the community (for example, outside the winter months) and include:

The common cold (coryza)

Causes more nasal problems, and fever, fatigue, and myalgia are less common and/or less severe.

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

Characterised by an acutely sore throat; usually, pain is more severe if infection with Streptococcus pyogenesis responsible. Cough, sneeze, and nasal congestion are absent.

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.

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.

Infectious mononucleosis (glandular fever)

Prevalent in young adults and adolescents. It is a chronic infection characterised by prolonged fever, severe sore throat, fatigue, and swollen lymph nodes.

Pertussis (whooping cough)

May cause prodromal symptoms similar to those of influenza, but it should be easily diagnosed once the characteristic cough develops.

Malaria

Should be suspected in people presenting with fever who have recently travelled to an area where malaria is endemic.

The diagnosis of influenza is generally made using clinical features alone when it is known to be circulating in the community.
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.
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.

Advise the person:
- To drink adequate fluids to avoid dehydration.
- To take paracetamol or ibuprofen for symptomatic relief.
- To rest in bed if they feel fatigued.
- To stay off work or school if they feel unable to attend — for most people, about 1 week will be adequate.
- That fever and associated systemic symptoms of uncomplicated influenza usually resolve after about 1 week, although some symptoms (such as cough and fatigue) may persist for up to 2 weeks after resolution of fever.
- About the symptoms of complicated influenza and to seek medical advice should their condition deteriorate.
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 person is in an 'at risk' group of people who are likely to suffer a worse prognosis from influenza than otherwise healthy people. '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 postpartum), and people with any of the following conditions:
  - Asplenia or dysfunction of the spleen
  - Chronic respiratory disease
  - Chronic heart disease
  - Chronic kidney disease
  - Chronic liver disease
  - Chronic neurological conditions
  - Diabetes mellitus
  - Immunosuppression due to disease or treatment
  - Morbid obesity (body mass index of 40 or more)
- 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.

Prophylaxis is normally not considered in at-risk groups who have been vaccinated against seasonal influenza at least 14 days before exposure.
Prescribe an antiviral drug for post-exposure prophylaxis if all of the following circumstances apply:
- The national surveillance scheme indicates that influenza is circulating.
- The person has been exposed to a person (in the same household or residential setting) with an influenza-like illness.
- The person is in an 'at risk' group and has not been adequately protected by vaccination, that is:
  - They have not been vaccinated since the previous influenza season.
  - The vaccination is not well matched to the circulating strain.
  - There has been less than 14 days between vaccination and date of contact with influenza.
- The person is able to start treatment within 48 hours of this contact (for oseltamivir) or 36 hours of this contact (for zanamivir).
Arrange for the person to receive influenza immunization if this has not already been done in the present influenza season.
Inform the person that although antiviral drugs help prevent influenza, they are not completely effective. Provide advice about symptomatic treatment and when to seek medical attention if influenza develops.

Respiratory complications:

Acute bronchitis
Exacerbations of asthma and chronic obstructive pulmonary disease
Otitis media
Pneumonia — this can be caused by a secondary bacterial infection (particularly with Staphylococcus aureus) or primary viral infection
Sinusitis

Non-respiratory complications include:

Cardiac complications — myocarditis, pericarditis, exacerbation of underlying cardiac disease
Febrile convulsions
Myalgia, myositis and rhabdomyolysis
Neurological complications — Reye's 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.

Influenza Summary

Virus Types

Key Symptoms

At-Risk Groups

Antiviral Treatment

Prevention

Complications