There are three basic types of influenza: A, B and C. Type A is the most pathogenic, influenza B is exclusively human, while A and C can infect multiple species, including birds and pigs.
The incidence of influenza increases during winter months. There is no gender bias and age is not a risk factor for acquiring disease, although transmission is higher in closed environments such as schools.
Influenza is transmitted through respiratory droplets and contact with infected surfaces. The virus invades airway epithelium and respiratory tract cells. After an incubation period of 18-72 hours, symptoms begin.
The patient is infectious from one to two days before and up to a week after symptoms start to develop.
Symptoms include pyrexia, although this can be absent in the elderly, myalgia, fatigue and tachycardia. Rarely, myositis can be severe enough to cause rhabdomyolysis.
There is usually a cough, which is initially minimal but progressive. It is not productive unless complicated by secondary infection.
Other symptoms include rhinitis, myocarditis, arrhythmias, frontal headache, ocular pain on eye movement and diarrhoea in children.
Haemoptysis and respiratory distress are not symptoms of normal circulating flu strains and should alert the clinician to possible infection with the H5N1 avian influenza virus.
Morbidity and mortality
The major cause of mortality is the exacerbation of underlying chronic disease with 85 per cent of mortality occurring in patients aged over 65 years.
Secondary bacterial pneumonias with Streptococcus pneumoniae and Haemophilus influenzae can occur two to three weeks post infection.
Other complications can include acute encephalopathy with coma, seizure and ataxia.
The mortality rate in patients aged 45-60 years is one in 5,000, while in those aged over 65 with two or more high-risk conditions, mortality dramatically increases to one in 125.
Diagnosis is clinical. Studies have shown that in the presence of locally circulating influenza the combination of acute onset of fever and cough within 48 hours has a positive predictive value of around 80 per cent. This drops to 4 per cent when influenza is not circulating.
A wide range of tests exist, including viral cell culture, which is the gold standard, and enzyme-based tests. None are clinically cost-effective and where prevalence rates are high a clinical diagnosis has an equivalent accuracy to the rapid enzyme antigen test.
Management includes fluid replacement, analgesia and pyrexia control, treatment of any secondary complications and antiviral agents.
Older antiviral agents such as amantadine are no longer recommended. Newer agents such as zanamivir or oseltamivir are recommended for use when influenza is circulating in the community.
These drugs reduce symptom duration by 1.5 days, decrease severity of symptoms by 40 per cent and exert a prophylactic effect, decreasing infectivity from 20 per cent to 4 per cent in family members.
The greatest effect is achieved if given within six hours of the onset of symptoms; effectiveness after 40 hours is minimal.
Post-exposure prophylaxis is recommended in non-vaccinated at-risk adults and children aged over one year. Prophylaxis or treatment with antiviral medication for the healthy is not a NICE recommendation.
The method of flu prevention is vaccination. The vaccine is changed yearly by the WHO.
A vaccine for the H5N1 strain is under development.
Influenza epidemics and pandemics occur on a cyclical basis. Experts agree that the next flu pandemic is imminent. GP practices are expected to have their own contingency plan.
Dr Hashmi is a GP and part-time tutor at St George's Hospital, south London
- Pyrexia, myalgia and a progressive cough are the main symptoms of influenza.
- Haemoptysis and respiratory distress are not symptoms of normal circulating flu strains and infection with H5N1 avian flu should be considered.
- When flu is circulating, acute onset of fever and cough within 48 hours has a positive predictive value of around 80 per cent.
- Eighty-five per cent of mortality associated with influenza occurs in patients aged over 65 years.
- NICE recommends antiviral treatment for infected patients, and antiviral prophylaxis for at-risk patients exposed to infection.