Major pandemics are caused by antigenic shift with significant changes in the neuraminidase and haemagglutinin antigens. Small changes are antigenic drift
About
- Orthomyxoviruses - Influenza A, B, C
- Most extensive and severe outbreaks are due to Influenza A
- Influenza B causes less severe pandemics and infections
Spread
- Influenza is highly infectious by respiratory droplet spray from the
earliest stages of infection.
Aetiology
- Virus has surface Haemagglutinin (H) and Neuraminidase (N) antigens
- Haemagglutinin binds erythrocytes and initiates infection
- Neuraminidase cleaves Haemagglutinin and allows release of virus from cells
Virology
- Antigen shift - a change to Haemagglutinin or Neuraminidase antigen and causes pandemics
- Antigen drift - minor changes in Haemagglutinin
- Avian Influenza - Influenza A H5N1 strain seen initially in SE Asia. Caused a more severe disease with pneumonia. Use Neuraminidase inhibitors
- Swine flu - Influenza A. H1N1 - began in Mexico. Use Neuraminidase inhibitors
Clinical
- Fever, malaise, rigors, cough, respiratory symptoms
- Main morbidity and mortality from Viral and secondary bacterial pneumonia
Complications
- Myositis
- Guillain-Barre Syndrome
- Myocarditis
- Pericarditis
- Pneumococcal and staphylococcal pneumonia
- Transverse myelitis
- Encephalitis
Investigations
- RNA reverse transcription PCR of nasopharyngeal swab
- CXR if pneumonia suspected
Management
- Amantadine has been superseded by Neuraminidase inhibitors
- Neuraminidase inhibitors - Oseltamivir 75 mg bd for 5 days or Inhaled Zanamivir inhibit neuraminidase and work against Influenza A and B but must be given within 48 hours
- Prevention by seasonal vaccination of those at high risk