Influenza virus

Background

Virology

• Enveloped, single-stranded, negative-sense RNA virus in the Orthomyxoviridae family
• Four genera corresponding to influenza A through D
  • A infects humans, pigs, horses, birds, and marine mammals
    • Most prone to antigenic shift
    • Most likely to cause epidemics and pandemics
    • Typed by hemagglutinin (HA) (H1, H2, H3) and neuraminidase (NA) (N1, N2) proteins
      • H1N1 and H3N2 are the two types that have caused the larges outbreaks in recent decades
  • B infects humans alone
    • Two major lineages, B/Victoria and B/Yamagata
  • C infects humans and pigs, and causes milk illness
  • D infects cattle alone, as far as we know

Epidemiology

• Worldwide
• Seasonal in temperate climates, lasting from fall through spring and peaking in late winter
• Birds are the main reservoir for influenza A
• Influenza mutations are classified as antigenic drift (small mutations) and antigenic shift (large changes due to reassortment)
  • Antigenic drift usually results in local outbreaks, while antigenic shift can result in epidemic or pandemics
  • Antigenic shift only really happens in influenza A, due to the reassortment that can happen in its multiple hosts
• In Canada, kills around 3500 people annually
• Thought to be infectious 1 day before to 5 days after symptom onset

Clinical Manifestations

• Incubation period of 1 to 2 days (range 1 to 4 days)
• Uncomplicated disease causes fevers, chills, rigors, headaches, myalgia, malaise, and anorexia
  • Often abrupt onset
  • The myalgias and headache are often quite notable
  • Can also have pain with ocular abduction, arthralgia without arthritis, tearing and burning in the eyes
  • Fever and other systemic symptoms typically lasts 3 days (range 4 to 8 days)
  • More prominence of systemic symptoms than many other respiratory infection
  • Respiratory symptoms can include non-productive cough, sore throat, and nasal symptoms
    • Cough and some of the other symptoms may persist after the systemic symptoms improve
• Viral pneumonia starts with typical influenza, followed by cough, dyspnea, cyanosis, and hypoxemia
  • Chest x-ray may show diffuse bilateral disease
• Secondary bacterial pneumonia starts with typical pneumonia, followed by a few days to 2 weeks of improvement before reemergence of fever and respiratory symptoms
  • However, it can overlap with the viral infection
• Extrapulmonary manifestations include myositis (mostly in children but can happen in adults), myocarditis, pericarditis, Guillain-Barré syndrome (influenza A), transverse myelitis, acute disseminated encephalomyelitis, encephalitis, Reye syndrome

Management

• Neuraminidase inhibitors started within 48 hours of symptom onset are associated with a shorter duration of illness
  • Oseltamivir
  • Zanamavir
  • Peramivir is IV only
• Monitor for secondary bacterial pneumonia

Prevention

• Annual influenza vaccination is recommended for everyone 6 months of age and older
  • Inactivate influenza vaccines (Fluzone, Fluviral, Influvac, Fluad), recombinant influenza vaccines, and live attenuated influenza vaccines (FluMist)
  • Egg-allergic patients can still be vaccinated using any influenza vaccine, including egg-based vaccines
• Chemoprophylaxis with oseltamivir may be indicated as post-exposure prophylaxis or during local outbreaks for all people at risk