Influenza virus

Background

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

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

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

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

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