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
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
