Influenza

Question 1

What are the general features of the flu virus? How are they classified/typed?

Answer 1

8 segment genome RNA virus of orthomyxoviridae family
Three main types - A (mammals and birds), B and C (mostly only humans) (also D - cattle)

A are classified by H/N:
18 different H antigens - haemagglutinin (facilitates viral attachment/entry to host cell)
11 different N antigens - neuraminidase (enables new virion to be released from host cell)

Question 2

What is the significance of antigenic drift and shift?

Answer 2

Drift

• Continual variation of the virus over time via ongoing point mutations in antibody-binding sites
• may change antigenic properties and eventually immune system will be less effective
• causes vaccine mismatch and worse epidemics

Shift

• abrupt major change in the virus
• this is what allows a strain to jump from one species to another
• the process by which two or more different strains combine to form a new H/N subtype
• re-assortment of virus gene segments
• new antigenic properties mean population is unprotected and at risk of pandemics

Question 3

Which types of flu are responsible for epidemic/pandemic outbreaks? How many people may be affected and how serious are the individual infections?

Answer 3

A - responsible for pandemics - can affect 25%+ of population and more serious an infection
- H1N1 responsible for last pandemic (2009)

A and B - responsible for seasonal (epidemic) outbreaks - affects 10-15% of population, not usually life-threatening
- avian flu (H5N1 Egypt, H7N9 China) current outbreaks

Question 4

How does influenza generally present - in terms of the most common symptoms, and other symptoms frequently seen? How long is the incubation?

Answer 4

Abrupt fever up to 41c lasting ~3 days
With 2 or more of:
- cough (/sore throat/rhinorrhoea)
- myalgia
- headache
- malaise

Systemic symptoms predominant

Other symptoms

• nausea
• vomiting
• diarrhoea

Incubation ~2-4 days

Question 5

What is an ‘Influenza-like illness’?

Answer 5

Fever >38 and Cough with onset in last 10 days

- defined as severe acute respiratory infection (SARI) if requires hospitalisation

Question 6

What are some common complications of flu?

Answer 6

Respiratory

• acute bronchitis
• secondary bacterial pneumonia (appears 4-5 days start of flu)

Question 7

What are some less common complications of flu?

Answer 7

Respiratory
- primary viral pneumonia - common in avian, rapid resp failure, mortality >40% within 7 days

Cardiac
- myocarditis/pericarditis

CNS

• transverse myelitis/Guillain-Barre
• Myositis and myoglobinuria

Question 8

Who are more at risk of complications?

Answer 8

Neurological, hepatic, renal, pulmonary and chronic cardiac disease
Diabetes Mellitus
Severe immunosuppression
Age >65
Pregnancy
Children <6month
BMI 40+

Question 9

How does influenza spread? When are individuals likely to infect others and how long does the virus survive on surfaces?

Answer 9

Airborne: person to person by large droplets >5 microns
Contact: direct (person to person) or indirect (person to person via fomite)

Virus shedding in first ~4 days of illness

Virus survival - 24-48hrs on non-porous, 8-12 on porous surfaces e.g. tissue

Question 10

How is influenza diagnosed, including investigations you may consider to understand the severity of the infection

Answer 10

Generally clinical diagnosis - ‘febrile respiratory illness during a known seasonal influenza outbreak’

Viral nose and throat swabs - viral culture or rt-PCR
CXR - pneumonitis/pneumonia/ARDS - urgent if symptoms and fever >4 days
Blood culture
Pulse oximetry - need O2 and ABG if <92
Respiratory rate
U+Es, FBC, CRP (should half in four days if pneumonia recovering)

Question 11

What is severity assessment for pneumonia? How does it relate to mortality?

Answer 11

CURB65

Confusion
Urea >7mM
Resp rate >30
BP - systolic <90 or diastolic <60
>65 years old

If 5/5, risk of death 57% in next 30 days
Big jump in mortality between 3/5 and 4/5 (17% vs 41%)

Question 12

Who gets treatment for influenza, and what/when would the treatment be?

Answer 12

Those with complications (hospitalised, and/or LRTI/CNS/exacerbation of other illnesses),
OR those in an at risk group for complications
- different if severely immunosuppressed

Oseltamivir first line
Zanamivir second line
Use ASAP and within 48 hours of symptom onset
- always give if complicated
- both are neuriminidase inhibitors

Consider oseltamivir resistance in immunosuppressed

Other treatments

• peramivir
• favipiravir

Question 13

When does an individual become non-infectious?

Answer 13

24 hours after last symptom (fever and cough) or when finished antivirals (whichever is longer)
- if immunocompetent, otherwise consider on an individual basis

Question 14

How is the flu vaccine prepared? Why should healthcare workers should have vaccinations?

Answer 14

Prepared each year using viruses considered most likely to be circulating in the forthcoming winter

Trivalent

• 2 type A
• 1 type B

Healthcare workers

• to protect themselves and their families
• to reduce the risk to ‘at risk’ patients
• to reduce absence from work during influenza ‘surge’ activity