Respiratory Tract Infections

Question 1

The common cold

Answer 1

5/7x per year in children, and 2/3x per year in adults.
Colds account for 40% of lost work time.
>200 viral subtypes associated - rhinoviruses are the most common - causes 30-50%.
Human coronaviruses cause 10-15% of common colds.
Transmission: hand contact, droplet transmission
IP 2-3 days; symptoms last 3-10 days & up to 2 weeks in 25%

Question 2

Causes of Symptoms

Answer 2

Bradykinin: intranasal administration of bradykinin causes a sore throat and nasal congestion, due to vasodilation.
Sneezing: mediated by stimulation of trigeminal sensory nerves - histamine mediated.
Nasal discharge: changes colour with increasing number of neutrophils (white->yellow->green) due to myeloperoxidase.
Cough mediated by Vagus nerve - inflammation must extend to larynx to trigger this.
Cytokines responsible for systemic symptoms.

Question 3

Difference between cold and flu

Answer 3

Onset: appears quickly/gradually
Location: nose and throat/whole body
Sense: feel unwell/feel so unwell you can't carry on
Fever: no fever/high fever
flu may have lower resp tract features

Question 4

Influenza

Answer 4

Caused by Influenza A or B.
Swaps hemispheres seasonally.
Uncomplicated - 1-4 days, fever (38-41), cough, myalgia, malaise.
Acutely debilitating

Question 5

Risk Groups and Complications

Answer 5

At risk: immunosuppressed/chronic medical conditions; pregnancy or 2 weeks postpartum; age <2 or >65; BMI: >40
Complications: primary viral pneumonia/secondary bacterial pneumonia, CNS disease, death

Question 6

Pathology of Influenza

Answer 6

The influenza virus haemagglutitin surface protein binds sialic acid on cell surface glycoprotiens/lipids in resp tract.
This allows influenza virus to enter the cell.
The neutaminidase (N_ on the surface of the virus allows the virus to escape by cleaving sialic acid bonds or escaping virions clump together.
The influenza virus has a segmented genome (8 parts) so can reassort if 2 different viruses infect the same cell.

Question 7

Options for treamtent and prevention

Answer 7

Active immunisation - against haemagglutinin and neuraminidase components.
Hand hygiene and droplet precaution.
Tamiflu = oseltamivir - a neuraminidase inhibitor.

Question 8

Pneumonia

Answer 8

Infection of lung parenchyma.
Alveoli full of inflammation - blocks O2 transfer.
Fever, breathlessness, cough, sputum production, hypoxia, increased resp rate, pleuritic chest pain, spesis

Question 9

Pneumonia Radiology

Answer 9

Diagnosis requires infiltrates on CXR.
Consolidation - alveoli and bronchioles completely filled with inflammation - debris/pus/pathogens.
Heart borders of diaphragm obscured (loss of solid-gas interface).
Air bronchograms - air in larger bronchi outlined by surrounding consolidations

Question 10

Streptococcus pneumonia

Answer 10

Most common organism overall.
Gram positive.
Risk factors - alcoholics, resp. disease, smokers, HIV, hyposplenism, CHD.
Acquired in nasopharynx.
Asymptomatic carriage in 40% - smokers > non-smokers.
Prevention: vaccine
Treatment: penicillin

Question 11

Mycoplasma pneumonia

Answer 11

Most common cause of ambulatory atypical pneumonia.
Classically young patient, lasts several weeks.
Extrapulmonary symptoms very common.
Lacks cell wall - penicillin-resistant - can’t grow on lab plates.
Diagnosis by PCR on throat swabs.
Treat with macrolides or tetracyclines).

Question 12

Legionella pneumonia

Answer 12

Sporadic infection or outbreaks associated with contaminated water (nosomical/travel).
Uncommon 350 cases/year in England and Wales.
Can cause severe, life-threatening infection.
Risk Factors: smoking, chronic lung disease.
Diagnosis: don’t grow on routine culture - need specific conditions & longer to grow.
Treatment: macrolides or quinolones.

Question 13

Health care associated pneumonia

Answer 13

New onset at least > 48 hours since admission.
Hospitalisation patients become colonised with hospital bacteria.
May be intrinsically more resistant to antibiotics. Broad spectrum antibiotics used empirically.

Question 14

TB facts and figures

Answer 14

2017: 8 countries accounted for 2/3 of TB.
WHO 2017 estimates 10 million new cases and 1.6 million deaths world-wide.
1/4 world’s population has latent TB.
In UK: 71% cases born outside UK, rates 13x higher than if born in UK; 12% have a social risk factor and 2.5% coinfected with HIV

Question 15

Microbiology of TB

Answer 15

Aerobic bacillus.
Divides every 16-20 hours (slow) - lab extends culture to 56 days.
Cell wall present, but lacks phospholipid outer membrane, so does not stain strongly with Gram stain (weakly positive) - retains stains after treatment with acid.
Referred to as fast bacillus (AFB).
Special stains - Ziehl-Neelsen or auramine rhodamine.

Question 16

Pathology of TB

Answer 16

Infection is initiated by the inhalation of aerosol droplets that contain bacteria.
Initial stages of infection are characterised by innate IR that involve recruitment of inflammatory cells to the lung.
Following bacterial dissemination to the draining lymph node, dendritic cell presentation of bacterial antigens leads to T cell priming and triggers an expansion of antigen specific T cells which are recruited to the lung.
The recruitment of T, B cells, activated macrophages and other leucocytes leads to establishment of granulomas which contain mycobacterium TB