Respiratory viruses are transmitted directly by aerosols or indirectly from
contaminated surfaces. The first site of attack is, not surprisingly, the epithelium
of the nose and throat. Indeed, the hundreds of corona and rhinoviruses
that cause the common cold replicate at 32 to 33oC, the temperature of the
mucosal surface lining the nose. The influenza viruses (Figures 2.4 and 2.8)
infect and replicate in respiratory epithelial cells causing cellular damage.
The generalized symptoms that present, such as muscular aches, malaise and
anorexia, suggest the virus may spread systemically from the respiratory tract
but there is no conclusive evidence for this.
The loss of ciliated and mucus producing epithelial cells impairs clearance of
invading microbes and creates conditions for secondary bacterial infections
of staphylococci, streptococci or Haemophilus influenzae. Bacterial proteases,
for example the V8 protease of Staphylococcus aureus, can enhance the
infectiveness of the influenza virus by improving virus adhesion.
Corynebacterium diphtheriae (Figure 3.7) and Bordetella pertussis (Figure
3.8) are obligate bacterial pathogens. Corynebacterium diphtheriae infects
the nasopharynx and the tonsils and may lead to a lethal systemic infection
affecting the heart, liver and kidneys. Bordetella pertussis is the causative agent
of whooping cough. It adheres to the epithelial cells lining the trachea and
bronchi where it interferes with ciliary action and releases toxins (Chapter 2)
and substances that damage and kill cells and irritate the surface, causing the
characteristic cough. Effective vaccines are available against both organisms
although 40 million infections of whooping cough occur annually worldwide. In
contrast, Streptococcus pneumoniae,Haemophilus influenzae,Staphylococcus
aureus and Moraxella catarrhalis, make up to 60% of the normal bacterial
population of the nasopharyngeal mucous membrane in healthy individuals.
They can become opportunistic pathogens in immunosuppressed individuals
INFECTIONS OF THE RESPIRATORY SYSTEMCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )*
SinusitisConjunctivitisRhinitisEpiglottitisTracheitisPneumoniaOtitisPharyngitisLaryngitisBronchitisFigure 3.6 Selected infections associated with the head and respiratory system. See text for
discussions.
Figure 3.7 Light micrograph of Corynebacterium
diphtheriae.Courtesy of School of Biochemistry and
Microbiology, University of Leeds, UK.