INTRODUCTION TO UPPER RESPIRATORY TRACT DISEASES

General Goal: To know the major mechanisms of defense in the URT, the major mechanisms invaders use to avoid the defenses of the LRT, the common modes of transmission and the most common microbes that infect a particular location of the respiratory tract.

Specific Educational Objectives: The student should be able to:

1. describe defense mechanisms the body uses to protect itself from infections.

2. identify the microbes normally found in the respiratory tract (normal flora).

4. the mechanisms microbes use to infect the respiratory tract.

5. The common microbial pathogens and the locations they usually infect.

Reading: Mosby's Color Atlas and Text of Infectious Diseases by Christopher P. Conlon and David R. Snydman. pp. 53-67.

Lecture: Dr. Neal R. Chamberlain

References: 

I. The respiratory tract is the most common site for infection by pathogens.

A. This site becomes infected frequently because it comes into direct contact with the physical environment and is exposed to microorganisms in the air.

B. The human respiratory tract is exposed to many potential pathogens via the smoke, soot, and dust that is inhaled from the air. It has been calculated that the average individual ingests about 8 microorganisms per minute or 10,000 per day.

A. The nasal cavity has a mucociliary lining similar to that of the lower respiratory tract. The inside of the nose is lined with hair which act to filter larger particles which are inhaled. The turbinate bones ("baffle plates") are covered with mucus which collect particles not filtered by nasal hairs. Usually, particles 5-10 µm in diameter are either trapped by nasal hairs, or impinged on the nasal mucosal surfaces.

B. The change in direction of the airway from the sinuses to the pharynx causes a large number of larger particles to impinge on the back of the throat. The adenoids and tonsils are lymphoid organs in the upper respiratory tract that are quite important in developing an immune response to pathogens.

C. A layer of mucus and ciliated cells covers the lower portion of the lower respiratory tract. Mucus is secreted by both single and subepithelial mucus-secreting cells.

D. Particles or respiratory pathogens which reach the lower respiratory tract are first trapped in the mucus layer and are driven upwards by ciliary action (the ciliary elevator) to the back of the throat.

E. There are two main obstacles a bacterium or virus must overcome in order to initiate an infection in the respiratory tract.

1. The microorganism must avoid being caught up in the mucus layers of the upper respiratory tract, being transported to the back of the throat, and eventually being swallowed.
2. If the invader has avoided the physical defense mechanisms of the upper respiratory tract, and is deposited in the lower respiratory tract or lung, it must either avoid phagocytosis, or be able to survive and multiply in the phagocytic cell.

A. Most of the surfaces of the upper respiratory tract (including nasal and oral passages, nasopharynx, oropharynx, and trachea) are colonized by normal flora. These organisms are usually regular inhabitants of these surfaces and rarely cause disease. The regular inhabitants of the nose include:
 
1. The most common bacteria found in the nose are staphylococci. These organisms are found just inside the nares and include Staphylococcus aureus and S. epidermidis.
2. In addition to the staphylococci, aerobic corynebacteria ("diphtheroids") can be cultured from the nasal surfaces.
3. Small numbers of Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae can also be found in the nasopharynx. However, most of these strains are not encapsulated or virulent. It should be noted that nonencapsulated, nontypable H. influenzae has been shown to play a significant role in the pathogenesis of otitis media.

 
B. The normal flora of the oropharynx also contains a large number of regular bacterial inhabitants.
1.  The nose and the oropharynx contains large numbers of S. aureus and S. epidermidis.
2.  The most important group of microorganisms native to this body niche are the alpha-hemolytic streptococci or viridans streptococci. This group includes S. mitis, S. mutans, S. milleri, and S. salivarius. It is believed that these bacteria act as antagonists against invasion by pathogenic streptococci.
3.  Additionally, cultures from this region usually show large numbers of diphtheroids, Moraxella (formerly Branhamella) catarrhalis, and small Gram-negative cocci related to Neisseria species.


C. The normal flora of these areas have two main functions that play a role in maintaining the healthy state of the host.

1. The organisms compete with pathogenic organisms for potential attachment sites.
2. These organisms often produce substances (toxins or acids) which are bactericidal.

A. Before a respiratory disease can be established, the following conditions need to be met.
1. There must be a sufficient number or sufficient "dose" of infectious agent inhaled.
2. The infectious particles must be airborne.
3. The infectious organism must remain alive and viable while in the air.
4. The organism must be deposited on susceptible tissue in the host.

B. Once a respiratory tract pathogen is in the respiratory tract, it is essential that it colonize these surfaces before it can cause obvious disease. Most microorganisms cause disease by only a few pathogenic mechanisms. A few of these mechanisms, especially those used by respiratory tract pathogens are discussed below.

1. Bacterial adherence factors = F and M proteins of Strep. pyogenes, Hemagglutinins of B. pertussis.
2. Extracellular toxins = diphtheria toxin; pertussis toxin.
3. Growth in host tissue = viruses, chlamydia sp.
4. Evasion of host defense mechanism = capsules of Strep. pyogenes (also M protein), S. pneumoniae and H. influenzae by inhibiting phagocytosis.

1. Viruses = Rhinoviruses, RSV, Adenoviruses, Influenza, Parainfluenza
2. Group A streptococci = pharyngitis
3. Other streptococci = S. pneumoniae = sinusitis, Group B = pneumonia of infants
4. Other microorganisms = C. diphtheriae, M. pneumoniae, Fungi Parasites

1. Common cold = mostly viruses
2. Acute otitis media = Dr. Tritz has mentioned.
3. Sinusitis = Bacteria = S. pneumoniae, H. influenza
4. Pharyngitis = 90% viruses, important bacteria = S. pyogenes and C. diphtheriae

VIII. Parenchymal Lung Disease Pneumonia = large number of bacterial infections in adults