Respiratory Tract Infections

RESPIRATORY TRACT INFECTIONS

Infections of the respiratory tract are among the commonest of infections, and account for much consultation in general practice and a high percentage of acute hospital admissions. They are divided into infections of the upper respiratory tract, involving the ears, throat, nasal sinuses and the trachea, and the lower respiratory tract (LRT), where they affect the airways, lungs and pleura.

a)   Upper Respiratory Tract Infections

Acute pharyngitis presents a diagnostic and therapeutic dilemma. The majority of sore throats are caused by a variety of viruses; fewer than 20% are bacterial and hence potentially responsive to antibiotic therapy. However, antibiotics are widely prescribed and this reflects the difficulty in discriminating streptococcal from non-streptococcal infections clinically in the absence of microbiological documentation. Nonetheless, Strep. pyogenes is the most important bacterial pathogen and this responds to oral penicillin. However, up to 10 days’ treatment is required for its eradication from the throat. This requirement causes problems with compliance as symptomatic improvement generally occurs within 2–3 days.

Although viral infections are important causes of both otitis media and sinusitis, they are generally self limiting. Bacterial infections may complicate viral illnesses, and are also primary causes of ear and sinus infections. Streptococcus pneumoniae and Haemophilus influenzae are the commonest bacterial pathogens. Amoxicillin is widely prescribed for these infections as it is microbiologically active, penetrates the middle ear and sinuses, is well tolerated and has proved effective.

b)   Lower Respiratory Tract Infections

Infections of the LRT include pneumonia, lung abscess, bronchitis, bronchiectasis and infective complications of cystic fibrosis. Each presents a specific diagnostic and therapeutic challenge, which reflects the variety of pathogens involved and the frequent difficulties in establishing an accurate microbial diagnosis. The laboratory diagnosis of LRT infections is largely dependent upon culturing sputum. Unfortunately this may be contaminated with the normal bacterial flora of the upper respiratory tract during expectoration. In hospitalized patients, the empirical use of antibiotics before admission substantially diminishes the value of sputum culture and may result in overgrowth by non-pathogenic microbes, thus causing difficulty with the interpretation of sputum culture results. Alternative diagnostic samples include needle aspiration of sputum directly from the trachea or of fluid within the pleural cavity. Blood may also be cultured and serum examined for antibody responses or microbial antigens. In the community, few patients will have their LRT infection diagnosed microbiologically and the choice of antibiotic is based on clinical diagnosis.

i)                    Pneumonia

The range of pathogens causing acute pneumonia includes viruses, bacteria and, in the immuno-compromised host, parasites and fungi. Table 14.2 summarizes these pathogens and indicates drugs appropriate for their treatment. Clinical assessment includes details of the evolution of the infection, any evidence of a recent viral infection, the age of the patient and risk factors such as corticosteroid therapy or pre-existing lung disease. The extent of the pneumonia, as assessed clinically or by X ray, is also important.

Streptococcus pneumoniae remains the commonest cause of pneumonia and still responds well to penicillin despite a global increase in isolates showing reduced susceptibility to this agent. So called ‘respiratory quinolones’ such as levofloxacin and moxifloxacin, which exhibit increased activity against Gram positive organisms compared to ciprofloxacin, are an alternative. A number of atypical infections may cause pneumonia and include Mycoplasma pneumoniae, Legionella pneumophila, psittacosis and occasionally Q fever. With psittacosis there may be a history of contact with parrots or budgerigars; while legionnaires’ disease has often been acquired during hotel holidays in the Mediterranean area. The atypical pneumonias, unlike pneumococcal pneumonia, do not respond to penicillin. Legionnaires’ disease is treated with erythromycin and, in the presence of severe pneumonia, rifampicin is added to the regimen. Mycoplasma infections are best treated with either erythromycin or tetracycline, while the latter drug is indicated for both psittacosis and Q fever.

ii)                 Lung abscess

Destruction of lung tissue may lead to abscess formation and is a feature of aerobic Gram-negative bacillary and Staph. aureus infections. In addition, aspiration of oropharyngeal secretion can lead to chronic low grade sepsis with abscess formation and the expectoration of foul smelling sputum that characterizes anaerobic sepsis. The latter condition responds to high dose penicillin, which is active against most of the normal oropharyngeal flora, while metronidazole may be appropriate for strictly anaerobic infections. In the case of aerobic Gram-negative bacillary sepsis, aminoglycosides, with or without a broad-spectrum cephalosporin, are the agents of choice. Acute staphylococcal pneumonia is an extremely serious infection and requires treatment with high dose flucloxacillin alone or in combination with fusidic acid.

iii)              Cystic fibrosis

Cystic fibrosis is a multisystem congenital abnormality that often affects the lungs and results in recurrent infections, initially with Staph. aureus, subsequently with H. influenzae and eventually leads on to recurrent Pseudomonas aeruginosa infection. The last organism is associated with copious quantities of purulent sputum that are extremely difficult to expectorate. Ps. aeruginosa is a co factor in the progressive lung damage that is eventually fatal in these patients. Repeated courses of antibiotics are prescribed and although they have improved the quality and longevity of life, infections caused by Ps. aeruginosa are difficult to treat and require repeated hospitalization and administration of parenteral antibiotics such as an aminoglycoside, either alone or in combination with an antipseudomonal penicillin or cephalosporin. The dose of aminoglycosides tolerated by these patients is often higher than in normal individuals and is associated with larger volumes of distribution for these and other agents. Some benefit may also be obtained from inhaled aerosolized antibiotics. Unfortunately drug resistance may emerge and makes drug selection more dependent upon laboratory guidance.