The clinical setting is a useful guide to the causal organism and hence to the 'best guess' early choice of antimicrobial, although in seriously ill patients cover for both 'typical' and 'atypical' pathogens should be included from the beginning. It is not possible reliably to differentiate between pneumonias caused by 'typical' and 'atypical' pathogens on clinical grounds alone.
Pneumonia in previously healthy people
Disease that is segmental or lobar in its distribution is usually caused by Streptococcus pneumoniae (pneumococcus). Haemophilus influenzae is a rare cause in this group, although it more often leads to exacerbations of chronic bronchitis and does cause pneumonia in patients infected with HIV. Benzyl-penicillin i.v. or amoxicillin p.o. are the treatments of choice if pneumococcal pneumonia is very likely; alternatively, use erythromycin/clarithromycin in a penicillin-allergic patient. Seriously ill patients are best given benzylpenicillin (to cover the pneumococcus) plus ciprofloxacin (to cover Haemophilus and 'atypical' pathogens). Where penicillin-resistant pneumococci are prevalent, i.v. cefotaxime is a reasonable 'best guess' choice.
Pneumonia following influenza is often caused by Staphylococcus aureus, and 'best guess' therapy is usually achieved by adding flucloxacillin to one of the regimens above. When staphylococcal pneumonia is proven, sodium fusidate p.o. plus flucloxacillin i.v. should be used in combination.
'Atypical' cases of pneumonia may be caused by Mycoplasma pneumoniae which may be epidemic, or more rarely Chlamydia pneumoniae or psittaci (psitta-cosis/ornithosis) Legionella pneumophilia or Coxiella burnetii (Q fever) and a tetracycline or erythromycin/ clarithromycin should be given by mouth. Treatment of ornithosis should continue for 10 days after the fever has settled and in mycoplasma pneumonia and Q fever a total of 3 weeks treatment may be needed to prevent relapse.
At the earliest possible stage, once a clinical improvement has been seen, initial i.v. administration of antibiotics for pneumonia should be switched to the oral route.
Pneumonia acquired in hospital
(Greek: nosokomeian, hospital) if it presents after at least 2 days in hospital. It occurs primarily among patients admitted with medical problems or recovering from abdominal or thoracic surgery or on mechanical ventilators. The common pathogens are Staphylococcus aureus, Enterobacteriaceae, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Haemophilus influenzae. It is reasonable to initiate therapy with ciprofloxacin, meropenem or ceftazidime (plus vancomycin if the local prevalence of MRSA is high) until the results of sputum culture and antimicrobial susceptibility tests are known.
Normal commensals of the upper respiratory tract proliferate in damaged lungs especially following viral infections, pulmonary congestion or pulmonary infarction. Mixed infection is therefore common, and since Haemophilus influenzae and Streptococcus pneumoniae are often the pathogens, amoxicillin or trimethoprim are reasonable choices, but if response is inadequate co-amoxiclav or a quinolone should be substituted.
Klebsiella pneumoniae rarely causes lung infection ('Friedlander's pneumonia') in the alcoholic and debilitated elderly. Abscesses form, particularly in the upper lobes: cefotaxime possibly with an aminoglycoside is recommended.
Moraxella (previously Branhamella) catarrhalis, a commensal of the oropharynx, may be a pathogen in patients with chronic bronchitis; because many strains produce ß-lactamase, co-amoxiclav or erythromycin/clarithromycin should be used.
Pneumonia in immunocompromised patients
Pneumonia is common, e.g. in acquired immunodeficiency syndrome (AIDS) or in those who are receiving immunosuppressive drugs.
Common pathogenic bacteria may be responsible (Staphylococcus aureus, Streptococcus pneumoniae) but often organisms of lower natural virulence (Enterobacteriaceae, viruses, fungi) are causal and strenuous efforts should be made to identify the microbe including, if feasible, bronchial washings or lung biopsy.
• Until the pathogen is known the patient should receive broad-spectrum antimicrobial treatment, such as an aminoglycoside plus ceftazidime.
• Aerobic Gram-negative bacilli, e.g. Enterobacteriaceae, Klebsiella spp., are pathogens in half of the cases, especially in neutropenic patients, and respond to cefotaxime or ceftazidime. Pseudomonas aeruginosa may also cause pneumonia in these patients; for treatment see Reference data on antimicrobial drugs of choice, page 211, Table 11.1.
• An important respiratory pathogen in patients with deficits in cell-mediated immunity is the fungus Pneumocystis carinii, which should be treated with co-trimoxazole 120 mg/kg/d by mouth or i.v. in 2-4 divided doses for 14 days, or with pentamidine (see p. 276).
Legionella pneumophila responds to erythromycin
2-4 g/d i.v. in divided doses but rifampicin may be added in more severe infections. Ciprofloxacin is also effective.
Pneumonia is often caused by aspiration of material from the oropharynx, or due to the presence of other lung pathology such as pulmonary infarction or bronchogenic carcinoma. As well as conventional microbial causes, the pathogens include anaerobic and aerobic streptococci, Bacteroides spp. and Fuso-bacterium, and the diagnosis may be missed unless anaerobic cultures of fresh material are performed. Treatment for several weeks with cefuroxime plus metronidazole may be needed to prevent relapse.
Pulmonary abscess is treated according to the organism identified and with surgery if necessary.
Empyema is treated according to the organism isolated and with aspiration and drainage.
When suspicion is high enough, three blood cultures should be taken over a few hours and antimicrobial treatment commenced; it can be adjusted later in the light of the results. Delay in treating only exposes the patient to the risk of grave cardiac damage or systemic embolism. Streptococci, enterococci and staphylococci are causal in 80% of cases, with viridans group streptococci the most common pathogens. In intravenous drug users, Staphylococcus aureus is the most likely organism. Culture-negative endocarditis (up to 20% of cases) is usually due to prior antimicrobial therapy or to special culture requirements of the microbe; it is best regarded as being due to streptococci and treated accordingly.
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