Microbiology
Pathogens
A 45 year old, with a history of chronic alcohol abuse, attends ED complaining of a 10 day history of fever, persistent cough and shortness of breath associated with vomiting and watery diarrhoea. Chest x-ray shows patchy alveolar infiltrates. You have recently completed an infectious diseases diploma and you are concerned about the possibility of Legionnaires disease. Which of the following clinical features does NOT fit with your suspected diagnosis.
Answer:
Rose spots describes the rash that occurs in up to 30% of people infected with typhoid fever caused by Salmonella typhi. The rash is characterised by groups of pink papules appearing mainly on the anterior chest and thought to be bacterial emboli to the skin. Legionnaires disease is characterised by a dry cough, progressive dyspnoea and a high fever, associated with systemic features such as nausea and vomiting, diarrhoea, headache, malaise and confusion. Hyponatraemia is a common finding in Legionella pneumonia. Rapid diagnosis can be made with a positive Legionella urinary antigen test. Culture will demonstrate Gram-negative rods.Legionella Pneumophila
Microbiology / Pathogens / Legionella
Last Updated: 21st April 2019
| Microorganism | Legionella Pneumophila |
|---|---|
| Gram stain | Gram negative |
| Shape | Rod (coccobacilli) |
| Oxygen requirements | Obligate aerobes |
| Additional features | Fastidious organisms |
| Reservoir | Contaminated water and air-conditioning units |
| Transmission | Aerosol |
| Disease | Pneumonia |
Legionella spp. are obligate aerobic, water-borne Gram-negative coccobacilli. Legionella pneumophila is the most frequent pathogen implicated in human disease, causing about 90% of pneumonia attributed to Legionella spp.
Transmission
Legionella spp. are predominantly found in environmental water sources such as warm springs, domestic water supplies, water-cooling towers, air-conditioning systems, swimming pools and hot tubs. Multiplication typically occurs at temperatures between 20 - 40°C. Transmission is via inhalation of aerosols generated from, for example, showers, taps and air-conditioning systems; person-to-person spread does not typically occur.
Host risk factors include:
- male patients
- older patients
- previous lung disease
- smoking
- high alcohol intake
- immunosuppression
Clinical Disease
Clinical features may vary in severity and include:
- Pontiac fever
- mild upper respiratory tract influenza-like illness
- Legionnaires disease
- severe pneumonia with high mortality
- presents with a dry cough, progressive dyspnoea and a high fever
- associated with systemic features such as nausea and vomiting, diarrhoea, headache, malaise, confusion and renal failure
- N.B. hyponatraemia is an important laboratory finding that occurs more often in Legionella pneumonia than in pneumonia caused by other bacteria
Diagnosis
Diagnosis may be by:
- Legionella antigen detection in urine (a rapid and specific method)
- Legionella culture of sputum or bronchoalveolar fluid
- Direct immunofluorescence of respiratory specimens
- Serology (Legionella antibodies take at least 8 days to develop - only really useful for retrospective diagnosis)
- Blood cultures for bacteraemia
Treatment
For low and moderate severity community acquired Legionella pneumonia, an oral fluoroquinolone or macrolide is recommended. For the management of high severity or life threatening Legionella pneumonia, both a quinolone and macrolide may be used together, or rifampicin may be added in for the first few days. Suggested duration of treatment is usually 7 - 10 days.
Prevention
Legionellosis is a notifiable disease. Legionellosis is prevented by adequate maintenance of air conditioning systems and ensuring hot-water supplies are above 45°C to prevent multiplication.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
