Microbiology
Pathogens
A patient who has recently returned from travelling to South America, presents to ED with a fever and a 'rose spot' rash shown below. He has been constipated over the past few days but today has developed diarrhoea. On examination you note splenomegaly. Which of the following pathogens is most likely responsible:
![By CDC/Armed Forces Institute of Pathology, Charles N. Farmer [Public domain], via Wikimedia Commons](https://primary-cdn.frcemsuccess.com/wp-content/uploads/2017/02/Rose-spots.jpg)
(Image by CDC/Armed Forces Institute of Pathology, Charles N. Farmer [Public domain], via Wikimedia Commons)
Answer:
The patient most likely has typhoid fever is caused by Salmonella typhi, a serotype of Salmonella enterica.Salmonella Enterica
Microbiology / Pathogens / Klebsiella and Enterobacteriae
Last Updated: 21st April 2019
| Microorganism | Salmonella Enterica |
|---|---|
| Gram stain | Gram negative |
| Shape | Rod |
| Oxygen requirements | Facultative anaerobe |
| Oxidase | Negative |
| Reservoir | Cattle, pigs, poultry, humans |
| Transmission | Faecal-oral route |
| Diseases | Food poisoning, enteric fever, septicaemia, osteomyelitis |
Salmonella species are facultatively anaerobic Gram-negative bacilli. Salmonella enterica is the primary pathogen in humans, and has a number of pathogenic serovars (serotypes) including Salmonella typhi (responsible for typhoid fever).
Transmission
Salmonella enterica is a commensal of animal gastrointestinal tracts. Transmission is primarily by the faecal-oral route, usually from ingestion of contaminated foods e.g. poultry, eggs, undercooked meat, milk or water.
Patients at risk of infection include those with reduced gastric acid secretion (for example following gastrectomy, or the use of acid-suppressing drugs) and those who are immunocompromised or asplenic.
Clinical Disease
There are three main types of Salmonella infections: enterocolitis, enteric fever (such as typhoid fever) and septicaemia with metastatic infections.
Enterocolitis:
Enterocolitis is characterised by invasion of the epithelial and subepithelial tissue of the small and large intestines, usually presenting as an acute self-limiting gastroenteritis with nausea/vomiting, abdominal pain, fever and diarrhoea, following an incubation period of about 12 - 48 hours. Usually the disease only lasts a few days and does not require medical care but fluid and electrolyte replacement may be required.
Enteric fever:
In enteric fever, infection begins in the small intestine but few gastrointestinal symptoms occur. Enteric fever is characterised by invasion of the intestinal wall, with spread to local lymph nodes and infection of the reticuloendothelial system and gallbladder, where the bacteria continue to multiply before passing into the blood causing a bacteraemia (associated with the onset of fever and other symptoms). Patients may present with fever, systemic upset, abdominal pain, alteration of bowel habit (both constipation and diarrhoea may occur), splenomegaly and a rash of 'rose spots'.
From the gallbladder, a further invasion of the intestine occurs; the bacteria multiply in the mononuclear phagocytes of Peyer's patches and cause ulceration that may be complicated by haemorrhage or perforation. Some patients may become chronic carriers, with gallbladder and bowel colonisation, with ongoing excretion of bacteria in the faeces for a long time. Cholecystectomy may be necessary to abolish the chronic carrier state.
Septicaemia:
Septicaemia accounts for only about 5 - 10% of Salmonella infections and typically occurs in patients with an underlying chronic disease (e.g. malignancy, sickle cell disease) or in a child with enterocolitis. Bacteraemia results in the seeding of many organs, with osteomyelitis, pneumonia and meningitis as the most common sequelae.
Treatment
Salmonella enterocolitis is usually self-limiting and does not require treatment. In severe or invasive infection, first line treatment is with ciprofloxacin or cefotaxime.
Infections from Middle-East, South Asia, and South-East Asia may be multiple-antibacterial-resistant and sensitivity should be tested. First line treatment of typhoid fever is with cefotaxime (or ceftriaxone). Azithromycin is an alternative in mild or moderate disease caused by multiresistant organisms. Ciprofloxacin is an alternative if the microorganism is sensitive.
Prevention
Salmonellosis is a notifiable disease and an investigation to determine its source should be undertaken.
Prevention of infection can usually be achieved with public health and personal hygiene measures: clean water, good animal husbandry and abattoir practices, proper sewage treatment, sanitation and hygienic handling of foodstuffs, pasteurisation of milk and proper cooking of poultry, eggs and meat.
A vaccination is available against Salmonella typhi, which should be given to individuals who are travelling to high-risk areas (Asia, Africa, Central and South America).
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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 |
