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Rat bite fever: an unusual cause of a maculopapular rash
  1. Andrew Rosser1,
  2. Martin Wiselka1,
  3. Manish Pareek2
  1. 1Department of Infection and Tropical Medicine, University Hospitals of Leicester, Leicester, UK
  2. 2Department of Infection, Immunity and Inflammation, University of Leicester, Leicester, UK
  1. Correspondence to Dr Manish Pareek, Department of Infection, Immunity and Inflammation, Maurice Shock Medical Sciences Building, University Road, Leicester, LE1 9HN;

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Rat bite fever (RBF) is a rare zoonosis caused worldwide by Streptobacillus monoliformis with Spirillum minus cases restricted to Asia.1 Transmission arises mainly by scratches or bites from colonised rats.1 At-risk groups include veterinarians, sewerage and laboratory workers or rat owners.1 Patients typically present within 7 days of exposure with an acute febrile illness characterised by headache, myalgia, migratory polyarthralgia and a rash.2 In the UK, one–two cases are diagnosed per year3 predominantly caused by S monoliformis.2 It is likely underdiagnosed due to a wide differential diagnosis and low culture sensitivity.2 If not suspected and treated with an appropriate antibiotic such as penicillin, mortality may reach 13%.1

Case history

A 28-year-old excavator operator presented with a 5 day history of fever, headache, rash, myalgia, symmetrical wrist arthritis and diarrhoea. He had received multiple bites to his fingers from a friend's pet rat 10 days previously. A painless non-blanching red maculopapular pustular rash developed affecting the lower extremities, 2 days before admission (figures 1 and 2). Examination revealed no lymphadenopathy, cardiac murmur, hepatosplenomegaly or septic arthritis. The rat bites had almost healed. Laboratory investigations showed a transient leucocytosis with neutrophilia and lymphopenia and an elevated C reactive protein. Clinically rat bite fever was diagnosed and intravenous co-amoxiclav commenced. Blood cultures were negative after 7 days. Serological testing for HIV, syphilis and leptospirosis, and urine screening for Neisseria gonorrhoeae and Chlamydia trachomatis were negative. He was discharged home on oral co-amoxiclav after 3 days of intravenous therapy to finish a 14 day course. When seen in clinic 1 month later, the patient was much better and had returned to work.

Figure 1

A close-up photograph of a maculopapular pustular rash of rat bite fever.

Figure 2

Photograph of the left ankle and foot showing a red maculopapular pustular rash.


Rash is a characteristic clinical feature of patients presenting with RBF due to S monoliformis and S minus, present in 75% and 50% of cases, respectively.1 In S minus cases it has a typical red-brown maculopapular appearance that is often confluent with occasional plaques or urticarial lesions.1 ,2 S monoliformis cases have a variable appearance including petechial,2 morbilliform1 and maculopapular with pustules4 as with our patient (figure 2). Although the rash may be generalised, it is typically acral affecting the palms and soles2 (figure 1). It may evolve by desquamation, the formation of purpura or vesicles.5 A skin biopsy typically shows a leucocytoclastic vasculitis.2

The differential diagnosis of the rash is wide including Henoch-Schönlein purpura, autoimmune vasculitis, meningococcaemia, gonococcaemia, staphylococcal and streptococcal infections and leptospirosis.5 The diagnosis can be hard to elucidate as rat bites heal quickly, often absent by the time of presentation1 and seemingly innocuous rat exposures such as kissing or faecal exposure can transmit S monoliformis resulting in RBF.5 The complications of RBF include septic arthritis, pericardial effusion, bronchopneumonia, overwhelming sepsis and endocarditis which carries a mortality of 53%.2 In patients with symptoms of fever, polyarthralgia and a rash predominantly affecting the extremities, it is important to seek a history of rodent exposure and consider RBF and to institute appropriate antibiotics to prevent unnecessary morbidity and mortality.



  • Contributors AR: drafted, wrote and revised the manuscript. MW: made substantial contributions to the conception of the article. MP: made substantial contributions to the conception of the article and revised it for critically important intellectual content.

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.