Streptobacillus moniliformis – Rat bite fever; Haverhill fever: Molecular diagnosis (PCR).


Streptobacillus moniliformis is a gram-negative bacillus, which can be observed in a filamentous, unbranched and very pleomorphic form, sometimes with lateral dilations, with long curved segments, non-mobile, and microaerophilic. It was initially classified in the family Fusobacteriaceae, but subsequently by genetic homology of 16S rDNA in the family Leptotrichiaceae. It was first isolated (1914) from a human rat bite and has been isolated from the respiratory system of domestic and wild rats. In 2014 a new species was identified in Hong Kong differentiated from the previous one by analysis of its 16S rDNA and the recA, groE1 and gyrB genes, called Streptobacillus hongkongensis.


Culture from blood is difficult because it is inhibited by an anticoagulant (polyamine sodium sulfonate) present in most aerobic commercial blood culture media. Alternatively, anaerobic blood culture bottles lacking such anticoagulants can be used, or trypticase-soy broth or agar with 10 to 20% blood, serum or ascites can be used. In solid media of blood agar 5% at 37°C it can develop between 2 and 7 days of incubation, giving small, convex, smooth, bright and gray colonies. In liquid media it produces cottony colonies. It can be identified by conventional metabolic identification tests. It is sensitive to penicillin, cephalosporins, macrolides, tetracyclines and other broad-spectrum antimicrobials.


Although this syndrome has been classically related to rat bite or rat scratch, both wild rats (Rattus norvegicus), colonized in 92% of cases, and domestic rats (Rattus rattus) colonized in 58% of cases, colonization has been described in other animals have also been the origin of this syndrome such as gerbils, guinea pigs, mice, ferrets, cats, dogs or nonhuman primates. All these species of animals can be colonized without presenting any symptoms.


Rat bite fever occurs after the bite of these animals, but in 30% of cases the existence of any bite or scratch is not remembered. In cases of rat bite fever syndrome, the presence of a lesion at the site of the bite or scratch is not observed, unlike what occurs with a related syndrome caused by Spirillum minus, which occurs preferably in Asia. However, this syndrome has also been related to contacts with colonized animals in which there has been no skin lesion, for example, animal caretakers in stalls, pet stores or individuals with intimate contact with their pets. Some published cases refer to a contaminated snake keeper through rats used to feed reptiles, or to children who are contaminated from rodents as pets. The possibility of transmission by infected water, close contact with rats or their handling, or even contaminated food (Haverhill fever -Erythema arthriticum epidermicum-, due to an outbreak of contaminated food in this US town) is admitted. Similar outbreaks have occurred due to consumption of raw milk, or water contaminated with rat feces. In these cases, patients show similar symptoms with vomiting, severe headache, cold sweating and high fever.


Symptoms of rat bite fever include high fever (38°C to 41°C) of sudden onset, which appears within 2 to 10 days of injury or exposure (bite, scratch or ingestion of food contaminated with rat feces), together with discomfort, muscle aches, occasional joint inflammation, accompanied in 78% of cases of maculopapular, petechial or pustular rash, which usually begins in the forearms and extends to the face, neck, legs, hands and feet, including palms and soles of the feet. Uncommon manifestations that occur occasionally such as arthritis, or general bilateral polyarthritis have been described, although not necessarily, which can affect the knees, ankles, elbows or wrist. Other complications may include: endocarditis, myocarditis, bronchitis, pneumonia, arteritis, septicemia, epidural abscess, vertebral osteomyelitis, spondylodiscitis, psoas abscess or cutaneous limb desquamation that can be confused with Kawasaki syndrome, among others. Mortality in untreated patients is estimated to be 13%, with increased risk in individuals with immunodeficiency (eg infected with HIV)


Due to its requirements for culture, its isolation is difficult,, and molecular diagnosis (PCR) is currently recommended.


Tests performed in IVAMI:

  • Molecular diagnosis (PCR).

Sample recommended:

  • Exudate obtained from vesiculation present in rash.
  • Joint puncture exudate - joint fluid - in case of arthritis.
  • Exudate obtained from puncture in cases of abscesses.
  • Pharyngeal exudate to rule out colonization in animals (not in people).

Storage and sending of the sample:

  • Refrigerated (preferred) for less than 2 days.
  • Frozen: more than 2 days. If frozen sample is available, send in this state without defrosting.

Interference with the test:

  • Complete blood hemolysis.
  • Repeated freeze-thaw.
  • Conservation at temperature above refrigeration.
  • Sample extraction during convalescence phase.
  • Extraction of blood in tubes with heparin.

Cost of the test:

  • Molecular diagnosis (PCR): Consult to