I often think of our medical knowledge as a collection of cross-referenced manilla folders that are stored in giant drawers in a large web of neurons. In the beginning, those folders have labels but are filled with sparse information. Through experiences, reading, learning,… forgetting, and relearning, those folders become abundant with details. Hopefully, the PedEM Morsels continue to assist in this process to some degree – I know that they help me anyway. For instance, while I learned early on that “fever + rash“ had a large Ddx, living in North Carolina has taught me that Rocky Mountain Spotty Fever is not just a theoretical possibility and should to be near the top of that Ddx. That “fever + rash“ file folder is filled with many malicious maladies (ex, meningococcemia) and our outstanding PEM Fellows at CMC recently helped me to add another one to my folder – Rat Bite Fever:

Rat Bite Fever: Basics

Rat Bite Fever is rare, but dangerous!

• Rats are only responsible for ~1% of the 2 million animal bites each year in the US. [PMID 17223620: 2007]
  • The “classic” victim used to be the young child who lived in poverty.
  • Now, rats are popular “pets” (I think we can blame Harry Potter for this.) so the demographics of victims have changed. [PMID 26698680: Vetter, 2016; PMID 23554193: Flannery, 2013]
  • ~ 10% of rat bites will develop Rat Bite Fever.

• Mortality of untreated Rat Bite Fever = ~ 13% [PMID 23554193: 2013; PMID 17223620: 2007]

• Complications include: [PMID 26698680: 2016; PMID 23554193: 2013; PMID 17223620: 2007]
  • Endocarditis (has highest associated mortality – ~50%)
  • Pericarditis
  • Hepatitis
  • Pneumonia
  • Meningitis
  • Periarteritis nodusa
  • Osteomyelitis [PMID 23554193: 2013]
  • Sepsis

Rat Bite Fever is due to Streptobacillus moniliformis.

• S. moniliformis is gram-negative non-acid fast rod. [PMID 17223620: 2007]
• It is very difficult to grow in culture! [PMID 17223620: 2007]
  • When considering this Dx, alert the lab, as alternative methods will be needed.
  • Even under ideal settings, it still is slow growing (may take as long as 7 days)
• S. moniliformis is the primary cause of Rat Bite Fever in the US, but a similar syndrome (sodoku – “rat poison”) in Asia is due to Spirillum minus.

Rat Bite Fever does NOT have to be from a Rat Bite. [PMID 26698680: 2016; PMID 17223620: 2007]

• Rats are the dominant natural reservoir for S. moniliformis.
• A bite is not required to transmit the bacteria… cases of kids “kissing” rats also well known.
• Mice also harbor the bacteria.
• Other animals that have been associated with cases are: guinea pigs, gerbils, and ferrets.

Rate Bite Fever: Presentation

• Rat Bite Fever classically consists of: [PMID 26698680: 2016; PMID 17223620: 2007]
  • Fever
  • Rigors
  • Rash (~75% develop a rash)
    • May vary in appearance.
    • Maculopapular
    • Petechial
    • Purpura
    • Hemorrhagic vesicles can develop on the extremities (including hands and feet)
  • Migratory Polyarthralgias (seen in > 50%)
    • May be misdiagnosed as arthritis.
    • Favors larger joints.
    • May persist after treatment.

• Many other non-specific symptoms can also be seen: [PMID 26698680: 2016; PMID 17223620: 2007]
  • Nausea / Vomiting
  • Sore throat
  • Headache
  • Severe myalgias

• Incubation = 3 days to > 3 weeks (typically < 7 days). [PMID 17223620: 2007]
  • May begin with prodrome of URI symptoms.
  • Skin wounds (bite marks) typically heal quickly without evidence of local infection.

Rat Bite Fever: Management

Laboratory Studies are of limited value. [PMID 26698680: 2016; PMID 17223620: 2007]

• WBC count is typically within the normal range. (once again, the WBC count can be normal despite infection!)
• ESR and CRP may be elevated later in the course.
• There is not a readily available PCR assay yet.
• Management is, therefore, predicated on clinical suspicion not lab values.

Cultures are important, but not perfect. [PMID 17223620: 2007]

• Decision to treat may need to be based on presentation and clinical suspicion, as growing the bacteria is difficult.
  • Alter the lab about the specific concern.
  • Blood Cultures should be obtained.
  • Aspiration of joint effusions can also be helpful.
  • Culture any other fluid collection as well (ex, ascites).
• Cultures should be obtained in bottles/tubes WITHOUT sodium polyanethol sulfonate (an anticoagulant that is in most aerobic culture bottles) as it can inhibit S. moniliformis growth. [PMID 26698680: Vetter, 2016]

Antibiotics are effective. [PMID 26698680: 2016; PMID 17223620: 2007]

• Penicillin is the treatment of choice.
  • Children dose = 12 – 30 mg/kg/Day of Pen G IV for 7 days, followed by 7 – 14 days of oral PCN VK (25-50 mg/kg/Day div qid).
  • Adult dose = 240 – 360 mg of Pen G IV for 7 days
  • S. moniliformis is susceptible to many antibiotics, but Penicillin is preferred.
  • Other options: Cephalosporins, Carbapenems, Aztreonam, Clindamycin, Erythromycin, Nitrofurantoin, and Vancomycin
• For Penicillin Allergic patients use Doxycycline (2 – 4 mg/kg/Day div BID; adult dose 100 mg BID) x 7 – 14 days.

Moral of the Morsel

• People have weird pets. Rats may be cool for Wizards… but… not sure regular people possess the magic to prevent contracting serious zoononitc infections.
• Fever + Rash? Ask about ANY rat exposure? Asking only about “rat bites” may lead you off the correct path.
• Alert the lab! This is a difficult to grow bacteria… so attempt to optimize the chances.
• Start IV penicillin empirically. Lab results will not define the presence or absence of the illness.

References