Rat Bite Fever


(Jane Reid, BA(Hons), MSc; & Karen Grant, RN)

1. How is Rat Bite Fever caused?

In North America and Europe, the bacterium responsible for Rat Bite Fever (RBF) is Streptobacillus moniliformis (S. moniliformis 1. In Asia, a different bacterium, Spirillum minus (S. minus), is most commonly responsible. Rats, both wild and fancy, are the primary hosts of these bacteria, but other rodents, and, more rarely, some other species, can also carry them 2. The bacteria live primarily in the upper respiratory tract and are non-pathogenic to rats, i.e., do not cause them to develop any disease. The rats are therefore asymptomatic carriers, i.e., do not show any signs of associated illness. Studies estimate that 50%+ of wild rats carry these bacteria, but there are no reliable statistics on the proportion of fancy rats that may be carriers.

People can become infected with these bacteria after close contact with carrier rats 3. The bacteria can enter the body through an open wound (e.g., from a rat bite or scratch) or through the mucous membranes (primarily the eyes, nose or mouth). Infection can occur by means of:

  • Direct contact with the saliva, urine or faeces (or, more rarely, the ocular or nasal secretions 4) of a carrier rat;
  • Contact with surfaces that are contaminated with the bacteria;
  • Consumption of food or drink contaminated with the bacteria.

A person who is infected with these bacteria may subsequently develop RBF. RBF is not transmissible person-to-person, i.e., cannot be passed directly from one person to another.

2. How widespread and serious a problem is RBF?

It is impossible to say definitively how widespread a problem RBF really is. The proportion of fancy rats that may be carrying the bacteria, the proportion of carrier rats that may be actively shedding bacteria, the size of bacterial load required for infectivity, and the subsequent risk level of developing RBF once infected are all unknown variables. In the absence of any compelling evidence to the contrary, the threat to humans is considered to be small and so RBF is not a reportable / notifiable disease in any country. This means in turn that data on individual cases is limited. RBF is therefore believed to be widely under-recognized and under-diagnosed 5, with the majority of documented cases following severe infection. From analysis of known cases, certain categories of people appear to be at higher risk 6:

  • Children, especially those 5 years and younger, as their immune systems are still developing;
  • Older adults, aged 65 and above;
  • Pregnant women;
  • Immuno-suppressed people, i.e., those who will have greater difficulty fighting off infection due to cancer, HIV/AIDS, organ implant, or taking certain medications.

You can greatly reduce the risk of infection by observing normal good husbandry and hygiene practices, including 7:

  • Maintaining a clean habitat for your rats;
  • Washing your hands thoroughly immediately after touching your rats or cleaning their habitat;
  • Disinfecting any surfaces outside their immediate habitat that they may have been in contact with. The organism does not form spores and is thought to have no persistence in the environment 4, so normal disinfection should serve to remove the bacteria from any surfaces touched by an infected rat.

3. Is RBF treatable?

The short answer is yes, RBF is treatable. It is susceptible to several commonly available antibiotics and typically responds very well to treatment 8. Early symptoms to look out for include but are not limited to 9:

  • Fever
  • Vomiting
  • Headache
  • Muscle pain
  • Joint pain or swelling (affects around 50% of people with RBF)
  • Rash (affects around 75% of people with RBF)

Onset of symptoms usually begins 3-10 days after initial bacterial infection, but can be delayed up to 21 days.

As with any medical issues, if you believe that you may have RBF, you should contact your healthcare provider ASAP – and remember to tell them that you have rodents. If left untreated, RBF can cause serious illness or even death.

4. How do I know if my rats carry these bacteria?

Since rats are asymptomatic carriers, you will not be able to tell from their health or behavior if they are carrying the bacteria or not. While laboratory testing can be carried out to detect the presence of the bacteria 4, this does not currently give a definitive indication of whether or not an animal is infected 2. This is because the bacteria can show sporadically in their saliva, urine and faeces, so a negative result does not necessarily mean that an animal is free of them (false negative). For example, bacteria may not show in a specific sample, although present in the animal, because the bacterial load may have been reduced by recent use of antibiotics. It is unknown if rats can still spread the bacteria if they are carrying them but at undetectable levels. In support of these hypotheses, we are already seeing empirical cases of groups of rats who have lived together for some time being PCR-tested, with only some of the group subsequently testing positive for RBF.

5. So how much should I worry about RBF?

We do not wish to dismiss the potentially serious consequences of RBF. It is important for owners and rescuers to be knowledgeable about all aspects of caring for their rats, including the potential risks. Further, we recognize that it may be desirable for some people, e.g., breeders, to identify what specific diseases are present in their colonies through laboratory testing. However, it is also important to take an objective and proportionate view of the risks. Eisenberg et al 10 state:

“RBF is occurring worldwide and is believed to be under-recognized and under-diagnosed in humans. The risk of infection with any organism following a rat bite is 1–10%, but the risk of developing streptobacillary RBF and the infectious dose are unknown. According to another survey 40,000 rat bites are noticed every year and approximately 2% of rat bites are followed by an infection.”

This would imply that around 800 infections per year might result in an infection. Of those, the vast majority will be resolved by timely and appropriate treatment. Eisenberg et al 10 further state:

“Untreated (emphasis added) RBF is associated with a case fatality rate of up to 13%.”

Assuming that 10% of all infections might be untreated (equivalent to 80 cases per year), that would imply a maximum of around 10 fatalities per year from RBF. It has also been suggested that there are only 1-2 cases of RBF in the UK every year 11.

While every death is tragic, this gives some much-needed perspective on the scale of RBF. In addition, there are some indications that the use of certain antibiotics may mitigate the already-small risks of bacterial transmission to humans by reducing the rats’ bacterial load, perhaps to undetectable levels (although it may not eliminate the bacteria completely). This is an avenue for further research investigation. You may wish to discuss the possible use of antibiotics in your rats for this purpose with your veterinarian.

In summary, there is much still to learn about RBF and the bacteria that cause it, but at present this group advises that, provided sensible husbandry and hygiene precautions are taken, there is currently insufficient scientific evidence to justify widespread concern.


  1. Centers for Disease Control and Prevention (CDC). Rat-bite fever (RBF). https://www.cdc.gov/rat-bite-fever/index.html
  2. Centers for Disease Control and Prevention (CDC). Rat-bite fever (RBF): Infection in animals. https://www.cdc.gov/rat-bite-fever/animals/infection-in-animals.html
  3. Centers for Disease Control and Prevention (CDC). Rat-bite fever (RBF): Transmission. https://www.cdc.gov/rat-bite-fever/transmission/index.html
  4. Charles River Laboratories. Streptobacillus moniliformis technical sheet. https://www.criver.com/sites/default/files/resources/StreptobacillusmoniliformisTechnicalSheet.pdf
  5. Eisenberg T, Ewers C, Rau J, Akimkin V & Nicklas W (2016). Approved and novel strategies in diagnostics of rat bite fever and other Streptobacillus infections in humans and animals. Virulence. 7:6, 630-648. https://doi.org/10.1080/21505594.2016.1177694
  6. Centers for Disease Control and Prevention (CDC). Rat-bite fever (RBF): People at higher risk for RBF. https://www.cdc.gov/rat-bite-fever/higher-risk/index.html
  7. Centers for Disease Control and Prevention (CDC). Healthy pets, healthy people: Pet rodents. https://www.cdc.gov/healthypets/pets/small-mammals/petrodents.html
  8. Centers for Disease Control and Prevention (CDC). Rat-bite fever (RBF): Treatment of rat-bite fever. https://www.cdc.gov/rat-bite-fever/treatment/index.html
  9. Centers for Disease Control and Prevention (CDC). Rat-bite fever (RBF): Signs and symptoms of rat-bite fever. https://www.cdc.gov/rat-bite-fever/symptoms/index.html
  10. Kache P A, Person M K, Seeman S M, McQuiston J R, McCollum J & Traxler R M (2020). Rat-bite fever in the United States: An analysis using multiple national data sources, 2001-2015. Open Forum Infectious Diseases. 7:6, June 2020. https://academic.oup.com/ofid/article/7/6/ofaa197/5854322
  11. Patient: Rat-bite fever. https://patient.info/doctor/rat-bite-fever-pro

Jane Reid, BA(Hons), MSc

Contributor and Editor

Karen Grant, RN


Adele Wharton, BVSc, MRCVS, CertGP (F&L) Veterinary Surgeon


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