American Fancy Rat & Mouse Association

This article is from the WSSF 2014 AFRMA Rat & Mouse Tales news-magazine.

Medical - Zoonotic Rodent Diseases - Rat Bite Fever

By Carmen Jane Booth, D.V.M., Ph.D.

RE: Death of San Diego boy from Rat Bite Fever

This was very sad to hear about. The reality is pet store rodents have lots of rodent viruses, bacteria, and parasites that can be problematic to rodents. Zoonotic agents are less common, but studies on the rate of this in pet store animals are limited. It is known that LCMV [lymphocytic choriomeningitis virus] in pet rodents can infect people and if that person dies and is an organ donor, the recipients can die from it since they are on immune suppression drugs. This happened from a pet hamster in MA, where the human died in a car accident and was an organ donor. They may test for that virus now. Pet mice can carry tapeworms that can infect humans.

Bacteria are more complicated since they are everywhere. Being exposed does not necessarily mean you will become infected or become ill. We are awash in a sea of bacteria. “Rat Bite Fever” is caused primarily by Streptobacillus moniliformis, a Gram-negative pleomorphic rod-shaped bacteria. This is considered a normal (commensal) bacteria in the nasopharynx of rats. It is common in wild rats and some laboratory rat colonies; although, most modern research rat colonies are negative for it in their colonies. It is unknown what percent of pet store rats have it. It requires special culture media to grow it; therefore, you won’t detect it on routine microbiology. However, you can do an oral swab and have your veterinarian submit it to a lab that has the specific media to grow it. Molecular diagnostics such as polymerase chain reaction (PCR) may be available. We use PCR for opportunistic or pathogenic bacteria that are difficult to grow in culture at work.

I have attached a good article on this. “Rat Bite Fever and Streptobacillus moniliformis. Clinical Microbiology Review. Jan 2007; 20(1): 13–22.doi: 10.1128/CMR.00016-06 [full text and PDF available to right on page]

Key Points from the article.

  1. Rat bite fever was first reported in the United States in 1839. It is a risk to pet rat owners.
  2. S . moniliformis exists in two variant types, the normally occurring bacillary form and the inducible or spontaneously occurring, cell wall-deficient L form, growing with a “fried-egg” colony morphology. The L form is considered nonpathogenic; spontaneous conversion between the two forms in vitro has been reported and is felt by some to be responsible for clinical relapses and resistance to therapy.
  3. Pathogenesis: Because of the relatively low incidence and low mortality rate of rat bite fever when recognized and treated, little information describing the pathogenesis of S. moniliformis exists. However, the organism appears to be capable of producing morphological findings not customarily associated with bacterial infections. In experimental infection in mice, it is of concern that persistence of organisms within joint spaces at 3 months of infection may occur despite the clearance of organisms from blood, liver, and spleen.
  4. Epidemiology: More than 2 million animal bites occur each year in the United States, and rats are responsible for approximately 1% of these. Historically, the typical victim of rat bite fever was a child under 5 years old living in poverty, and over 50% of reported cases in the United States were children. Now that rats have become popular pets and study animals, the demographics of potential victims have broadened to include children, pet store workers, and laboratory technicians. Over 200 cases of rat bite fever have been documented in this country, but this represents a significant underestimate because neither the disease nor its causative organism is reportable to health departments. The youngest reported case of rat bite fever was in a 2-month-old infant, and the oldest reported case occurred in an 87-year-old man. The risk of infection after a rat bite appears to be 10%, and the mortality rate of untreated rat bite fever is approximately 13%.
  5. The rat appears to be the dominant natural reservoir of S. moniliformis, which likely is a member of the commensal flora of the rat’s upper respiratory tract. Healthy rats may demonstrate the organism in cultures of the nasopharynx, larynx, upper trachea, and middle ear. Healthy domesticated or laboratory rats demonstrate S. moniliformis colonization 10% to 100% of the time, while wild rats appear to be 50% to 100% colonized. Most rats are asymptomatically colonized but occasionally may demonstrate signs and symptoms of disease (vague respiratory; usually co infected with other agents: Klebsiella, Mycoplasma, viruses perhaps). There are reports of infection or colonization in such potential pets as guinea pigs, gerbils, ferrets, cats, and dogs. However, no confirmatory evidence exists to prove the risk of transmission from either cats or dogs. More likely, the latter two animals are colonized only transiently after attacking or eating a rodent colonized with S. moniliformis. Rat bite fever in nonhuman primates (rhesus macaque and titi monkey) has been reported, and streptobacillary disease in turkeys and koalas has been demonstrated.
  6. Conclusions: Rat bite fever, caused by S. moniliformis, is an under-recognized and under-reported disease characterized by abrupt onset of fever, rigors, and migratory polyarthralgias; it carries a mortality rate of approximately 10%. Although S. moniliformis is exquisitely susceptible to penicillin, most patients experience treatment delays due to the nonspecific nature of the clinical features, a broad differential diagnosis list, and difficulties in culture diagnosis. However, the changing epidemiology of rodent exposure, together with the risk of severe, invasive disease if left untreated, suggests that rat bite fever and S. moniliformis should occupy a more prominent place in our diagnostic thinking.

Having had pet rats on and off since the mid ’70s as well as other pets, and being a veterinarian/scientist/pathologist and rat enthusiast, having any pet or interacting with animals is a risk. To put the risk of bite in perspective: In NY city (Public Health Rep. 2012 Mar-Apr; 127(2): 195–201.) during the study period (one year), more than 6,000 animal bite patient visits were recorded per year. The proportion of visits for animal bites did not appear to change over time. Dog bites accounted for more than 70% and cat bites accounted for 13% of animal bite patient visits. Animals can and do bite when stressed, sick, handled improperly, or not socialized. Rats are small and have limited defense options. Children should be supervised with pets regardless of size. Bites or scratches should be treated immediately. This disease should be on the list for any rodent bites. Hand washing after handling pets is recommended; although, this is often unrealistic. I wash my hands very frequently and wear gloves in my work. I wash my hands a lot when cooking or handling food. Do I wash my hands every time after petting my dogs? Um, I plead the 5th.

While the risk is low that a rat bite or exposure will result in infection by S. moniliformis, it is not zero. Can pet rats or rodents be treated with antibiotics to eliminate it? Hmmm, I can’t find any research on this. Does antibiotic treatment work on humans with this bacteria? Yes, but not 100%. I would guess the same is true for rodents. The loss of a child or family member (regardless of cause), is heart breaking. People have to make their own decisions regarding the risk of having pets, including rats. *

Other articles on diseases and problems:
“Rat Bite Fever” - by Carmen Jane Booth, D.V.M., Ph.D.
“Contagious Rat Diseases?; Zoonotic Diseases” - by Carmen Jane Booth, D.V.M., Ph.D.
For more on zoonotic diseases see “Zoonotic Diseases” from the UC Santa Barbara Office of Research web site (archived page)
“Illness in Rat Bites and Feces?” - by Carmen Jane Booth, D.V.M.
“Leptospirosis in Pet Rats?”
“Papilloma Wart Virus or Rat Pox Virus”
“Bugs” - By Nichole Royer, Edited by Carmen Jane Booth, D.V.M.
“Ringworm & Rats” - by Carmen Jane Booth, D.V.M.
“Ringworm on Mice” - by Carmen Jane Booth, D.V.M., Ph.D.
“Animal Allergies” - by Nichole Royer

There is an article by the HPA (part of Public Health England) “Reducing the risk of human infection from pet rodents.” (PDF file; and in a PDF brochure) that goes into detail on caring for pet rodents that may be suspect to harboring disease such as leptospirosis, hantavirus, rat bite fever, or lymphocytic choriomeningitis (LCMV).

Guide to Diseases Spread by Animals by Wildlife Removal.

Rat Bite Fever from The Center for Food Security & Public Health with section on Infections in Animals and treatment. New!

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Updated July 13, 2021