Rat bite fever (RBF) is an uncommon disease in people, but one that I nonetheless spend a lot of time talking about with owners, veterinarians, and physicians (and sometimes lawyers). It’s a bacterial infection spread by (you guessed it) rats. The causative agent of RBF is an obscure bacterium called Streptobacillus moniliformis, which lives in the mouth of most healthy rats.  It’s typically introduced into a person’s body via a bite from a rat. Usually people don’t get an infection after such a bite, but sometimes they do, and the consequences can range from mild to fatal. Most cases aren’t too bad and respond to treatment, even though treatment is often started later than it should have been (because many physicians don’t ask about animal contact and many patients don’t mention if they’ve been exposed to a rat).

It’s not clear how common RBF really is, especially because it’s not reportable (so no one formally tracks it) and it’s probably under-diagnosed anyway.

A recent study in Open Forum Infectious Diseases provides a bit more information about RBF, but it’s still pretty limited. The paper, “Rat bite fever in the United States: an analysis using multiple national data sources, 2001-2015” (Kache et al. 2020), concludes that it’s a rare disease. That’s presumably true; however, what they looked at were hospitalizations and emergency room visits. So, they can conclude that it’s a rare cause of hospitalizations, but that’s not necessarily the same as telling us how often it causes milder disease. Regardless, the study still provides useful information about the more serious cases of RBF.

Here are a few highlights:

Rat bite injury visits were 10.5 per million people. Hospitalization for rat bite injury was 0.27 per million.

  • That’s rare, but not unexpected.  Visits to an ER after a rat bite would be unusual since they don’t cause the same degree of trauma as for example, a dog bite.

Rat bite injury hospitalization rates were highest in females, people over 60 years of age and Blacks, as well as people from the Northeast.

  • It’s maybe easiest to explain the higher rate for older individuals, as they may be more likely to seek care after an injury. The others are tougher to explain. Knowing more about the type of rat bites would help. Pet rat vs wild rat exposures weren’t differentiated, and it’s certainly plausible that someone bitten by a wild rat would be more likely to go to an ER than someone bitten by their pet. There could be socioeconomic-related reasons for racial disparities in exposure to wild rats in urban areas.

The ER visit rate for RBF was 0.33 per million.

  • Again, that will be an underestimate of RBF, since it only includes people with RBF that go to the ER and are diagnosed and properly documented in the medical record (both potential issues).

RBF hospitalization was most common in people 19 years of age or younger.

  • That’s fully expected and consistent with the cases with which I get involved. One reason is that kids are presumably more likely to be bitten, based on how and how often they interact with animals. Young kids are more likely to get sick after exposure. Put those together, and an age predisposition is expected, as with many other pet-associated diseases.

Outcomes weren’t reported. Rat bite fever is typically easily treatable, but serious disease can occur. Sometimes that’s because the diagnosis is missed, but it can also cause rapidly progressive severe disease in rare cases, and even death.

For anyone interested, the paper has a lot more data about hospitalization rates, duration of hospitalization and costs, but I won’t get into those here.

The conclusion states: “For the medical community, clinical recognition involves enhancing awareness of RBF and the implications of this disease among pediatrics.” That’s a critical point, since RBF is a niche disease that can be overlooked by physicians (and veterinarians counseling owners about zoonotic disease risks). One of my typical talking points is about the need for physicians to query animal exposure ALL the time. This disease gets missed time and again because rat bites and rat contact get missed. Rat bite fever won’t jump to mind for most physicians examining a kid with a fever and rash, but if they ask about animal contact or the parent mentions it, there’s a good chance the diagnosis will be made. If not, it’s very unlikely. Asking a question about animal contact is free and takes a few seconds – there’s no reason not to do it.