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Rat-bite Fever

by Debbie "The Rat Lady" Ducommun

Rat-bite fever (RBF) is the common name for two similar diseases caused by different bacteria. The most common form is caused by Streptobacillus moniliformis, a gram negative rod-shaped bacteria. This is also called Streptobacillary fever. The less common cause of rat-bite fever is Spirillum minus, sometimes called Spirillum minor, a gram negative spiral-shaped bacteria. This is also called Sodoku, a name that comes from Japan where the disease is more common. I found different references that variously said the Spirillum is only carried in wild rats, not in domestic rats, or that it is only found in Asia and not in the U.S.

The most common way for people to contract rat-bite fever is by being bitten or scratched by either a wild or domestic rat, or being scratched by an item exposed to rats, such as a cage. Occasional cases have been reported among children who live in wild rat-infested homes, but who had no direct rat contact. The disease can also be contracted from mice, squirrels, gerbils, and animals who eat wild rodents, including dogs and cats.

The Strep form of the disease can also be contracted from ingesting food or liquid contaminated by rat feces or urine. In fact, when the strep form of the disease includes nausea, abdominal pain, and/or vomiting and a sore throat, it is called Haverhill fever, named after a 1926 outbreak in Haverhill, MA attributed to contaminated raw milk. There was another epidemic of the disease that occurred due to contaminated milk in England in 1983.

There was a case in 1998 in New Mexico where two boys who were friends became sick. One was diagnosed as having rat-bite fever. The other boy also became ill and was treated with antibiotics, but rat-bite fever was not specifically diagnosed in his case. Both cases are thought to have been contracted either from the boys being licked by a dog, or from them drinking water from an open irrigation ditch. Neither boy had any contact with rats.

Both bacteria that cause the disease in humans can be carried in the mouth of rats without causing disease. The occurrence of Streptobacillus moniliformis in healthy laboratory rats ranges from 10% to 100% depending on the population. In wild rats the occurrence ranges from 50% to 100%. There are no statistics of the occurrence of the bacteria in pet rats. Since so many pet rats are treated with antibiotics, I would think the occurrence of the bacteria might be very low.

A tragic case of rat-bite fever occurred in Santa Barbara, CA in August 2001. A young man apparently contracted the infection from a scratch from a rat cage he received while working in a pet shop. He did not know he had the infection until it damaged his heart valves and he had to be rushed to the hospital. Despite surgery he died. A newspaper article about this story made the statement that about 10% of rat bites result in some form of rat-bite fever. This is an example of the faulty use of statistics. After doing a web search about rat-bite fever, I located the website from which they got this information which is www.kcom.edu/faculty/chamberlain/Website/lectures/lecture/ratfever.htm

The statement on the website is actually “It has been estimated that 10% of rat bites result in some form of RBF.” I also found the same statement repeated on other websites.

I believe the original resource for this statement is a study published by someone named Richter in 1946. (I was not able to find the actual study, but only a summary of it.) This study reported on the incidence of rat bites and rat-bite fever in inner city Baltimore from 1939-1943 and concluded that during this time, in an area of less than 2 square miles, at least 93 people were bitten by rats. Obviously these bites would have been from wild rats, not pet rats. Sixty-five of the people with rat bites were treated in the hospital and of these, 7 developed rat bite fever. None of them died.

So, if this is the origin of the statement that “10% of rat bites result in rat-bite fever” it has been taken out of context. The correct conclusion from this study is that 50 years ago, 10% of urban people who went to the hospital after being bitten by a wild rat developed symptoms of rat-bite fever. I’m sure the incidence of the disease among people bitten by pet rats is much lower. Another factor is that the symptoms of rat-bite fever are similar to those of other diseases, especially Rocky Mountain Spotted fever, coxsackie B virus and meningoccemia, and it’s difficult to diagnose, so it’s possible that some cases reported to be rat-bite fever were something else.

Richter’s study found that 60% of those bitten were under one year of age, so it is likely that young children are more susceptible to the disease. Of the 6 cases I know of that have occurred up through 1998 that were probably from pet rats, 5 of them were in children, including a 5-year-old, a 4-year-old and a 2-year-old.

I contacted the Center for Disease Control to ask them about the incidence of rat-bite fever and they told me that it isn’t a reportable disease, so there are no statistics on it. They only receive a few calls about it each year from health workers who want to know how the disease is diagnosed.

Symptoms and Treatment

In the Strep form, if the disease is caused by a rat bite, the bite usually heals up, although occasionally there may be swelling or infection at or near the bite site. The incubation period can range from 1 to 22 days, but onset usually occurs 2-10 days after exposure. The first symptoms are usually like the flu including fever and chills, headache, muscle aches, and weakness. There may or may not be nausea, abdominal pain, vomiting and a sore throat. Then, 2-4 days later a rash of small red bumps usually forms on the hands and feet, mostly on the palms and soles. However, the rash can also occur on the arms, legs and trunk. Arthritis in the knees and hips is also a very common symptom. Sometimes there is swelling in the lymph nodes nearest the bite.

In most cases, the disease goes away on its own within 2 weeks, but symptoms can continue for several months and 13% of untreated cases are fatal. In rare cases the infection can cause pneumonia and can travel to the heart and brain causing damage and abscesses. In cases that are treated promptly the disease is almost always cured without any residual effects.

The only way to diagnose the Strep form for sure, either in humans or rats, is to do a blood culture. Because the bacteria is very difficult to grow, the culture must be done in a very specific manner.

The Spirillus form has a longer incubation period, 4 to 28 days, and usually longer than 10 days. The initial wound may persist with swelling and ulceration or may heal only to reappear at the onset of systemic symptoms. The main symptom is a cycle of fever lasting from 2 to 4 days. Malaise, headaches, and enlargement of the lymph nodes adjacent to the wound are also common. Symptoms usually continue for 4 to 8 weeks but may continue for up to one year. A rash of larger red wheals sometimes develops but is generally less prominent than the rash produced by the Strep form. Arthritis is rare. Complications may include infection of the heart, meningitis, hepatitis, and enlarged spleen. If left untreated death results in 6% to 10% of the cases.

The Spirillus form cannot be cultured and must be diagnosed by finding the bacteria itself in samples of blood, or in tissue or fluid from the lesions or adjacent lymph nodes.

The treatment for both types of the disease recommended by the Center for Disease Control is intravenous penicillin for 5 to 7 days followed by oral penicillin for 7 days. For people allergic to penicillin, tetracycline or streptomycin can be used. Other antibiotics such as erythromycin, chloramphenicol, clindamycin and cephalosporins have been used with some success in some cases.

To summarize, rat-bite fever is a very rare disease that is easily treated if the symptoms are recognized early enough. Although I don’t know of any studies that show whether rats treated with antibiotics can still carry the bacteria, it seems likely that treating your rats with antibiotics will reduce the chance that they could pass on the disease.

If you have more questions, you can contact the Center for Disease Control at:

CDC Public Inquiries
(404) 639-3534
(800) 311-3435

http://www.cdc.gov/netinfo.htm

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