Rats belong to group of rattus and related genera. These are found both in habitations and wild. Occasionally a person may be bitten by rat while sleeping or dealing with the animal in laboratory.

The risk of infection after a rat bite is usually low but in certain people Rat Bite fever may develop. It is caused by two spirochaetes, strepto bacillus moniiformis and spirilum minus.

Streptobacillus moniliformis is a gram negative organism which grows slowly on liquid medium giving typical ‘puff ball colonies’ in a period ranging from 2 to 7 days. It consists of short rods, filaments and yeast like swellings. On the other hand spirillum minus is a short, thick and closely coiled gram negative spiral organism. Both organisms reside in city and wild rats.

Infection is acquired by contact with other animals as well as by ingestion of contaminated milk. In humans the organisms are found in the bitten tissues and in the lymph glands draining that area. They can be demonstrated in peripheral blood but with difficulty. Degenerative changes in the kidney and liver are seen.

Symptoms of Rat bite

After being bitten by a rat which usually is either in the feet, face or upper limbs, the wound heals uneventfully. There is no regional lymphadenopathy but after a period of 2-3 weeks, there is sudden rise in fever with rigors, headache and myalgia.

In the streptobacillary form a specific rash usually appears as dusky colored purplish red spots or patchy erythema over the palms and soles or it may become generalized. This lasts for some time and slowly disappears.

Rat bite

The body temperature after remaining high for periods ranging from 2 to 7 days, falls by crisis. After a varying period of time the temperature reappears and this is followed by further relapses. Repeated febrile illnesses coupled with toxaemia leads to weight loss and poor health. Endocarditis, pericarditis and pneumonia may occur uncommonly. Severe degree of arthralgia or arthritis involving large joints can be seen either in one or multiple joints.

Cases with spirillum minus infection where the wound has healed, in 2 to 6 weeks pain and swelling appear at the site of old Rat bite and the scar breaks down. The lymphatics draining the area become inflamed and there is regional lymphadenopathy.

A picture of spreading inflammation, myositis and vesicular eruptions may develop and when this has continued for some time, general symptoms of toxaemia appear; fever rising to 103°F with rigors, joint pains, headache, nausea vomiting and diarrhea.

Large brown macular rash appears. Arthritis is not commonly seen in this kind of infection. There is another type of infection associated with wild rodents. Hantavimses are found in wild rats and are spread by aerolised excreta. The severe type of infection results in hemorrhagic fever characterized by shock, rise in body temperature, hemorrhage and oliguria with a mortality rate of 5-10%. Diagnosis of this condition is made by ELISA.


It is based on history of rat bite. The incubation period of strepto bacillus fever is 1-5 days and of S. Minus 1-4 weeks. Sterpto bacillus moniliformis infection manifests with prominent joint involvement and high temperature while spirillum minus infection is usually associated with suppuration at healed site with lymphangitis and lymph adenopathy.

Diagnosis is made by demonstration of spirochaetes in aspirates of joint fluid, from a regional lymph node (spirillum minus) or from the blood (streptobacillus moniliformis). Isolation of S. minus requires intre peritoneal inoculation of blood or infected material into mice or guinea pigs. This is followed by dark field’s examination of animal’s blood and peritoneal fluid 1 to 3 weeks later.

In the presence of high body temperature, rash and arthritis, diagnosis of rat bite fever has to be differentiated from syphilis, viral exanthema and collagen disorders.


Penicillin is the drug of choice. Dose is 5 million units I/M every 6 hly for 7 to 10 days. Alliteratively procaine penicillin 6 lac units twice a day for 7 to 10 days. Other drugs which are employed are tetracycline 500 mg 6 hry or Ampicillin I Amoxycillin (500 mg 6 hrly / 8 hrly). In any case with suspected rat bite, prophylactic administration of Amoxycillin for 5 days (500 mg 8 hrly) shall abort infection.