RAT BITE FEVER

There are two known etiologic agents of rat bite fever: Spirillum minus and Streptobacillus moniliformis. The streptobacillary form is more common.

STREPTOBACILLARY RAT-BITE FEVER:

Etiology: This form of rat bite fever is caused by Streptobacillus moniliformis. It is a gram negative, non-spore forming, non-motile, facultative anaerobic bacillus which reverses to L form spontaneously. It is called monoliformis due to its necklace-like appearance, mainly consisting of series of oval or elongated bulbous swellings when grown under certain conditions. The bacterium is stained better by Giemsa and Wayson than Gram stain.

Habitat: It is a part of normal flora of oropharynx in rodents.

Mode of infection: Human infection follows bite of rats (or rarely cats), weasels and other rodents. Human infections by S.monoliformis with no history of rat bite have been associated with consumption of contaminated unpasteurized milk. This form is known as Haverhill fever.

Incubation period: Two weeks, usually less than ten days.

Signs and symptoms: Sudden onset of fever with chills, vomiting, severe headache, back and joint pains are usually observed features. A morbilliform, petechial rash appears in about 3 days on the hands and feet of most patients. Alternate remission and febrile episodes may last for weeks to months. Polyarthralgia or arthritis, involving wrists, elbows, knees, ankles called erythema arthriticum epidemicum develops in many patients within a week and may persist for several days or months if untreated. Bacterial endocarditis, pneumonia or brain abscess may occur in immunocompromised patients.

Laboratory diagnosis: Specimen collected is blood, synovial fluid or pus from bite wound for culture.

• Microscopy: Blood smears may be stained with Wayson, Gram or Giemsa stain.

• Culture: Citrated blood is centrifuged and the deposit is inoculated on to Rogosa agar. Samples may also be inoculated on basal medium with 15-20% defibrinated rabbit blood. It may also be cultivated on RCM or thioglycollate broth.

• Animal inoculation: Citrated patient's blood may also be inoculated intraperitoneally into mouse, which results in septicemia and death.

• Serology: Antibodies can be detected in serum after 2-3 weeks by tube agglutination test using formalin-killed suspensions. A titre of 1 in 80 is suggestive but a four-fold rise in titre is significant.

Treatment: Treatment is by procaine penicillin G. Erythromycin may be used for patients allergic to penicillin.

SPIRILLARY RAT BITE FEVER:
It is also known as sodoku or spirillar fever.

Etiology: This form of rat-bite fever caused by Spirillum minus. It is a gram negative, curved-spiral, aerobic-microaerophilic motile bacillus with bipolar tufts of flagella.

Habitat: It is a part of normal flora of oropharynx in rodents.

Mode of infection: Infection is acquired through the bite of a rat or mouse.

Incubation period: Usually longer than streptobacillary type; may range from 10 days to 4 weeks.

Signs and symptoms: Disease is characterized by induration, inflammation and chancre-like ulceration at the site of bite. This is accompanied by a relapsing fever, lymphangitis and regional lymphadenitis. Sometimes rashes may be seen but arthritis is absent. Complications include endocarditis, meningitis, hepatitis, nephritis, epidydimitis, myocarditis, anemia etc.

Laboratory diagnosis: Specimen collected is blood, pus from bite lesion or lymph node.

• Microscopy: Spirilla may be demonstrated in blood smears or exudate from the bite lesions or lymph nodes by Wright or Giemsa stain. They can also be demonstrated by phase contrast or dark-ground microscope.

• Culture: It has not been cultivated on any culture medium so far.

• Animal inoculation: Patient's blood may be inoculated intraperitoneally into mouse or guinea pig and spirilla demonstrated in its peritoneal fluid in 1-4 weeks.

• Serology: The VDRL test results are false-positive in half the patients. There are no specific serologic tests.

Treatment: Treatment using Penicillin with Streptomycin or tetracycline. Erythromycin may be used for patients allergic to penicillin.