Rat bite fever: causes, symptoms, diagnosis, and treatment

Rat bite fever (RBF) is an infection caused by Spirillum minus or Streptobacillus moniliformis as a result of bites or scratches of infected rodents.


Causes

The pathogens are Spirillum minus and Streptobacillus moniliformis.

Spirillum minus, also known as Spirochaeta morsus muris, is spiral, with pointed ends, 1.7 -5μm long, 0.2 - 0.5μm wide, Gram-negative, with 2 - 6 regular spirals, with one or more flagella. The pathogen can rotate and bend along its long axis, and can also move quickly in multiple directions by means of flagella. The bacteria cannot grow on the artificial culture media.

Streptobacillus moniliformis is short, rod-shaped, 1 - 5μm long, 0.1 - 0.7μm wide, Gram-negative, nonmotile, and intolerant to acid. The pathogen can only grow on culture media containing 10% - 20% of blood, serum, or ascitic fluids other than ordinary culture media. In the initial isolation, the pathogen is mostly obligate anaerobic, but in the subculture, it is facultative anaerobic. On the liquid media, the bacterial colony is puffball-like or breadcrumb-like.

Source of infection

The main source of infection is house rats, pet mice, and laboratory rats, including rats, mice, guinea pigs, squirrels, voles, and weasels. Cats and dogs can occasionally be the sources of infection.

Route of transmission

The main route is bites or scratches from rats or mice.


Signs and Symptoms

Rat bite fever caused by Spirillum minus

The incubation period is several days to weeks. Redness and edema occurs on the healed rat bite site, resembling erysipelas or cellulitis. Sometimes, blisters or necrosis occurs on the red, swollen skin, forming sharply demarcated ulcers. The systemic symptoms are chills, fever with body temperature up to 39 - 40℃, general malaise, arthralgia, and lymphadenopathy. The body temperature drops to normal levels in 3 - 7 days, and fever occurs again in an interval of 2 - 7 days, resembling relapsing fever. There may be extensive erythema, mostly erythema multiforme or erythema nodosum, or nodules on the extremities and trunk, often aggravated or relieved with the rise and fall of body temperature. Repeated attacks can last for several months, eventually spontaneously healing.

The pathogen is present in the rat bite site, skin lesions, lymph nodes, and blood. With inoculation to guinea pigs or mice in the course of fever, large amounts of spirochetes can be seen on the blood smears after staining. The result of serologic test for syphilis can be positive.

Rat bite fever caused by Streptobacillus moniliformis

This condition is also known as erythema arthriticum epidemicum or Haverhill fever. The incubation period is few days to one month, averagely 12 days. Inflammation occurs on the healed bite site, resembling cellulitis or erysipelas. There are fever, general malaise, anorexia, arthralgia, and lymphangitis. The punctate erythema occurs on the abdomen initially, expanding extensively, particularly on the face and chest. The purplish red skin lesion gradually develops into enlarged, solid plaques. Nonsuppurative migratory polyarthritis and endocarditis can occur. Without treatment, the disease can persist for 6 months.


Diagnosis

If there are clinical presentations and pathogen isolated from the animal inoculation, the disease can be diagnosed.


Treatment

The treatment regimen is amoxicillin 1g orally once every 8 hours, penicillin G procaine 600,000 units intramuscularly once every 12 hours, or penicillin V 500mg orally 4 times a day, for 7 - 10 days. Those who are allergic to penicillin can take orally erythromycin ethylsuccinate 500mg 4 times a day or doxycycline 100mg once every 12 hours for 14 days.