Spirillary Rat-Bite Fever

Spirillary rat-bite fever (RBF), also known as Sodoku (Japanese for so: rat and doku: poison), is caused by the Gram-negative bacillus Spirillum minus and is transmitted to humans through the bites and scratches of rats. The disease is mostly present in Asia.

Epidemiology

The exact incidence is unknown.

Clinical description

The bite is often small and heals quickly. However, after an incubation period of around 14 to 18 days, an inflammation appears at the site of the bite that becomes painful, indurated, edematous and may ulcerate. The inflammation is followed by fever, vomiting and chills and is associated with local lymphadenopathy. In 50% of the cases, a macular rash develops and in rare cases, swollen, red, and painful joints can appear. Occasionally, diarrhea, vomiting, neuralgias and complications, such as endo- and myocarditis, hepatitis and meningitis, can occur.

Etiology

Spirillum minus is present in the saliva of rats and is only transmitted through bites and scratches. A few cases of transmission by other animals (monkeys, mice) have been reported.

Diagnostic methods

Without a noticeable bite, diagnosis is based solely on detection of the germ. However, this is difficult due to its poor growth on culture media.

Differential diagnosis

The differential diagnosis includes streptobacillary RBF and Haverhill fever (see these terms) and several bacterial and viral infections (Lyme disease, leptospirosis, brucellosis, Rocky Mountain spotted fever, malaria, typhoid fever (see these terms), S. pyogenes and S. pyogenes-associated diseases, S. aureus infection, disseminated gonorrhea, meningococcemia, viral exanthems, secondary syphilis, Epstein-Barr virus, and coxsackieviruses).

Management and treatment

Management requires a prophylactic (avoiding direct or indirect contact with host-animals) and therapeutic approach (local treatment and antimicrobial therapy). Treatment of this form of rat-bite fever is primarily based on penicillin G administration as little is known about the susceptibility of this germ to other antibiotics.

Prognosis

Without treatment, symptoms disappear within 3-4 days but regular relapses can occur 3-10 days later. The initial lesion can become necrotic and can desquamate. The relapses can go on for a year but normally the symptoms disappear within two months. If left untreated, RBF carries a mortality rate of 6.5% due to complications.