S. minus is a short, spiral, Gram-negative bacillus and measures0.2–0.5 3 3–5 mm in size. It is a motile bacterium with the pres-ence of 1–5 amphitrichous flagella. The bacteria stain poorly with Gram stain, but stain better with Giemsa or Fontana stains. S. minus also causes rat-bite fever similar to that caused by S. moniliformis, but with few differences in the clinical manifestations:
· Rat-bite fever caused by S. minus has a long incubation period of 2–4 weeks.
· The condition may manifest as suppuration of the rat-bite wound, and the onset of fever is associated with regional lymphadenopathy.
· Mortality is relatively less (6%) in untreated cases.
Other clinical manifestations of rat bite are similar to that caused by S. moniliformis. S. minus organisms like the S. moniliformis are found in the nasopharynx of rats and other small rodents. They are transmitted to humans by the bite of these animals but not by contaminated water or milk as seen with S. moniliformis.
Dark field microscopic examination of blood, pus, or lymph node aspirate is useful to demonstrate S. minus. Microscopy of Giemsa- or Wright-stained blood smear is also useful. Animal inoculation is a frequently used method for diagnosis. In this method, clinical specimens are inoculated intraperitoneally into rodents following which these pathogens can be demon-strated in the blood and the peritoneal fluid of these rodents, 1–3 weeks after inoculation.
Serology is not useful. Rat-bite fever caused by S. minus can be treated with both penicillin and tetracycline. Table 47-1 shows a comparison between rat-bite fever caused by Streptobacillus and Spirillum species.