Streptobacillary Rat Bite Fever

Rat bites are fairly common among the poor people of large cities and among workers who handle laboratory rats. In the past, as many as one out of 10 bites resulted in rat bite fever. Currently, the disease is not reportable in any state and the incidence of the disease in the United States is unknown.

Symptoms

Usually, the bite wound heals promptly without any problem noted. Two to 10 days later, however, chills and fever, head and muscle aches, and vomiting develop. The fever characteristically comes and goes. A rash usually appears after a few days, followed by pain on motion of one or more of the large joints.

Causative Agent

The cause of streptobacillary rat bite fever is Streptobacillus moniliformis, a facultatively anaerobic Gram-negative rod. Stained smears show a multiplicity of forms ranging from small coccobacilli to unbranched filaments more that 100 mm long. The organism is unique in that it spontaneously develops L-forms. L-forms are cell wall deficient variants, first identified

708 Chapter 27 Wound Infections at the famous Lister Institute (hence, the L in L-form). As might be expected, L-form colonies resemble those of mycoplasmas, bacteria that lack a cell wall. ■ mycoplasmas, p. 291

Pathogenesis

The details of how this bacterium causes streptobacillary rat bite fever are not yet known. The organisms enter the body through a bite or scratch, and sometimes by ingestion. There is typically little enlargement of the local lymph nodes, and S. moniliformis quickly enters the bloodstream and spreads throughout the body. Fever often subsides and reappears in an irregular fashion. The majority of cases recover without treatment in about 2 weeks, but others develop serious complications such as brain abscesses or infection of the heart valves. About 13% of untreated cases are fatal.

Epidemiology

From 50% to 100% of wild, and 10% to 100% of healthy laboratory rats, as well as mice and other rodents carry the organism in their nose and throat. Rat bites and scratches are the usual source ofhuman infections and can occur while the victim is sleeping. Epidemics of the disease, however, have arisen from ingesting milk, water, or food contaminated with S. moniliformis from rodent droppings. The foodborne disease is called Haverhill fever from a 1926 epidemic in Haverhill, Massachusetts. A more recent epidemic occurred in an English boarding school in 1983, when 208 students acquired the disease from drinking raw milk. Cases of rat bite fever have also been associated with exposure to animals that prey on rodents, including cats and dogs.

Prevention and Treatment

Wild rat control and care in handling laboratory rats are reasonable preventive measures. Penicillin, given by injection, is the treatment of choice for cases of rat bite fever. This shows that either S. moniliformis L-forms do not occur in vivo, or if they do, they are avirulent.

The main features of streptobacillary rat bite fever are presented in table 27.9.

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