Rat-Bite Fever -- New Mexico, 1996

Rat-bite fever (RBF) is a systemic bacterial illness caused by Streptobacillus moniliformis that can be acquired through the bite or scratch of a rodent or the ingestion of food or water contaminated with rat feces. Cases are rarely reported in the United States and the true incidence of disease is unknown. On August 7, 1996, a physician in a rural community reported a case of infection with S. moniliformis bacteremia in a 15-year-old boy to the New Mexico Department of Health. This report summarizes the results of the investigation of this case and indicates the need for ongoing surveillance for this illness.

On July 29, the patient presented to the local emergency department because of a 3-day history of irregularly relapsing fever (104.0 F {40.0 C}), shaking chills, progressive myalgia, nausea, and vomiting. Findings on physical examination included fever (102.0 F {38.9 C}), tachycardia, and pharyngeal erythema without exudate. He was hospitalized for further evaluation. On admission, the patient's white blood cell count was 8000/mm3 (4,300-10,800/mm3) with a differential of 46 neutrophils, 47 bands, 3 lymphocytes, and 4 monocytes. Possible sepsis was presumptively diagnosed and treatment was initiated with intravenous cefuroxime and gentamicin followed by a 7-day course of an oral second-generation cephalosporin; no relapse was reported. Blood cultures were obtained before initiation of antibiotic therapy and incubated in trypticase soy broth (1). On August 5 (day 7 of incubation), the hospital laboratory isolated a Gram-negative rod from one aerobic blood culture sample; the organism was later confirmed by Gram stain and biochemical tests as S. moniliformis at the New Mexico Department of Health's Scientific Laboratory Division.

To determine possible risk factors for S. moniliformis and identify possible modes of transmission, interviews were conducted with family members, close contacts of the boy, and local physicians. Active surveillance was initiated to detect cases of febrile illness among the boys' teammates on a local youth baseball team and at area hospitals and laboratories to identify suspected cases of Gram-negative bacteremia.

On July 29, a baseball teammate of the patient presented to his physician because of a 2-day history of irregularly relapsing fever (102.0 F {38.9 C}), severe backaches, and a sore throat. The boy was treated empirically for pharyngitis with oral amoxicillin; he discontinued therapy after 3 days.

Blood cultures obtained on July 29 were incubated for 5 days, then discarded because they were negative. On August 7, symptoms recurred and treatment was reinitiated with a second course of oral amoxicillin for 7 days; he recovered completely. A rickettsial panel and a monospot test were both negative. A diagnosis of RBF could not be confirmed.

Both boys had participated in baseball-related trips in the month before onsets of illness. On both trips, team members stayed in motels and played baseball for 5 days. Neither boy camped or slept outdoors, and there was no history of contact with any rodents or wild animals or of insect bites. However, both boys had played with and were licked by the same domestic dog of the second boy at his home, and both had consumed water from an open irrigation ditch at the site of the baseball field in Farmington, New Mexico, approximately 3-4 days before onset of symptoms. No additional cases were detected among team members or coaches who had shared living quarters and meals with the boys; no other team members were known to have consumed water from the irrigation ditch.

Reported by: WM Ryan, MD, Espaĺola Hospital, Espaĺola; L Nims, MS, DW Keller, MD, CM Sewell, DrPH, State Epidemiologist, New Mexico Dept of Health. Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases; Div of Applied Public Health Training (proposed), Epidemiology Program Office; and an EIS Officer, CDC.

Editorial Note

Editorial Note: RBF refers to two similar diseases caused by different gram-negative facultative anaerobes: streptobacillary RBF caused by infection with S. moniliformis and spirillary RBF by Spirillum minus (2,3). The incubation period of RBF caused by S. moniliformis can range from 1 to 22 days, but onset usually occurs 2-10 days after the bite of a rat. The clinical syndrome is characterized by irregularly relapsing fever and asymmetric polyarthritis followed within 2-4 days by a maculopapular rash on the extremities, palms, and soles. The wound from the bite heals spontaneously. Headache, nausea, vomiting, myalgia, minimal regional lymphadenopathy, anemia, endocarditis, myocarditis, meningitis, pneumonia, and focal abscesses have been reported (2-6). Although most cases resolve spontaneously within 2 weeks, 13% of untreated cases are fatal (2). A second form of RBF caused by Spirillum minus occurs worldwide, but is most common in Asia; this form is characterized by a longer incubation period (1-3 weeks), rare arthralgia, and an inoculation wound which can reappear at the onset of symptoms or persist with edema and ulceration (2-4).

Streptobacillary RBF can be diagnosed by blood culture only, and the organism is characterized by strict growth requirements and slow growth, making it difficult for most laboratories to culture (1,2,5). No serologic test is available for S. moniliformis; the previous slide agglutination test is no longer available because of performance limitations.

RBF is rare in the United States, and accurate data about incidence rates are unavailable because the disease is not reportable in any state (3). Most cases in the United States are caused by S. moniliformis acquired through rat bites or scratches (2). Nasopharyngeal carriage rates in healthy laboratory rats range from 10% to 100%; carriage rates in wild rats range from 50% to 100% (2,3). Cases of RBF also have been associated with the bites of mice, squirrels, and gerbils and exposure to animals that prey on these rodents (e.g., cats and dogs) (2). Sporadic cases have been reported in children without histories of direct rodent contact but who lived in rat-infested dwellings (2,4). Ingestion of food or water potentially contaminated with rat feces also can result in S. moniliformis bacteremia (i.e., Haverhill fever) (7). Two large outbreaks of Haverhill fever have occurred worldwide; implicated sources were raw milk and contaminated drinking water (6,7). Based on the investigation in this report, potential sources of infection included common exposures to the same dog and consumption of surface water. In particular, both boys in this report ingested water from an open irrigation ditch that could have been contaminated with rat feces.

S. moniliformis and Spirillum minus are susceptible to penicillin. Recommended treatment is intravenous penicillin for 5-7 days followed by oral penicillin for 7 days. Mild cases can be treated with oral penicillin alone (2). Other appropriate therapies include tetracycline and streptomycin (2,3). Although other antibiotics have been used (i.e., erythromycin, chloramphenicol, clindamycin, and cephalosporins) with some success, the effectiveness of these agents has not been assessed rigorously (2). The efficacy of prophylactic antibiotic therapy against RBF following a rodent bite is unknown (2,4).

References

1. CDC. Streptobacillus moniliformis. In: Clark WA, Hollis DG, Weaver RE, Riley P, eds. Identification of unusual pathogenic gram-negative aerobic and facultatively anaerobic bacteria. Atlanta, Georgia: US Department of Health and Human Services, CDC, 1984:288-9.

2. Washburn RG. Streptobacillus moniliformis (rat-bite fever). In: Mandell GL, Bennett JE, Dolin R, eds. Principles and practice of infectious diseases. Vol 2. New York: Churchill Livingstone, 1995:2084-6.

3. CDC. Rat-bite fever in a college student -- California. MMWR 1984;33:318-20.

4. Benenson AS, ed. Control of communicable diseases in man. 16th ed. Washington, DC: American Public Health Association, 1995:391-2.

5. Rupp ME. Streptobacillus moniliformis endocarditis: case report and review. Clin Infect Dis 1992;14:769-72.

6. McEvoy MB, Noah ND, Pilsworth R. Outbreak of fever caused by Streptobacillus moniliformis. Lancet 1987;2:1361-3.

7. Place EH, Sutton LE Jr. Erythema athriticum epidemicum (Haverhill fever). Arch Intern Med 1934;54:659-84.

   
   

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