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Clinical Microbiology Reviews, January 2007, p. 13-22, Vol. 20, No. 1
0893-8512/07/$08.00+0 doi:10.1128/CMR.00016-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Rat Bite Fever and Streptobacillus moniliformis
Sean P. Elliott*
Department of Pediatrics and Steele Children's Research Center, University of Arizona Health Sciences Center, Tucson, Arizona

SUMMARY
Rat bite fever, caused by
Streptobacillus moniliformis, is a
systemic illness classically characterized by fever, rigors,
and polyarthralgias. If left untreated, it carries a mortality
rate of 10%. Unfortunately, its nonspecific initial presentation
combined with difficulties in culturing its causative organism
produces a significant risk of delay or failure in diagnosis.
The increasing popularity of rats and other rodents as pets,
together with the risk of invasive or fatal disease, demands
increased attention to rat bite fever as a potential diagnosis.
The clinical and biological features of rat bite fever and
Streptobacillus moniliformis are reviewed, providing some distinguishing features
to assist the clinician and microbiologist in diagnosis.

INTRODUCTION
Disease following the bite of a rat has been known in India
for over 2,300 years, but it has been described worldwide much
more recently as rat bite fever. This term describes two similar
yet distinct disease syndromes caused by
Streptobacillus moniliformis or
Spirillum minus. Rat bite fever caused by
S. moniliformis is more common in North America, while
S. minus infection, also
known as sodoku, is more common in Asia. Streptobacillary rat
bite fever, the subject of this review, is a systemic illness
classically characterized by relapsing fever, rash, migratory
polyarthralgias, and a mortality rate of 13% when untreated.
Often associated with the bite of a wild or laboratory rat,
rat bite fever historically has affected laboratory technicians
and the poor. As rats have become popular as pets, this has
changed such that children now account for over 50% of the cases
in the United States, followed by laboratory personnel and pet
shop employees. Over 200 cases of rat bite fever have been documented
in the United States, but this is likely a significant under-representation
because rat bite fever is not a reportable disease. Further,
rat bite fever has a nonspecific presentation with a broad differential
diagnosis, and isolation and identification of its causative
organism,
S. moniliformis, is not straightforward. Thus, the
challenges of diagnosis and broadened demographic exposure demand
close attention to this disease and its causative organism by
clinicians.

HISTORICAL ASPECTS
Rat bite fever was first reported in the United States in 1839
(
89). An association with a specific pathogen was not reported
until 1914, when Schottmüller described
Streptothrix muris ratti, isolated from a rat-bitten man (
71). This association
was confirmed in the United States in 1916 (
9). In 1925, the
organism was renamed
Streptobacillus moniliformis (
48), a name
that has remained in general use since, although some reports
refer to
Actinomyces or
Actinobacillus muris (
41,
87). A milk-associated
outbreak of disease occurred in Haverhill, MA, in 1926 and was
described by Place and Sutton (
60). The organism found at this
time was named
Haverhillia multiformis by Parker and Hudson
(
55), although this most likely represents
S. moniliformis disease.
Any review of the literature regarding rat bite fever is complicated
by the near-simultaneous description of
Spirillum minus, the
primary cause of rat bite fever in Asia, which is known by many
as sodoku. Unfortunately, some reports discuss both organisms
simultaneously, blurring the distinction between the two diseases
and epidemiologic distributions that are, in fact, distinct.

BIOLOGY
Morphology
Streptobacillus moniliformis is a highly pleomorphic, filamentous,
gram-negative, nonmotile, and non-acid-fast rod. It usually
appears straight but may be fusiform and may develop characteristic
lateral bulbar swellings. The organism is typically arranged
in chains and loosely tangled clumps (Fig.
1). It varies in
its dimensions, from 0.1 to 0.5 µm by 2.0 to 5.0 µm,
up to 10 to 15 µm, with long, curved segments up to 100
to 150 µm (
65).
S. moniliformis exists in two variant
types, the normally occurring bacillary form and the inducible
or spontaneously occurring, cell wall-deficient L form, growing
with a "fried-egg" colony morphology. The L form is considered
nonpathogenic (
28); spontaneous conversion between the two forms
in vitro has been reported and is felt by some to be responsible
for clinical relapses and resistance to therapy (
72).
Spirillum minus, the other etiologic agent of rat bite fever,
was discovered during the 19th century and initially named
Spirocheta morsus muris or
Sporozoa muris. It was renamed
Spirillum minus in 1924. The organism is a short, thick, gram-negative, tightly
coiled spiral rod which measures 0.2 to 0.5 µm and has
two to six helical turns. Because
Spirillum minus cannot be
cultured on synthetic media, initial diagnosis relies on direct
visualization of characteristic spirochetes with Giemsa stain,
Wright stain, or dark-field microscopy (
86).
Growth Characteristics
S. moniliformis is an extremely fastidious organism that needs
microaerophilic conditions to grow, making microbiological diagnosis
difficult. Optimal growth requires Trypticase soy agar or broth
enriched with 20% blood, serum, or ascitic fluid. The bacteria
grow slowly (2 to 3 days) and may take as long as 7 days. Typical
colonies have a "cotton ball" appearance on media, while colonies
on agar appear circular, convex, grayish, smooth, and glistening
(
69). After 5 days of growth, some colonies may demonstrate
the "fried-egg" appearance seen with the L form. Importantly,
the 0.05% sodium polyanethol sulfonate ("Liquoid") that is added
to most commercial aerobic blood culture bottles as an anticoagulant
inhibits the growth of
S. moniliformis at a concentration as
low as 0.0125% (
74). However, Trypticase soy agar or broth,
resin bead culture systems, and anaerobic culture bottles may
demonstrate growth because sodium polyanethol sulfonate typically
is not added (
46,
68,
75). Once the organism has grown, confirmation
of its identity occurs by conventional biochemical and carbohydrate
fermentation analysis (Table
1). Serologic testing and gas-liquid
chromatographic analysis of the fatty acid profile have also
been used and are discussed below.
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TABLE 1. Results of biochemical tests performed on the parent strain and an L-phase variant of Streptobacillus moniliformisa
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Pathogenesis
Because of the relatively low incidence and low mortality rate
of rat bite fever when recognized and treated, little information
describing the pathogenesis of
S. moniliformis exists. However,
the organism appears to be capable of producing morphological
findings not customarily associated with bacterial infections.
Autopsy of rat bite fever victims demonstrates pronounced erythrophagocytosis,
hepatosplenomegaly, interstitial pneumonia, and lymph node sinus
hyperplasia (
72). Endocarditis and myocarditis have also been
demonstrated, along with degenerative changes in the kidneys
and liver (
1). Radiological data suggest that rat bite fever
may be considered a cause of damage to physes and acrophyses,
mimicking frostbite damage (
53), and clinical data suggest that
S. moniliformis may have a predilection for synovial and serosal
surfaces (
67). Biopsy of skin lesions seen in rat bite fever
has demonstrated leukocytoclastic vasculitis (
90). Experimental
infection in mice (
70) results in a progressive polyarthritis,
beginning with fibrinopurulent exudate within the joint space
and adjacent periostea in the first 24 h of infection. This
changes to a predominately macrophage presence on day 4, followed
by periarticular abscess and necrosis on day 7. Periostitis
develops by 2 weeks and is followed by fibrous connective tissue
proliferation after 3 weeks. The degree of polyarthritis depends
on the size of the inoculum. It is of concern that persistence
of organisms within joint spaces at 3 months of infection may
occur despite the clearance of organisms from blood, liver,
and spleen (
70).

EPIDEMIOLOGY
More than 2 million animal bites occur each year in the United
States, and rats are responsible for approximately 1% of these
(
30). Historically, the typical victim of rat bite fever was
a child under 5 years old living in poverty, and over 50% of
reported cases in the United States were children (
37,
65).
Now that rats have become popular pets and study animals, the
demographics of potential victims have broadened to include
children, pet store workers, and laboratory technicians. Over
200 cases of rat bite fever have been documented in this country,
but this represents a significant underestimate because neither
the disease nor its causative organism is reportable to health
departments. The youngest reported case of rat bite fever was
in a 2-month-old infant (
72), and the oldest reported case occurred
in an 87-year-old man (
82). The risk of infection after a rat
bite appears to be 10% (
23,
35), and the mortality rate of untreated
rat bite fever is approximately 13% (
65,
91).
Geographic Distribution
Most reports of
S. moniliformis originate from the United States,
although other Western Hemisphere reports have come from Brazil,
Canada, Mexico, and Paraguay. Most European reports come from
the United Kingdom and France, but sporadic reports from Norway,
Finland, Germany, Spain, Italy, Greece, Poland, Denmark, and
The Netherlands also exist. Australia has also demonstrated
some cases. Few reports from Africa exist, other than one report
of sodoku from Kenya (
8) and two episodes of squirrel bite-associated
disease in Nigeria (
33), probably underestimating the presence
of
S. moniliformis. Most reports from Asia document cases of
sodoku, caused by
Spirillum minus and not discussed here (
91).
Within the United States, most early reports originate from
the eastern half of the country. However,
S. moniliformis now
appears to have migrated to the West Coast (
13,
32), and cases
are documented nationwide (
34).
Animal Infectivity
Rats.
The rat appears to be the dominant natural reservoir of
S. moniliformis,
which likely is a member of the commensal flora of the rat's
upper respiratory tract. Healthy rats may demonstrate the organism
in cultures of the nasopharynx, larynx, upper trachea, and middle
ear (
56). Healthy domesticated or laboratory rats demonstrate
S. moniliformis colonization 10% to 100% of the time, while
wild rats appear to be 50% to 100% colonized (
16). Most rats
are asymptomatically colonized but occasionally may demonstrate
signs and symptoms of disease.
Mice.
Because mice are a preferred animal model for research, a significant amount of effort has been expended to identify their risk of colonization and disease from S. moniliformis, as summarized by Wullenweber (91). It is well documented that laboratory mice may show symptoms of infection with S. moniliformis, ranging from septic lymphadenitis to polyarthritis and multiorgan microabscesses leading to septicemia, cachexia, and death (30, 91). However, it appears that not all strains of mice are equally susceptible to streptobacillosis and, in fact, many inbred strains demonstrate mild to no disease whatsoever. This is important from a laboratory personnel health risk perspective, as some animals may be asymptomatic carriers with the potential to transmit disease via exposure to saliva, as are rats. The persistence of S. moniliformis in mice is debated in the literature and ranges from none (91) to 6 months (70). Overall, there appears to be a low risk of rat bite fever from the bite of a healthy, inbred laboratory mouse. However, this may be different if the bite is by an outbred strain or wild mouse.
Other animals.
There are reports of infection or colonization in such potential pets as guinea pigs (44), gerbils (90), ferrets (31), cats (82, 91), and dogs (16, 57, 82). However, no confirmatory evidence exists to prove the risk of transmission from either cats or dogs. More likely, the latter two animals are colonized only transiently after attacking or eating a rodent colonized with S. moniliformis (57). Rat bite fever in nonhuman primates (rhesus macaque and titi monkey) has been reported, and streptobacillary disease in turkeys and koalas has been demonstrated (83).
Human Infectivity
The reported incidence of rat bite fever caused by
S. moniliformis from laboratory rat bites is low. Of 65 cases of documented
rat bite fever since 1938 that were reviewed for this article,
only 8 (12%) were attributed to a laboratory rat exposure. This
likely does not represent the true incidence of disease in humans
because of low clinical suspicion by clinicians and the organism's
strict growth requirements. Similarly, the incidence of wild-rat-associated
disease is seriously underestimated, as not all cases of rat
bite fever are associated with an actual bite.
S. moniliformis may also be acquired by handling of the animal or by exposure
to its excreta or saliva. Nineteen of the 65 reviewed cases
(29%) documented no bite or known exposure, consistent with
literature reports that 30% of patients report no known bite
(
15,
32). However, as stated above, 10% to 100% of domestic
rats and 50% to 100% of wild rats carry
S. moniliformis, and
a known bite causes infection approximately 10% of the time.
Thus, rat bite fever and rat colonization with
S. moniliformis represent a significant public health threat that remains unrecognized.

CLINICAL FEATURES
Rat bite fever is associated with three clinical syndromes in
the literature. Rat bite fever caused by
S. moniliformis infection
is the predominant form seen in the United States. Disease caused
by
Spirillum minus is known as sodoku and occurs primarily in
Asia. Ingestion of
S. moniliformis via contaminated food causes
Haverhill fever, so named for the first description of an outbreak
in Haverhill, MA.
Initial Symptoms
S. moniliformis-associated rat bite fever is a systemic illness
classically characterized by fever, rigors, and migratory polyarthralgias.
After exposure, the incubation period ranges from 3 days to
over 3 weeks but typically is less than 7 days. Many patients
report symptoms suggestive of an upper respiratory tract infection
during this time. If a bite has occurred, it typically heals
quickly, with minimal residual inflammation and no significant
regional lymphadenopathy. Persistence of significant induration
at the bite site should suggest an alternate diagnosis, including
sodoku.
At disease onset, fevers begin abruptly and may range from 38.0°C to 41°C. Rigors associated with fevers are prominent. Fever may resolve in 3 to 5 days but can relapse. Other frequently reported symptoms in the initial phase of illness include headache, nausea, vomiting, sore throat, and severe myalgias.
Disease Progression
As rat bite fever progresses, over 50% of patients develop migratory
polyarthralgias. The severity of pain and the presence of swelling
and erythema indicate arthritis (
38,
67,
80). Reports also document
the presence of synovitis and nonsuppurative arthritis suggestive
of rheumatoid arthritis (
40,
47,
67). The joints involved include
both large and small joints of the extremities. Many patients
experience arthritis of at least the knee and ankle during their
illness. Migratory polyarthralgia is the most persistent finding
of rat bite fever, lasting several years in some patients.
Nearly 75% of patients develop a rash that may appear maculopapular, petechial, or purpuric (20) (Fig. 2). Hemorrhagic vesicles may also develop on the peripheral extremities, especially the hands and feet, and are very tender to palpation (Fig. 3). Appearance of this rash, especially the hemorrhagic vesicles, in the setting of an otherwise nonspecific set of disease signs and symptoms should strongly suggest the diagnosis of rat bite fever. The rash may persist beyond the other, more acute, symptoms. Approximately 20% of rashes desquamate, especially those with hemorrhagic vesicles (20).
Outcome
Untreated, rat bite fever has a mortality rate of approximately
10%, ranging from 7% to 13% (
15,
54,
65,
80,
91). Reported causes
of death include endocarditis, refractory pericardial effusion,
bronchopneumonia, pneumonitis, periarteritis nodosa, volvulus,
and overwhelming septicemia, with organisms found in both the
adrenal glands and bone marrow at autopsy (
14,
15,
62,
65,
72,
76). Although some patients appear to show spontaneous recovery
from serologically confirmed disease (
5,
13), a lack of effective
antibiotic treatment is highly associated with death. Initiation
of an appropriate antibiotic regimen usually precipitates rapid
resolution of acute symptoms. However, some patients experience
prolonged migratory polyarthralgias, fatigue, and slowly resolving
rash.
Review of English Literature
Epidemiology.
A review of the English language literature reveals 65 discrete
case reports that provide full descriptions of the clinical
presentation (
2,
3,
5-
7,
13-
16,
18,
21,
22,
24-
27,
29,
31,
33-
36,
38-
43,
45-
47,
49-
52,
54,
58,
61-
65,
67-
69,
77,
79,
80,
82,
84,
87,
90). Many additional cases are described within case series
in which signs and symptoms specific to each case are not detailed
(
3,
54,
65,
69,
88). The 65 detailed cases were reported from
1938 to 2005 and primarily come from the United States, although
the United Kingdom, Europe, Canada, Australia, and Nigeria are
also represented. The patient ages range from 2 months to 87
years. Fifty (77%) of the rat bite patients described were male.
Twenty-six (40%) of the exposures occurred from a wild rat,
8 (12%) were from a laboratory rat, and 3 (5%) were from a pet
shop rat. Twenty-two (34%) of the patients described a nonbite
or nonrat exposure. The remaining cases occurred in association
with bites from a ferret (one), mouse (one), squirrel (two),
gerbil (one), and dog (one).
Clinical findings.
Symptoms described include fever (92%), rash (61%), polyarthralgias (66%), myalgias (29%), nausea and vomiting (40%), headache (34%), and sore throat (17%). The mean temperature achieved during the cases was 39.4°C. Patients' laboratory values reveal an average white blood cell count of 12,200 per cubic millimeter, with a polymorphonuclear cell and band form predominance. Only five patients demonstrated white blood cell counts higher than 15,000 per cubic millimeter. More significantly, the average erythrocyte sedimentation rate was 69 mm per hour. Only four patients had erythrocyte sedimentation rates below 15 mm per hour; these patients all had laboratory values obtained either very late in their clinical course or after recovery. Seven (10.8%) of the patients died, consistent with the published average mortality rate of 10%.
Complications
Published complications of rat bite fever include endocarditis,
myocarditis, pericarditis, systemic vasculitis, polyarteritis
nodosa, meningitis, hepatitis, nephritis, amnionitis, pneumonia,
and focal abscesses (
14,
24,
35,
54,
62,
78,
79,
83). Of these,
endocarditis is the best described and carries the highest mortality
rate (
64). Seventeen patients with endocarditis associated with
S. moniliformis infection have been described (
14,
50,
58,
64,
68). A 1992 review of 16 of these patients (
68) revealed that
8 of them had valvular disease prior to the onset of endocarditis,
most commonly rheumatic heart disease. Most cases were defined
by multiple positive blood cultures and had typical symptoms
of rat bite fever accompanied by murmur (100%), petechiae (13%),
Osler's nodes (13%), hepatosplenomegaly (33%), anemia (33%),
and cardiac dysrhythmia (13%). Echocardiography was performed
for four patients and demonstrated valvular vegetations in only
two patients. The reported mortality rate associated with
S. moniliformis endocarditis is 53% (
68), and death may occur from
2 weeks to 3 years after symptom onset (
58). However, a majority
of these deaths occurred in the absence of effective antimicrobial
therapy (
14).
Differential Diagnoses
Diseases.
The differential diagnosis of symptoms typical of rat bite fever
(fever, rash, polyarthralgias) is extensive (
27,
54,
63,
78).
Possible bacterial causes include sepsis from such bacteria
as
Streptococcus pyogenes and
Staphylococcus aureus, disseminated
gonorrhea, meningococcemia,
Streptococcus pyogenes-associated
diseases (scarlet fever, rheumatic fever, post-streptococcal
reactive arthritis), Lyme disease, ehrlichiosis, and brucellosis.
Rickettsial infections, especially Rocky Mountain spotted fever,
must be considered in areas where such infections are endemic.
Such spirochetal infections as leptospirosis and secondary syphilis
are also possible. It is of note that up to 50% of rat bite
fever patients have a falsely positive Venereal Disease Research
Laboratory (VDRL) test; however, a negative treponemal test
can be used to rule out syphilis. Many potential viral causes
exist, although Epstein-Barr virus, parvovirus B19, and coxsackieviruses
are especially prominent. Relapsing fevers may suggest
Borrelia recurrentis, malaria, and typhoid fever. Noninfectious causes
include collagen vascular diseases and drug reactions.
Sodoku.
Infection caused by rat bites in Asia is likely to be caused by Spirillum minus and is designated sodoku (so, rat; doku, poison). This entity differs from rat bite fever not only in geographic distribution but also clinically. After an incubation period of approximately 14 to 18 days, the bite site becomes indurated and may ulcerate, with associated regional lymphadenopathy. Fevers have regular relapses separated by afebrile periods lasting 3 to 7 days. Approximately 50% of patients develop a violaceous red-brown macular rash which occasionally has plaques or urticarial lesions. Joint manifestations are rare (1, 27).
Haverhill Fever
Haverhill fever refers to an outbreak of epidemic disease resulting
from
S. moniliformis-contaminated milk. The first reports were
from a 1926 outbreak in Haverhill, MA, and described an illness
termed erythema arthriticum epidemicum caused by an organism
named
Haverhillia multiformis (
55,
60). This organism was later
shown to be identical to
S. moniliformis (
10). Patients with
Haverhill fever develop signs and symptoms identical to those
of rat bite fever. However, the absence of rat exposure and
the presence of a large number of patients with common temporal
and geographic exposure should suggest Haverhill fever.
. U.S. experience.
Although Haverhill fever is named for the site of the first published description of epidemic S. moniliformis-associated disease, an earlier outbreak likely occurred in 1925 in Chester, PA (60). In this episode, approximately 400 cases occurred with striking similarity of onset, symptoms, course, and epidemiologic relation to a milk supply. The following year, 86 cases developed over a 4-week period in Haverhill, a small manufacturing city. These cases were investigated by Place and Sutton (60), and the organism responsible was described by Parker and Hudson (55). The ages of the patients ranged from 8 months to 54 years, and 41% were male. The patients came from 39 families, representing 231 people and an attack rate of 36%. The milk supply in every case came from one dairy, either directly or through four stores selling the dairy's milk products. The milk from the dairy was not pasteurized. Although no cultures from the milk demonstrated S. moniliformis, pasteurization of the milk products was associated with the end of the outbreak.
The onset of symptoms in the 1926 outbreak was acute and associated with sudden development of rigors, emesis, or severe headache. Initial symptoms resolved after 3 to 4 days and included fever (97%), vomiting (62%), headache (56%), chills (55%), dizziness (16%), and irritability (8%). However, fever recurred 2 to 3 days later and was associated with the onset of polyarthralgias and polyarthritis. Rash appeared from the first to the fifth day of disease, was most marked at the distal extremities, and was mostly "rubelliform" in nature. The rash progressed over 3 days to include hemorrhagic lesions and lasted an average of 6 days. Polyarthritis was the most persistent symptom, lasting from 1 week to several months and severely limiting activity and weight bearing. Wrists and elbows were most frequently involved, followed, in order, by knees, shoulders, fingers, and ankles. Associated laboratory findings demonstrated an average white blood cell count of 11,500 per cubic millimeter, with 70% polymorphonuclear cells. Blood cultures in 11 of 17 cases and joint fluid aspirate cultures in 2 of 2 cases demonstrated an organism subsequently named Haverhillia multiformis, with characteristics identical to S. moniliformis. Outcomes were uniformly excellent, with no deaths and few permanent sequelae. Several patients, however, experienced chronic, recurring arthralgias.
United Kingdom experience.
A second reported outbreak of Haverhill fever occurred in 1983 in 208 children at a boarding school in Chelmsford, Essex, United Kingdom (59). In a description of four cases from this outbreak (74), the clinical features were described as abrupt onset of fever with headache, peripheral erythematous rash, polyarthralgias, and subsequent sore throat. Initial diagnoses included viral illness (especially coxsackievirus), meningococcal septicemia, and erythema multiforme. The point source of the outbreak appeared to be raw milk, ingested by many students at the school. Students developed symptoms at school and were sent home to recover from an apparent viral epidemic, thus explaining the subsequent appearance of cases in London, Leeds, and Nottingham. Blood cultures from the four described patients demonstrated S. moniliformis, and the information was provided to health care workers caring for other students to assist with diagnosis and management.

DIAGNOSIS
Culture
Growth characteristics are discussed separately (see "Biology,"
above).
Fatty Acid Profiles
Fatty acid profiles obtained by gas-liquid chromatography, together
with characteristic growth, can be used for rapid identification
of
S. moniliformis. The major cellular fatty acid peaks are
tetradecanoic acid (14:0), palmitic acid (16:0), octadecanoic
acid with linoleic acid (18:2) and oleic acid (18:1), and stearic
acid (18:0) (
66,
69). High-resolution polyacrylamide gel electrophoresis
in conjunction with computer analysis has also been used to
distinguish and confirm strains of
S. moniliformis (
19).
Other Methods
Serologic assays and slide hemagglutination tests, although
used historically (
12,
55,
70) and in some animal research (
10,
91), are currently not available for use with humans. Although
these assays are sufficiently sensitive, the increasing demand
for rapid, more-sensitive tests likely has detracted from their
utility. Molecular methods such as PCR show promise and have
been used successfully with humans and laboratory animals (
4,
7,
11,
43,
85). A PCR assay specific for
S. moniliformis has
been described by Boot et al. (
11); it uses primers designed
on the basis of 16S rRNA gene base sequence data of human and
rodent strains of
S. moniliformis (forward primer, 5' GCT TAA
CAC ATG CAA ATC TAT 3'; reverse primer, 5' AGT AAG GGC CGT ATC
TCA 3'). These primers showed 100% complementarity to
S. moniliformis ATCC 14674
T and
S. moniliformis ANL 370-1. The PCR assay generated
a 296-bp product which, when discriminated by BfaI restriction
enzyme treatment, generated three fragments (128, 92, and 76
bp) specific to
S. moniliformis. This assay has been used by
others to examine both human- and animal-derived specimens and
has been found to distinguish
S. moniliformis from other organisms
with great accuracy (
4,
11,
85). However, until such an assay
becomes more readily available, a diagnosis of
S. moniliformis-associated
rat bite fever requires a high clinical index of suspicion coupled
with the appropriate use of culture and attention to ruling
out alternate diagnoses.

TREATMENT
Penicillin is the treatment of choice for proven or highly suspected
cases of rat bite fever. Tests of
S. moniliformis antibiotic
susceptibility by the disk diffusion method usually demonstrate
sensitivity to penicillins, cephalosporins, carbapenems, aztreonam,
clindamycin, erythromycin, nitrofurantoin, bacitracin, tetracycline,
teicoplanin, and vancomycin; intermediate susceptibility to
aminoglycosides, fluoroquinolones, and chloramphenicol; and
resistance to trimethoprim-sulfamethoxazole, polymyxin B, and
nalidixic acid (
69,
91). Antibiotic susceptibility tests performed
by broth macrodilution usually demonstrate the following MICs:
penicillin, <0.03 µg/ml; cephalothin, <0.03 µg/ml;
ceftriaxone, <0.03 µg/ml; vancomycin, 0.5 µg/ml;
tetracycline, 0.25 µg/ml; erythromycin, 2 µg/ml;
streptomycin, 8 µg/ml; and gentamicin, 1 µg/ml (
68).
Only one penicillin-resistant
S. moniliformis strain has been
demonstrated (
81), and that was over 50 years ago.
Adults with rat bite fever should receive 400,000 to 600,000 IU/day (240 to 360 mg) of intravenous penicillin G for at least 7 days, but this dose should be increased to 1.2 million IU/day (720 mg) if no response is seen within 2 days (65). Children should receive 20,000 to 50,000 IU/kg of body weight/day (12 to 30 mg/kg/day) of intravenous penicillin G for 5 to 7 days, followed by 7 days of oral penicillin V, 25 to 50 mg/kg/day (maximum, 3 g/day) divided four times per day (27, 73). For penicillin-allergic patients, both streptomycin and tetracycline appear to be effective (61, 68), but erythromycin use has been associated with treatment failures (35). Cephalosporins have also been used successfully (16, 20) and may be considered if cross-allergenicity with penicillin is felt to be unlikely. Other antimicrobials may be considered, based on the in vitro susceptibility data presented above, but no published evaluations of their effectiveness exist.
Patients with S. moniliformis endocarditis require dual therapy with high-dose penicillin G in combination with streptomycin or gentamicin (50). The currently recommended dose for adults is 4.8 million IU/day (4.8 g) of intramuscular procaine penicillin G if the isolate is susceptible to 0.1 µg/ml. If the isolate is more resistant, 20 million IU/day (12 g) of intravenous penicillin G should be used (65, 68) for adults. Children should receive 160,000 to 240,000 IU/kg/day (96 to 144 mg/kg/day), up to the adult maximum of 20 million IU/day (12 g) (68, 73). Successful treatment of adults with a 4-week regimen has been demonstrated (50). The appropriate treatment length for children is not known, although 6-week regimens generally are considered effective for other causes of bacterial endocarditis. The use of streptomycin appears to enhance activity against the cell wall-deficient L forms of S. moniliformis (68); one might anticipate that other aminoglycosides would provide the same benefit.

CONCLUSIONS
Rat bite fever, caused by
S. moniliformis, is an under-recognized
and under-reported disease characterized by abrupt onset of
fever, rigors, and migratory polyarthralgias; it carries a mortality
rate of approximately 10%. Although
S. moniliformis is exquisitely
susceptible to penicillin, most patients experience treatment
delays due to the nonspecific nature of the clinical features,
a broad differential diagnosis list, and difficulties in culture
diagnosis. However, the changing epidemiology of rodent exposure,
together with the risk of severe, invasive disease if left untreated,
suggests that rat bite fever and
S. moniliformis should occupy
a more prominent place in our diagnostic thinking.

FOOTNOTES
* Corresponding author. Mailing address: Department of Pediatrics, University of Arizona Health Sciences Center, 1501 N. Campbell Ave., Tucson, AZ 85724-5073. Phone: (520) 626-6507. Fax: (520) 626-5652. E-mail:
selliott{at}peds.arizona.edu.


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