Lessons from practice - Meningitis and pneumonitis caused by pet rodents

It is possible that our patient was co-infected with an arenavirus such as lymphocytic choriomeningitis virus (LCMV). Although LCMV is rare, it can be acquired by handling pet rodents or their excreta and can cause a flu-like illness with aseptic meningitis.1 Diagnosis of LCMV infection would require molecular testing of blood or CSF in a reference laboratory.

S. moniliformis, a causative agent of rat bite fever, is part of the commensal flora of the rat’s oropharynx.2 Another form of rat bite fever — known as “sodoku” — is caused by Spirillum minus, a spirochaete-like organism.2 Sodoku generally differs from S. moniliformis infection by causing induration at the site of the rodent bite and having an incubation period of more than 10 days. The incubation period for S. moniliformis infection is typically fewer than 7 days.2

Rat bite fever, as the name suggests, is usually acquired through a rat bite. However, the disease can result from handling and exposure to excreta or saliva of rodents such as rats or guinea pigs.2,3 S. moniliformis infection can also be caused by ingestion of contaminated milk; in such cases it is known as Haverhill fever, as the first known outbreak (in 1926) occurred in Haverhill, Massachusetts.2

Clinical manifestations of S. moniliformis infection include fever, headache, rash and polyarthritis. The rash typically involves the extremities, including the palms and soles, with a leukocytoclastic vasculitis seen on histology.4 The disease is often mild or self-limiting. However, infection is potentially lethal and can cause rapid death in previously healthy adults.2,5 The overall mortality rate is estimated to be 13%.2 Although less frequently reported than fever, headache, rash and polyarthritis, pneumonitis and endocarditis are common autopsy findings in those who succumb to the disease.5-7 Meningitis due to S. moniliformis is rare, but it has been described in Dutch and Portuguese literature.8,9

Classically, this organism has been described as slow and difficult to grow in the laboratory. This was not the experience in our laboratory as growth occurred within 24 hours, although enriched media and incubation with supplemental carbon dioxide was required. The initial Gram stain was not distinctive, but with subculture the bacteria became irregular and formed bulbar swellings, a feature highly suggestive of S. moniliformis.

The standard recommended therapy is penicillin, but S. moniliformis is susceptible to a number of other antibiotics including cephalosporins and macrolides. In cases where the organism has been isolated, in-vitro antibiotic susceptibility testing should be done for any antibiotic being considered for clinical use.2

Systemic illness following rat bite has been recognised for thousands of years. Historically, the disease was associated with squalor and slum-dwelling. However, victims in recent years have included laboratory workers, pet shop employees and, increasingly, owners of pet rats.2,4,5,10 As this case demonstrates, a bite is not necessary for infection — close contact with rodents may be sufficient. As rodents become more popular as household pets, more cases of S. moniliformis infection due to affectionate contact are likely to occur.

1 Chest radiograph (mobile, erect) taken on Day 2, showing diffuse bilateral infiltrates consistent with a pneumonic process

2 Gram stain of Streptobacillus moniliformis cultured from the patient’s blood (100 × magnification)

3 Characteristic rash on the patient’s palms