To the Editor: Lymphocytic choriomeningitis virus (LCMV) is a rodent-borne pathogen that causes a spectrum of disease in humans, ranging from self-limiting meningoencephalitis to congenital birth defects to severe disseminated illness in organ transplant recipients (1). It is not known how frequently cases of LCMV infection are diagnosed in the United States. We performed a retrospective case analysis of American Indian and Alaska Native (AI/AN) patients treated within the Indian Health Service (IHS) health care system who had an LCMV-associated diagnosis defined by the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM). Our goal was to estimate the incidence of LCMV-associated aseptic meningitis and encephalitis diagnosed within a well-defined population.

Inpatient and outpatient visit data from fiscal years 2001–09 were obtained from the IHS National Patient Information Reporting System (2). For each fiscal year, records for AI/AN patients with at least 1 inpatient or outpatient visit within the year that listed the ICD-9-CM code 049.0 (lymphocytic choriomeningitis and meningoencephalitis) were selected (3,4). A subset of medical records was reviewed for patients with the diagnosis code of interest in the IHS Southwest and Southern Plains regions. Health care facilities were located in Arizona, New Mexico, and Oklahoma, USA. Suspected LCMV infection was defined as a diagnosis of meningitis, choriomeningitis, encephalitis, or meningoencephalitis not explained by another etiologic agent. A confirmed case of LCMV infection required laboratory detection of antibodies, virus antigen, or virus. A patient with no evidence of suspected LCMV infection in his or her medical record was not considered to have a case of LCMV infection.

Annual population denominators were determined by using annual IHS Southwest and Southern Plains region user populations, which includes all registered AI/ANs who received IHS-funded health care at least once during the previous 3 years (5). The annual average incidence rate for diagnosed cases of infection with LCMV was determined. Rates were also determined for viral meningitis not otherwise specified (ICD-9-CM code 047.9); for unspecified causes of encephalitis, myelitis, or encephalomyelitis (323.9); and for unspecified non-arthropod–borne viral diseases of the central nervous system or viral encephalitis not otherwise specified (049.9) (3). Annual numbers were calculated on a patient basis, whereby the first time a diagnosis was coded for a given patient during each fiscal year was counted.

Twenty-six AI/AN patients received the diagnosis code of 049.0 during fiscal years 2001–09. Of these patients, 16 received the diagnosis in the Southwest or Southern Plains regions. Fourteen available medical charts from these 2 regions were reviewed, and 4 patients were classified as having signs and symptoms consistent with suspected LCMV infection (Table), although no patients were confirmed by diagnostic testing as having LCMV infection. All 4 suspected cases of LCMV infection were in women (age range 16–43 years) who had a 1–2-day history of headache, nausea, and vomiting. Photophobia and neck or back pain were present in patients 1–3. Cerebrospinal fluid from patients 2–4 had increased leukocyte counts that were lymphocytic.

The diagnoses of the 10 remaining patients were classified as miscodes because the written diagnoses in the charts did not mention lymphocytic meningitis. Two patients had central nervous system disease (Lyme encephalitis and tuberculosis meningitis) although an etiologic agent was confirmed that was not LCMV. Additional diagnoses mistakenly coded are shown in the Table.

Among the 4 patients identified as having clinical signs and symptoms of suspected LCMV infection in the Southwest and Southern Plains regions during fiscal years 2001–09, the average annual incidence rate was estimated to be 0.06 cases/100,000 persons. In the same population, viral meningitis not otherwise specified was reported for 971 patients (incidence rate 13.69 cases/100,000 persons/year). Unspecified causes of encephalitis, myelitis, or encephalomyelitis were diagnosed for 444 patients (incidence rate 6.26 cases/100,000 persons), and unspecified non-arthropod–borne viral diseases of the central nervous system or viral encephalitis not otherwise specified were diagnosed for 56 patients (incidence rate 0.78 cases/100,000 persons).

Using a population-based analysis of diagnoses for patients who visited the IHS system at least 1 time as an inpatient or outpatient, we found that LCMV-associated aseptic meningitis and encephalitis were infrequently diagnosed and that confirmatory testing was not conducted. We also found that the code for LCMV was used incorrectly for several patients, probably because the virus is named ambiguously, and LCMV could be mistaken for other unrelated diseases with a lympho prefix. In addition, the definition for the code does not specify LCMV infection. We recommend that description of the ICD-9-CM and ICD-10 codes should be adjusted accordingly to clarify and reduce coding errors.

LCMV infection is considered to be underdiagnosed (6,7), although the virus was not detected in several studies that prospectively tested for LCMV in large numbers (>90) of clinical encephalitis or meningitis patients (8–10). Despite infrequently causing aseptic meningitis or mild disease in immunocompetent persons, LCMV has the potential to cause severe disease and should be considered for patients with a compatible illness and potential rodent exposure.