Prognostic importance of lymphopenia in West Nile encephalitis.

To the Editor:

During the summer months of 2003, the infectious disease service was consulted on 18 patients with viral (non–herpes simplex virus) meningitis/encephalitis. Of these patients, 6 (33%) had West Nile encephalitis. Among the 12 patients with non–West Nile viral meningitis/encephalitis, mild/transient relative lymphopenia (≤21% lymphocytes) was present in 5 patients (42%) on admission to the hospital. All 6 patients with West Nile encephalitis had relative lymphopenia. We found that even in this small sample relative lymphocyte counts were higher in patients with non–West Nile viral meningitis/encephalitis. This prompted us to review our 2001–2003 experience with West Nile encephalitis.

Of the 13 patients with West Nile encephalitis seen during the 3-year period, all had relative lymphopenia (range, 2% to 20%). However, patients with severe/fatal West Nile encephalitis had a lower relative lymphopenia (range, 2% to 9%) than did patients with nonsevere/fatal disease (range, 10% to 20%).

In addition to measles and human immunodeficiency virus, lymphopenia is an important nonspecific laboratory finding in typhoid fever, babesiosis, malaria, viral hepatitis, tuberculosis, histoplasmosis, brucellosis, and severe acute respiratory syndrome. Lymphopenia may also be induced by steroids, antilymphocyte globulin, alcohol, radiation, or chemotherapeutic agents for cancer. Lymphopenia may be present in a variety of noninfectious disorders, including rheumatoid arthritis, systemic lupus erythematosus, Hodgkin lymphomas, CD4 lymphocytopenia, severe combined immunodeficiency, ataxia-telangiectasia, Wiskott-Aldrich syndrome, and myasthenia gravis (1, 2, 3).

Many patients with viral illnesses have leukopenia, lymphopenia, or thrombocytopenia on presentation. Lymphopenia is one of the nonspecific clinical laboratory manifestations of West Nile encephalitis. We previously reported that prolonged lymphopenia in a patient with viral meningitis/encephalitis should be suggestive of West Nile encephalitis (4).

However, in our review of cases at the Winthrop-University Hospital from 2001 to 2003, we found that the degree of relative lymphopenia not only differentiated non–West Nile viral meningitis/encephalitis from West Nile encephalitis, but also was predictive of a severe/fatal outcome in patients with West Nile encephalitis (Table).

Table

Lymphocyte Counts in Patients With West Nile Encephalitis

Patient No.	Age (years)	Sex	Leukocyte Count (/mm3)	Lymphocytes (%)	Comments
1	80	M	13,000	7	Died
2	75	M	8900	10	Recovered
3	74	F	5900	2	Died
4	77	M	6900	11	Recovered
5	68	F	8600	13	Recovered
6	41	M	9500	20	Recovered
7	46	M	11,400	10	Recovered
8	61	F	8100	18	Recovered
9	85	M	17,300	14	Recovered
10	54	M	11,100	16	Recovered
11	69	M	5000	16	Recovered
12	17	M	17,200	9	Severe neurologic deficits
13	80	M	6900	16	Recovered

F = female; M = male.

The lymphocyte count, particularly the degree of relative lymphopenia, is a readily available test, and its diagnostic role in many disorders is underappreciated (5). The degree of relative lymphopenia (≤10%) appears to have prognostic importance in West Nile encephalitis.