West Nile virus infection in 2002: morbidity and mortality among patients admitted to hospital in southcentral Ontario

Abstract

Background: In August and September 2002 an outbreak of West Nile virus (WNV) infection occurred in southern Ontario. We encountered a number of seriously ill patients at our hospitals. In this article we document the clinical characteristics of these cases.

Methods: We conducted a retrospective chart review of patients who came to the attention of infectious disease or neurology consultants or the microbiology laboratories at 7 hospitals in the municipalities of Toronto, Peel and Halton, Ont. Patients were included if they had been admitted to hospital or stayed overnight in the emergency department, had serological evidence of WNV infection and had clinical evidence of WNV fever, aseptic meningitis, encephalomyelitis or motor neuronopathy.

Results: In all, 64 patients met the inclusion criteria; 57 had encephalitis or neuromuscular weakness or both, 5 had aseptic meningitis, and 2 had WNV fever. The mean age was 61 years (range 26-87). The patients were predominantly active, middle-aged or elderly people living independently in the community. Seven patients were immunocompromised A febrile prodromal illness preceded the neurological symptoms in almost all cases. The most common neurological abnormality was decreased level of consciousness; this frequently evolved to severe lower motor neuron neuromuscular weakness. Ataxia and swallowing disorders were frequent and important problems. Sixteen patients (25%) required intubation and mechanical ventilation because of a decreased level of consciousness, inability to clear secretions or respiratory muscle weakness; 9 others had disabling muscle weakness of one or more limbs. Ten patients died. The study patients were in hospital a total of 1856 patient-days, including 532 patient-days in an intensive care unit. Only 28% (13/47) of the patients who survived encephalitis or neuromuscular weakness, or both, were discharged home without additional support. Slow turnaround time for serological test results resulted in delayed diagnosis.

Interpretation: The 2002 WNV infection outbreak in Ontario caused serious morbidity and mortality in the subset of patients who had encephalitis or neuromuscular weakness severe enough to require hospital admission.

Fig. 1: Number of people with West Nile virus infection admitted to 7 Toronto-area hospitals in 2002, by week of onset.

Fig. 2: Venn diagram, showing 3 principal types of neurological manifestations of West Nile virus infection: decreased level of consciousness (yellow circle), brainstem and cerebellar signs (pink circle) and neuromuscular weakness (blue circle). Most patients had abnormalities in all 3 areas (n = 18), decreased level of consciousness alone (n = 15) or a combination of decreased level of consciousness and brainstem/cerebellar signs (n = 12).