Epidemiology and aetiology of encephalitis in Canada, 1994-2008: a case for undiagnosed arboviral agents?

Abstract

Encephalitis is a clinical syndrome often associated with infectious agents. This study describes the epidemiology and disease burden associated with encephalitis in Canada and explores possible associations with arboviral causes. Encephalitis-associated hospitalizations, 1994-2008, were analysed according to aetiological category (based on ICD-9/ICD-10 codes) and other factors using multivariate logistic regression for grouped (blocked) data and negative binomial regression. A discrete Poisson model tested spatio-temporal clustering of hospitalizations associated with unclassified and arboviral encephalitis aetiologies. Encephalitis accounted for an estimated 24028 hospitalizations in Canada (5·2/100 000 population) and unknown aetiologies represented 50% of these hospitalizations. In 2003, clusters of unclassified encephalitis were identified in the summer and early autumn months signifying potential underlying arboviral aetiologies. Spatio-temporal patterns in encephalitis hospitalizations may help us to better understand the disease burden associated with arboviruses and other zoonotic pathogens in Canada and to develop appropriate surveillance systems.

Fig. 1.

Encephalitis-associated hospitalizations per year for all causes, known aetiologies and unknown aetiologies, 1994–2008.

Fig. 2.

Encephalitis-associated hospitalizations per month for all causes, known aetiologies and unknown aetiologies, 1994–2008.

Fig. 3.

Monthly rates of hospitalization associated with encephalitis of unknown (solid line) and arboviral (dotted line) aetiologies, by province/region, 1994–2008.

Fig. 4.

Encephalitis-associated hospitalizations by age group for all causes and different aetiological categories, 1994–2008.

Fig. 5.

Distribution of hospitalizations associated with encephalitis due to unknown and mosquito-borne viral aetiologies in Canada, 2003. AB, Alberta; BC, British Columbia; MB, Manitoba; NB, New Brunswick; NL, Newfoundland; NS, Nova Scotia; NU, Nunavut; NWT, Northwest Territories; ON, Ontario; PEI, Prince Edward Island; QC, Quebec; SK, Saskatchewan; YK, Yukon.

Fig. 6.

Space–time clusters of hospitalizations associated with encephalitis due to unknown and arboviral aetiologies, 2003.