Toxoplasmosis

There have been several studies of seroprevalence of toxoplasmosis in the general Canadian population, but most are dated from the 1960s and 1970s. Population-based seroprevalence estimates vary from as low as 2.2% in Cree and Ojibway Indians in Ontario and

Table 7.4 Seroprevalence studies of toxoplasmosis in pregnant women in Canada

First author

Year Canadian location

Sample size

Testa Cut-off titre

Seroprevalence (%)

Grimard

1996Québec

2 141

Agg1 4 IU

11.2

Karim

1977British Columbia

305

DT 1:8

25.0

Mackenzie

1974Nova Scotia

65

IHA —

13.7

Martineau

1974Québec

1 162

IFA 4 IU

48.1

McDonald

1990North Québec

30

IFA 4 IU

49.9

Proctor

1994British Columbia

49

ELISA—

20.4

Tanner

1987North Québec

131 (Inuit)

IHA 1:32

72.0

31 (Indian)

19.0

Viens

1977Québec

4 136

IFA 4 IU

40.8

Notes a ELISA: Enzyme-linked immunosorbent assay; IHA: indirect haemagglutination assay; DT: dye test; Aggl: agglutination test; IFA: indirect immunofluorescence assay. —Not reported.

12% among Indian patients in northern Québec to as high as 48% among Inuit patients in northern Québec and 53% in Montréal, Québec. While the variability in the estimates of seroprevalence is great, the absence of large-scale or representative population-based studies using standardized serological tests makes it difficult to characterize the true magnitude of infection.

The primary focus of study on seroprevalence has been in women of reproductive age. Seroprevalence estimates reported in this population sub-group (Table 7.4) indicate not only an important variation in the magnitude of infection but also that congenital toxoplasmosis continues to be an important health concern. From available Canadian data, it has been estimated that between 140 and 1 400 cases of congenital toxoplasmosis occur annually, with severe manifestations at birth in 70-280 infants. Many of the remaining cases will experience sequelae over several years (Carter and Frank 1986). Toxoplasma infection is generally acquired in one of two ways: from ingestion of raw or undercooked meat containing infective bradyzoïtes or from ingestion of oocysts excreted by felines, typically via oocyst contamination in cat litter or soil. In North America, there is evidence of both these types of transmission but the contribution of each has been difficult to assess. However, in northern native communities, the likelihood is that transmission is almost exclusively related to ingestion of raw and undercooked meat of various kinds. An epidemiologic study conducted in northern Québec in 1988 among 22 pregnant women revealed that seroconversion during pregnancy was related to skinning of animals and the frequent consumption of caribou meat while seropositivity was related to a history of ingestion of dried seal meat, fresh seal liver, and the frequent consumption of caribou meat (McDonald et al. 1990).

Oocyst contamination of water and other foods, particularly those which are water-washed, is possible and may play a role in transmission. It is suspected that the winter 1995 outbreak of toxoplasmosis in the greater Victoria region in British Columbia was due to contamination of the water supply, likely by free-roaming cougars (Stephen et al. 1996).

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