Modelling toxoplasma incidence from longitudinal seroprevalence in Stockholm, Sweden

Abstract

Age-stratified data on toxoplasma seroprevalence in pregnant women in Stockholm, Sweden for the years 1969, 1979 and 1987 provide the basis for an analysis of temporal patterns of Toxoplasma gondii infection, and estimation of the risk of maternal toxoplasmosis, in this population. A catalytic infection model, in which the rate or force of infection is assumed to be a function of time (and not, as is more usual, age), was employed to describe the observed changes in levels of toxoplasma seropositivity. A range of simple incidence functions (up to 3 parameters) were fitted using a method of maximum likelihood. The data were significantly better described by a linear or an exponential decay in the rate of infection through time compared with a constant level. More complex incidence functions gave no better data description. Thus, whilst there is strong evidence for declining incidence in Stockholm over the past 4-5 decades, the data do not allow discrimination between different possibilities for the nature of this decline. Based on these modelling results, best estimates of the force of infection in 1987 acting on susceptible women are within the range 0 to 0.0045/susceptible/year (95% confidence limits), yielding a possible risk of maternal toxoplasmosis of between 0 and 2.7 cases/1000 pregnancies. These values are shown to be significantly lower than estimates based upon an assumption of temporal stability in toxoplasma incidence, which may be of practical significance to public health policy.

Visually impaired Swedish children. Longitudinal comparisons 1980–1999

“…Regarding chorioretinitis, the decrease of the problem is in accordance to the earlier comparison between data published by Lindstedt (1972) and the present Swedish register (Blohmé & Tornqvist 1997 c). A reduction of the toxoplasmosis frequency in pregnant women, in Sweden as well as in Denmark, during the last 3-4 decades has been noted (Forsgren et al 1991;Nokes et al 1993;Lebech et al 1993), and this must be the explanation. The reason for this encouraging finding is unclear.…”

Toxoplasma gondii: from animals to humans

“…As a consequence, no two tests produce the same results in all cases, even when carried out in the same laboratory [45][46][47][48][49][50][51][52][53][54]. In addition, prevalence rates vary over time and with the age of the individuals included in the study [55][56][57][58][59][60][61][62][63][64][65][66][67][68].…”

Sero-epidemiology of rubella in the urban population of Addis Ababa, Ethiopia

“…Catalytic infection models were used to investigate age-dependence in the instantaneous incidence of rubella infection ; also known as the force of infection [13][14][15][16]. The analysis is based upon the following assumptions : (i) the proportion serologically positive at age a, p(a), defines the cumulative proportion infected by age a, and further assumes that no individual dies from post-natal rubella infection ; (ii) seropositivity is non-reversible, i.e.…”

“…Maximum-likelihood (ML) estimates (implicitly weighting for sample size) for parameters in each model were obtained using the procedure described by Nokes and colleagues [15] within Microsoft  using the Quasi-Newton search method. In deriving the 95 % confidence intervals (CI) for each force of infection estimate in the PWC model we assumed the log-likelihoods approximate to a χ# distribution [21].…”

“…where pZ(a) was the proportion seropositive by age a predicted by the model [equations (1) or (2)], and n was the sample size in age group a [15]. Comparison of goodness of fit to the data between models was made using the likelihood ratio test [21].…”