Abstract
Ingestion of undercooked meat has been proposed as an important source of human Toxoplasma gondii infection. To ascertain the contribution of meat consumption to the risk of human infection, estimates of the prevalence of infection in meat-producing animals are required. A cross-sectional study was conducted to assess T. gondii infection in pigs raised in England, to identify risk factors for infection, and to compare performance of two serological tests: modified agglutination test (MAT) and enzyme-linked immunosorbent assay (ELISA). Blood samples from 2071 slaughter pigs originating from 131 farms were collected and 75 (3.6%) were found to be positive by MAT. Positive pigs originated from 24 farms. A subset of samples (n = 492) were tested using ELISA, and a significant disagreement (p < 0.001) was found between the two tests. An empirical Bayes approach was used to estimate the farm-level prevalence and the probability of each individual farm having at least one positive animal, considering the uncertainty arising from the sampling strategy and the imperfect test performance. The adjusted farm-level prevalence was 11.5% (95% credible interval of positive farms 8.4-16.0%). Two different criteria were used for classifying farms as infected: (1) ≥50% probability of having at least one infected pig (n = 5, 6.8%) and (2) ≥10% probability (n = 15, 20.5%). Data on putative risk factors were obtained for 73 farms. Using a 10% cutoff, the relative risk (RR) of infection was higher in farms where cats have direct access to pigs' food (RR = 2.6; p = 0.04), pigs have outdoor access (RR = 3.0; p = 0.04), and farms keeping ≤200 pigs (RR = 3.9; p = 0.02), with strong collinearity between the three variables. The findings suggest a low level of T. gondii infection in the farms studied, most of which are likely to send to slaughter batches comprising 100% uninfected pigs. These results provide key inputs to quantitatively assess the T. gondii risk posed by pork to consumers.
Highlights
Toxoplasmosis is a worldwide distributed zoonosis caused by the protozoan parasite Toxoplasma ee gondii (T. gondii)
The objectives of this study were (i) to assess, by means of an empirical Bayes estimation, the probability of T. gondii infection in selected commercial farms in England, (ii) to identify factors associated with a higher risk of T. gondii infection at farm level and (iii) to compare the performance of the reference serological test for T. gondii in pigs (MAT), with a commercially available enzymelinked immunosorbent assay (ELISA)
The proportion of farms deemed positive was 1.5% higher using results given by ELISA when considering a ≥50% cut-off
Summary
Toxoplasmosis is a worldwide distributed zoonosis caused by the protozoan parasite Toxoplasma ee gondii (T. gondii). Ie ev Humans can become infected via three main routes: (i) congenital, (ii) ingestion of sporulated oocysts present in cats’ litter trays or contaminated soil, water and vegetables and (iii) consumption of raw or undercooked meat containing T. gondii bradyzoites clustered in tissue cysts (‘infective cysts’). (Andreoletti et al, 2007; Tenter et al, 2000) The latter has been considered the most important route of infection in developed countries by the World Health Organization (WHO, 2015). It is estimated that up to a third of the world’s population is currently infected with T. gondii with important differences between and within countries (Pappas et al, 2009; Tenter et al, 2000). Estimates of the overall incidence of human toxoplasmosis in England are lacking, as records of the number of confirmed cases (on average 330 cases per year) represent a small proportion of the total number of cases in the population given the asymptomatic nature of the infection in healthy individuals (PHE, 2015, 2016)
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