Toxoplasma gondii prevalence

Adriaanse, Katherine, Firestone, Simon M., Lynch, Michael, Rendall, Anthony R, Sutherland, Duncan R., Hufschmid, Jasmin & Traub, Rebecca, 2020, Comparison of the modified agglutination test and real-time PCR for detection of Toxoplasma gondii exposure in feral cats from Phillip Island, Australia, and risk factors associated with infection, International Journal for Parasitology: Parasites and Wildlife 12, pp. 126-133 : 129-130

3.1. Demographics of feral cats sampled and Toxoplasma gondii prevalence

A total of 161 feral cats (91 males and 69 females) was collected between July 2016 and December 2017. Body weight ranged from 0.5 to 6.2 kg (mean = 2.7, SD = 1.4). Sex and weight data were not available for one cat. Cats were either live trapped (135), shot (22), hand caught (2), or found as roadkill (1). Capture data was not recorded for one cat. Serum was available from 97 cats. Toxoplasma gondii seroprevalence, as detected by MAT, was 91.8% (95% CI 84.6–95.8). Toxoplasma gondii qPCR results were available for all cats (n = 161). Toxoplasma gondii apparent prevalence, as detected by qPCR, was 79.5% (95% CI 72.6–85.0). Capture locations were distributed across most of the island and T. gondii qPCR positive individuals were widespread ( Fig. 1 ). DNA extraction controls confirmed amplifiable eukaryotic DNA was present in all T. gondii -negative samples (data not shown).

3.2. Evaluation of the modified agglutination test and real-time PCR as diagnostic tests

The overall agreement between MAT and qPCR was 87.6% ( Table 2). Given the extremely high prevalence of T. gondii infection in the present study, PABAK was calculated in preference to Cohen's kappa. PABAK was 0.753 (95% CI 0.588 –0.869), indicating substantial agreement between the two tests ( Byrt et al., 1993) .

Bayesian estimations of the diagnostic test parameters for qPCR and MAT, and the estimated true prevalence of T. gondii infection in the population are outlined in Table 3. On sensitivity analysis, no substantial changes were observed in the point estimates for the true prevalence of T. gondii , the sensitivity and specificity of the qPCR method, or the sensitivity of the MAT. When the lower credible limit of the prior distribution of the specificity of the MAT (SpMAT) was lowered to 50%, the point estimate of the specificity was reduced to 74.3% (95% CrI 44.5–95.7). Visual assessment confirmed that posterior distributions for all parameters except SpMAT were informed by the data, rather than overly influenced by the prior distributions. The plot of prior and posterior distributions of SpMAT suggested that this parameter may have been strongly influenced by the prior distribution inputs. When the data and prior distributions were analysed using the dependent tests model, the 95% CrI for the correlation terms crossed zero, satisfying the assumption that the tests were conditionally independent and justifying the use of the independent test model.

Figures

Fig. 1. Location and Toxoplasma gondii infection status, as detected by real-time PCR (qPCR), of feral cats trapped on Phillip Island (Victoria) from July 2016 to December 2017. Map shows the distribution of different location types (Park, Agricultural, Residential) used in multivariable regression analysis. A circular spread of points around a location marked with ‘x’ indicates multiple animals were sampled at the same site (i.e. same GPS coordinates). Red = T. gondii qPCR positive, white = T. gondii qPCR negative. Map created using Quantum GIS, version 3.8. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)