A standardized enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to Toxoplasma gondii

Prevalence and risk factors for exposure to Toxoplasma gondii in slaughterhouse workers in western Kenya

Background

Toxoplasma gondii is a zoonotic protozoan parasite infecting warm-blooded animals. Infection in people can occur through ingestion of oocysts passed in the faeces of the definitive hosts; ingestion of bradyzoites in the tissue of infected intermediate hosts; or exposure to tachyzoites in raw milk and eggs. Slaughterhouse workers are considered a high-risk group for T. gondii exposure because of their contact with raw meat, although a positive relationship between handling raw meat and T. gondii seropositivity has not been demonstrated in all studies. This study aimed to determine the seroprevalence of antibodies to T. gondii in slaughterhouse workers in Kenya and identify risk factors associated with seropositivity.

Methods

A survey of slaughterhouse workers was conducted in 142 slaughter facilities in the study area. Information regarding demographics, contact with livestock, meat consumption, and practices in the slaughterhouse was collected using structured questionnaires. Commercial ELISAs were used to detect IgM and IgG antibodies against T. gondii and a multi-level logistic regression model was used to identify potential risk factors for seropositivity in slaughterhouse workers.

Results

The apparent prevalence of antibodies to T. gondii was 84.0% (95% Confidence Interval (CI) 81.2–86.5%) for IgG and 2.2% (95% CI 1.3–3.5%) for IgM antibodies. All IgM positive individuals were IgG positive. Risk factors for exposure to T. gondii were: increasing age (Odds Ratio (OR) 1.03; 95% CI 1.01–1.05); owning poultry (OR 2.00; 95% CI 1.11–3.62); and consuming animal blood (OR 1.92; 95% CI 1.21–3.03).

Conclusions

The seroprevalence of antibodies to T. gondii was very high in this population and considerably higher than published values in the general population. Risk factors included age, owning poultry and drinking animal blood which were consistent with previous reports but none were specifically associated with working in the slaughterhouse. In this instance slaughterhouse workers may represent a useful sentinel for the general population where the level of exposure is also likely to be high and may signify an unidentified public health risk to vulnerable groups such as pregnant women. A detailed understanding of the epidemiology of infection is required, which should include an assessment of incidence, mortality, and burden since T. gondii infection is likely to have life-long sequelae.

Toxoplasma gondii: sensitive and rapid detection of infection by loop-mediated isothermal amplification (LAMP) method

Loop-mediated isothermal amplification (LAMP) method amplifies DNA with high specificity, sensitivity and rapidity. In this study, we used a conserved sequence in the 200- to 300-fold repetitive 529 bp gene of Toxoplasma gondii to design primers for LAMP test. Detection limit of T. gondii LAMP assay with the primers is 1 pg/microL of T. gondii DNA, which was evaluated using 10-fold serially diluted DNA of cultured parasites. Furthermore, LAMP and conventional PCR methods were applied for amplification of the T. gondii DNA extracted from the lymph nodes taken from pigs which were suspected to be Toxoplasma infection. As a result, 76.9% (70/91) and 85.7% (78/91) of the samples were positive on PCR and LAMP analyzes, respectively. Therefore, the LAMP has a potential to be applied as an alternative molecular diagnostic tool for detection of T. gondii infection from veterinary samples. This is the first study, which applies the LAMP method to diagnose Toxoplasma from veterinary samples.

Evaluation of kinetics and single-read enzyme-linked immunoassays for detection of Toxoplasma gondii antibodies in sheep

A kinetics enzyme-linked immunosorbent assay (ELISA) and a single-read ELISA for the detection of ovine anti-Toxoplasma gondii IgG were developed and optimized. During the kinetics assay, 3 optical densities were obtained for each serum sample at intervals of 45 seconds, and the results were presented as average slopes (replicates of 2) of the reaction rate between bound enzyme and substrate solution. The kinetics ELISA was stopped 5 minutes after dispensing the substrate to constitute the single-read ELISA, and the results were presented as average optical densities for duplicates of each sample. Performance of the assays was evaluated using the modified agglutination test (MAT) as the "gold standard." There was a high level of agreement between both ELISAs and the MAT, as measured by Pearson correlation coefficients, kappa statistics, and the area under the receiver operating characteristics curves. The single-read ELISA was as accurate as the kinetics ELISA, with a sensitivity of 89% and a specificity of 96%.

High incidence of toxoplasmosis during pregnancy in Nigeria

Sera from 606 women of child-bearing age from the Benue River Basin area of Nigeria were screened by serological tests to determine the incidence of toxoplasmosis in Nigeria; 43.7% had serologic evidence of infection, increasing from 25% in the group of 15 to 18-year-old women to 71.4% in the 39-42 year age group. Using a previously published epidemiological model, the incidence rate for each age group varied from 0.5% for 15 years old women to 5.3% in the age group 37 years, with an overall rate of 1.8%. Extending this model as it applies to the estimation of incidence of infection during pregnancy, a 5.4% expected rate of seroconversion was found.

Investigation of two commercially available kits for determination of IgG and IgM to Toxoplasma gondii by enzyme-linked immunosorbent assay (ELISA)

Enzyme-linked immunosorbent assay (ELISA) kits for determination of IgG and IgM to Toxoplasma gondii provided by Labsystems Oy, Finland, were tested on a number of human sera. It was found that the IgG kit agreed well with the dye test (DT) in differentiating positive and negative results. A deviation from a straight line relationship in the positive results obtained was considered due to the different antigens involved in the two tests. The correlation between the results obtained in the IgM kit and DS-ELISA was remarkably good. Only a small fraction of the false negative results due to high IgG titres and false positive results due to rheumatic factor, experienced in the indirect fluorescent antibody test, were apparent in the IgM kit. False positive results due to the presence of rheumatic factor could be corrected for by the use of an IgM confirmatory test kit. The influence of antinuclear factor was not apparent in any of the tests used in this work.