Recent developments in the diagnosis of toxoplasmosis

Determination of the viability of Toxoplasma gondii in cured ham using bioassay: influence of technological processing and food safety implications

Toxoplasmosis is a zoonotic disease caused by the protozoan Toxoplasma gondii and distributed worldwide. Ingestion of viable cysts from infected raw or undercooked meat is an important route of horizontal transmission of the parasite to humans. Little information is available concerning the effect of commercial curing on cysts of T. gondii. This study is the first in which the influence of processing of cured ham on the viability of T. gondii has been evaluated, using bioassay to assess the risk of infection from eating this meat product. Naturally infected pigs were selected for the study, and a mouse concentration bioassay technique was used to demonstrate viable bradyzoites of T. gondii in porcine tissues and hams. No viable parasites were found in the final product (14 months of curing) based on results of the indirect immunofluorescence assay and histological and PCR analyses. Our results indicate that the consumption of hams cured as described here poses an insignificant risk of acquiring toxoplasmosis. However, additional studies are required to evaluate the safety of ham products cured under different conditions of curing time, salt, and nitrite concentration.

Optimization and evaluation of a PCR assay for detecting toxoplasmic encephalitis in patients with AIDS

Toxoplasma gondii is a common life-threatening opportunistic infection. We used experimental murine T. gondii infection to optimize the PCR for diagnostic use, define its sensitivity, and characterize the time course and tissue distribution of experimental toxoplasmosis. PCR conditions were adjusted until the assay reliably detected quantities of DNA derived from less than a single parasite. Forty-two mice were inoculated intraperitoneally with T. gondii tachyzoites and sacrificed from 6 to 72 h later. Examination of tissues with PCR and histology revealed progression of infection from blood to lung, heart, liver, and brain, with PCR consistently detecting parasites earlier than microscopy and with no false-positive results. We then evaluated the diagnostic value of this PCR assay in human patients. We studied cerebrospinal fluid and serum samples from 12 patients with AIDS and confirmed toxoplasmic encephalitis (defined as positive mouse inoculation and/or all of the Centers for Disease Control clinical diagnostic criteria), 12 human immunodeficiency virus-infected patients with suspected cerebral toxoplasmosis who had neither CDC diagnostic criteria nor positive mouse inoculation, 26 human immunodeficiency virus-infected patients with other opportunistic infections and no signs of cerebral toxoplasmosis, and 18 immunocompetent patients with neurocysticercosis. Eleven of the 12 patients with confirmed toxoplasmosis had positive PCR results in either blood or cerebrospinal fluid samples (6 of 9 blood samples and 8 of 12 cerebrospinal fluid samples). All samples from control patients were negative. This study demonstrates the high sensitivity, specificity, and clinical utility of PCR in the diagnosis of toxoplasmic encephalitis in a resource-poor setting.

Antibody maturation in Trypanosoma cruzi-infected rats

The study of antibody avidity changes during infection has improved the understanding of the pathologic processes involved in several infectious diseases. In some infections, like toxoplasmosis, this information is being used for diagnostic purposes. Results of the evolution of antibody avidity for different specific antigens in Trypanosome cruzi-infected rats are presented. A Western blotting technique, combined with avidity analysis to identify antigens that elicit high-avidity antibodies, is suggested. In this system, antibodies showed high avidity values only during the chronic phase of infection and only in relation to antibodies against 21-, 33-, 41-, 42-, 56-, 58-, 66-, and 72-kDa antigens. Finally, a 97-kDa T. cruzi antigen, which was recognized by high-avidity antibodies and occurred in noninfected rats, was identified. These results allow us to evaluate the different antigens in chagasic infection. Our results show that with the correct choice of antigen it is possible to detect differences in maturation of antibodies and to discriminate, in an experimental model, between recent (acute) and chronic infections.

Quantitation of Toxoplasma gondii DNA in a competitive nested polymerase chain reaction

AIM

To quantify Toxoplasma gondii DNA using a specially constructed artificial template as competitor in a nested polymerase chain reaction (PCR).

METHODS

The diagnostic assay was a nested PCR employing four primers that amplify part of the single copy gene for the P30 major surface antigen in T gondii. An artificial competitor containing the four primer binding sites was made first by creating a 216 bp deletion in the native 914 bp full length PCR product using restriction enzyme digestion, ligation of selected digestion fragments, and cloning the ligation product into an E coli plasmid vector for production. Competitive nested PCR using three different quantities of T gondii genomic DNA with four corresponding 10-fold dilutions of the artificial competitor was then performed, and the results visualised with agarose gel electrophoresis. A standard curve was drawn by plotting the T gondii to competitor ratio readings against log10 of the competitor readings.

RESULTS

The band intensities on agarose gel showed quantitative amplification in competitive nested PCR. The amount of competitor required to achieve equal molar amounts of PCR products is calculated by reading off the value of the competitor where the T gondii to competitor ratio equals 1 on the standard curves.

CONCLUSIONS

Competitive PCR is possible with a nested assay, and quantitative amplification is well preserved. The use of an artificial competitor containing the same primer binding sites as the target enables the absolute amount of T gondii DNA in unknown samples to be estimated. In addition, the competitor simultaneously serves as a control for detecting false negative results of failed reactions in individual assay runs.

Do IgA, IgE, and IgG avidity tests have any value in the diagnosis of toxoplasma infection in pregnancy?

AIM

To determine the value of tests for specific IgA, IgE, and IgG avidity in diagnosing Toxoplasma gondii infection during pregnancy.

METHODS

In a retrospective study, current serological tests (dye test and three IgM assays with different sensitivities) were compared with immunosorbent agglutination assays (ISAGA) for specific IgA and IgE and an IgG avidity enzyme linked immunosorbent assay (ELISA). Patient group 1 comprised six women with definite or probable infection during pregnancy determined by congenital toxoplasmosis or laboratory results. Group 2 comprised seven women infected during or before 11 pregnancies (two consecutive pregnancies in two patients and three in a third).

RESULTS

One patient in group 1 seroconverted during pregnancy. IgA ISAGA and avidity confirmed acute infection when confirmatory IgM ELISA remained negative. In five of six patients from group 1, IgA and IgE ISAGA and avidity confirmed acute infection. In group 2, the dye test titre was raised in seven of 11 pregnancies (six of seven patients). Specific IgM and IgA were positive during all 11 pregnancies. IgE ISAGA was positive in only four of 11 pregnancies (three of seven patients), but negative results in the remainder may exclude acute infection. High avidity antibodies indicative of past infection were found in four of 11 pregnancies (two of seven patients).

CONCLUSIONS

Each test improved diagnosis or timing of infection but no single test was ideal. The IgA ISAGA was sensitive and detected seroconversion. Positive IgE ISAGA and low avidity both confirmed infection, whereas negative IgE may exclude acute infection. High avidity diagnosed past infection but persistence of low avidity reduced its value to differentiate acute and past infection. Further studies with larger patient groups are needed to determine the optimum diagnostic strategy. These techniques are valuable in complementing existing tests.

Improved diagnosis of reactivated toxoplasmosis

AIMS

To identify antigens detected by western blotting in primary Toxoplasma gondii infection and determine their role in diagnosis of reactivated toxoplasmosis.

METHODS

Twenty three immunocompromised patients were tested by IgG western blotting. Patients were grouped retrospectively. Group 1 comprised 15 human immunodeficiency (HIV)/AIDS patients and included: group 1A (six patients with clinical and/or serological evidence of reactivation), group 1B (five patients with clinical evidence only), and group 1C (four asymptomatic patients). Group 2 comprised eight non-HIV/AIDS immunocompromised patients with clinical and/or serological evidence of reactivation. Immunocompetent patients (n = 23) with primary toxoplasmosis were a control group used to determine the progression of the antigens detected.

RESULTS

In primary toxoplasmosis, antibodies against 6, 20, 22, 23, 25, 28, 29, and 36 kDa antigens predominated. Detection of four or more of the 6, 20, 22, 23, 25, and 36 kDa antigens was considered to be western blot positive. In two group 1A patients, western blotting indicated past infection. During reactivation, this reverted to being western blot positive. Three other group 1A patients were western blot positive. In three of five group 1B patients, western blot positive results improved serological diagnosis of reactivated toxoplasmosis (p < 0.05). In two of five group 1B patients and all four group 1C patients, western blot indicated past infection. In group 2, two of eight patients reverted from a pattern of past infection to western blot positive. Five other patients from group 2 were western blot positive.

CONCLUSIONS

Detection of some low molecular weight antigens is diagnostic of reactivated toxoplasmosis. These antigens can be detected even with normal dye test titres and their detection improves the diagnosis of reactivated toxoplasmosis. They might be the result of the release of bradyzoites from ruptured tissue cysts.

Comparison of cell culture, mouse inoculation, and PCR for detection of Toxoplasma gondii: effects of storage conditions on sensitivity

The sensitivity of detection of a wild-type strain of Toxoplasma gondii by cell culture, mouse inoculation, and PCR was determined following sample storage under conditions to which clinical specimens may be subjected during transport to the testing laboratory. Sample storage at -20 degrees C significantly decreased the sensitivity of mouse inoculation. The sensitivity of cell culture decreased with sample storage at 4 and -20 degrees C. The sensitivity of PCR was reduced by storage at 4 degrees C for 48 h, freezing, and heating. These findings have implications for the selection of appropriate methods for the direct detection of T. gondii organisms in suboptimally transported clinical samples.