Biotin-labelled antigen screening test for toxoplasma IgM antibody

Success in the toxoplasma dye test

OBJECTIVES

To compare the success rate of the toxoplasma dye test using different accessory factors (human serum as a source of complement) and different batches of tachyzoites produced in vivo and in vitro.

METHODS

Twenty-five accessory factors were used in the dye test to assess both types of tachyzoite. Batches of tachyzoites were produced in vivo (n = 49) and in vitro (n = 23) and their performance assessed against panels of accessory factors. Performance was recorded as success or failure (incorrect results, total killing or no killing).

RESULTS

With in vivo tachyzoites 21/25 accessory factors were successful in P > or = 1 dye test runs, whereas with in vitro tachyzoites all 25 were successful. One or more failure was recorded for 19/25 and 12/25 accessory factors using in vivo and in vitro tachyzoites, respectively (P < 0.05). The number of successful dye test runs was less for in vivo (92/141, 65%) than in vitro (140/163, 86%) tachyzoites (P < 0.001). This was due to a higher success rate in citrated accessory factors used for in vitro tachyzoites compared to the corresponding uncitrated accessory factors used for in vivo tachyzoites (P < 0.001). Success in the dye test was recorded for 48/49 and 23/23 batches of in vivo and in vitro tachyzoites, respectively. The number of successful dye test runs was lower with in vivo (156/234, 67%) than in vitro (116/142, 82%) tachyzoites (P < 0.01).

CONCLUSIONS

Success in the dye test may be due to the accessory factor, tachyzoites, or a combination of both. Problems due to the accessory factor can be minimized by careful quality control or use of modification procedures. Tachyzoites produced in vitro may also increase success in the dye test. Careful selection of accessory factor/tachyzoite combination makes it possible to use the dye test in a district general hospital.

Immunoblotting can help the diagnosis of ocular toxoplasmosis

AIMS

To determine whether IgG immunoblotting can improve the diagnosis of ocular toxoplasmosis.

METHODS

Samples of serum were tested from patients with ocular lesions that could be caused by toxoplasmosis. All such samples from Scotland and Northern Ireland are usually referred to the Scottish Toxoplasma Reference Laboratory. From questionnaires filled out by the clinicians, two groups of sera were identified: ocular toxoplasmosis (active and quiescent), n = 54 (group 1); and eye disease as a result of other causes, n = 36 (group 2). Control groups were made up of sera from patients with no eye disease and a normal dye test result (< or = 125 IU/ml), n = 16 (group 3); and toxoplasma seronegative, cytomegalovirus (CMV) positive, and herpes simplex virus (HSV) positive sera (group 4), n = 18.

RESULTS

Immunoblots with an active pattern could be identified (IgG antibodies against at least four antigens with molecular weight of 6, 20, 22, 23, 25, and 36 kDa). Significantly more of this pattern was found in group 1 (33 of 54; 61.1%) compared with group 2 (nine of 36; 25%) or group 3 (six of 16; 37.5%). Within group 1, significantly more sera with an active pattern had dye test results > or = 65 IU/ml compared with those without. More sera from patients < 30 years of age were found with the active pattern in group 1 compared with group 2. No group 4 sera had active immunoblot patterns.

CONCLUSIONS

The immunoblot result adds more support to the diagnosis of ocular toxoplasmosis. In cases where the clinical diagnosis is difficult, immunoblots are particularly indicated; if negative, other causes of eye disease should be sought.

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.

Specificity and usefulness of an IgE immunosorbent agglutination assay for toxoplasmosis

AIMS

To develop an immunosorbent agglutination assay for the detection of Toxoplasma gondii IgE antibodies (IgE-ISAGA); to assess its specificity; and to determine the role of specific IgE in the diagnosis of current toxoplasma infection.

METHODS

Rabbit antihuman IgE capture antibody was adsorbed onto microtitre plates and formaldehyde fixed tachyzoites were used to identify specific antibody. Specificity was assessed in 513 serum samples (blood donor, potentially interfering and difficult, elevated and low total IgE and myeloma). Serum samples (n = 108) from 65 patients with documented toxoplasmosis were tested, as were 26 serum samples from nine pregnant women positive for specific IgM and 30 from 20 HIV positive patients.

RESULTS

IgE-ISAGA was highly specific with only three of 513 (0.58%) positive reactions amongst the control groups, one of which (0.19%) was regarded as a false positive. Elevated total IgE did not influence specific IgE results nor did the presence of abnormal immunoglobulin concentrations. Sixty (92.3%) patients with toxoplasma associated lymphadenopathy had specific IgE in one or more samples. Positive or borderline results were obtained in 68 of 77 (88.3%) serum samples taken up to four months after onset and were also detected for up to 11 months in 21 of 31 (67.7%) sera. Of the nine pregnant women with detectable specific IgM, specific IgE was absent in five (12 specimens). Specific IgE was also detected in 10 of 30 (33.3%) serum samples from the 20 HIV positive patients, which was similar to the number with specific IgM. Neither the specific IgE nor IgM tests could distinguish symptomatic from asymptomatic HIV positive patients.

CONCLUSIONS

IgE-ISAGA is sensitive, specific, and easy to perform. Although results suggest that specific IgE may be less helpful than previously claimed, specific IgE has a useful role in the diagnosis of current toxoplasma infection when used in conjunction with other tests.

Use of the polymerase chain reaction to detect Toxoplasma gondii in human blood samples

AIMS

To assess the value of detecting Toxoplasma gondii in human blood samples using the polymerase chain reaction (PCR).

METHODS

Blood samples in lithium heparin were examined from 34 patients with suspected toxoplasmosis, and six healthy volunteers with or without the addition of doubling dilutions of toxoplasma tachyzoites. Products of a nested PCR, using oligonucleotide primers of the B1 gene, were analysed by electrophoresis and stained by ethidium bromide. The primary product was 194 base pairs in length; the nested products were 160 or 97 base pairs.

RESULTS

When toxoplasma tachyzoites were added to the leucocytes of six different volunteers, eight to 16 parasites were detected by nested PCR in one experiment and one to four parasites in eight experiments. All nine experiments were negative in samples without added tachyzoites. Of 34 patients, PCR was negative in 13 with recent lymphadenopathy; nine with persisting IgM, including two pregnant patients; four with reactivated infection due to immunodeficiency; and five with no evidence of active infection. Positive PCR results were found in three patients with reactivated infection. There was only one discrepancy between PCR and animal culture results; this was in an immunocompromised patient with a positive PCR and negative culture.

CONCLUSIONS

The PCR technique was rapid, sensitive, and specific in human blood samples. Negative PCR results in patients with persisting IgM suggested that the fetus was not at risk, or that treatment was not indicated in myalgic encephalomyelitis-like illness. PCR results in immunocompromised patients permitted appropriate management--no treatment if negative, treatment if positive.

Comparison of relative uses of commercial assays for Toxoplasma gondii IgM antibodies

AIMS

To compare the sensitivity and user friendliness of seven commercially available enzyme linked immunoabsorbent assay (ELISA) kits for toxoplasma specific IgM.

METHODS

Five antibody capture assays supplied by Abbott, Mercia, Northumbria, Organon and Sorin, and two indirect ELISA assays from Biostat and Mast, were assessed. Using defined dilutions of Toxoplasma gondii specific IgM, the performance and sensitivity of each assay was established. They were further assessed on a panel of 27 sera with a range of dye test and IgM results (as determined by the Scottish Toxoplasma Reference Laboratory). All of the assays were performed by three experienced operators and assessed for user satisfaction.

RESULTS

The Mast, Organon, and Abbott assays were of low sensitivity; the Mercia and Northumbria were of high sensitivity; and the Biostat and Sorin assays produced too many false positive results. The Mercia kit provided most user satisfaction; the Mast and Abbott assays were most difficult to use.

CONCLUSIONS

Local laboratories investigating toxoplasma infection should have three tests: one IgG and two IgM (high and low sensitivity) to help in the timing of infection. Alternatively, one sensitive IgM assay, such as that of Mercia, could be used by selecting appropriate high and low thresholds.

Comparison of IgG antibody profiles by immunoblotting in patients with acute and previous Toxoplasma gondii infection

The IgG antibody profile to Toxoplasma gondii proteins of less than 37 kilodaltons in sera from patients with acute and previous infection was studied by immunoblotting. Bands at 28, 29, and 36 kilodaltons were more common in acute infection (10 out of 10, nine out of 10, and nine out of 10, respectively) compared with previous infection (five out of 10, four out of 10, and one out of 10, respectively). The 6 kilodalton band was present in 10 out of 10 sera from patients with acute infection and five out of 10 sera from those with previous infection. A new observation is a doublet of bands of 22-25 kilodaltons present only in sera from patients with acute infection. This doublet may be a more reliable indicator of acute infection than the 6 kilodalton band.

Congenital toxoplasmosis: to screen or not to screen?

We have reviewed the present day quantifiable cost to society of the 73 cases of congenital toxoplasmosis which are estimated to occur annually in Scotland with the cost of preventing the disease by screening and treatment. Our analysis includes advances in laboratory techniques. The cost of screening would depend on its scale and if in-house or commercial tests are used. If only 2 specimens were screened, at booking and at delivery, the screening costs are estimated to be between 0.5-0.9 times the preventable costs. If a third specimen were tested in the second trimester, to maximise scope for remedial action during pregnancy, the screening costs are 0.7-1.2 times preventable costs. As likely screening costs in most of the schemes we consider are now less than the preventable costs, a screening programme should be adopted.

Simultaneous diagnosis of toxoplasmosis in goats and goatowner's family

Four out of 6 goats from a small British goat herd gave birth to weak or stillborn kids. All were seropositive for Toxoplasma gondii. The parasite was isolated from the tissues of one kid and the milk of one goat. Concurrently, one of the goatowner's sons developed a mononucleosis-like illness with serological evidence of current toxoplasma infection. Investigation of the family showed past infection in the other son, but both parents were seronegative. The source of infection for both boys appeared to be the consumption of unpasteurized goat's milk.

Diagnosis of congenital toxoplasmosis in Scotland

During a period of 10 years (1976-1986) 853 cases of toxoplasmosis were identified serologically in the Toxoplasma Reference Laboratory in Scotland. Suspected cases of infection in pregnancy (29 cases) and of congenital infection (27 cases) were rare but ophthalmic disease was common (224 cases). In two of the congenital cases, maternal infection was identified during pregnancy. The other 25 babies were investigated because of suspected congenital infection. Toxoplasmosis was confirmed retrospectively in 11/25 babies and excluded in two of them. The other 12 cases remain unclassified. Diagnosis of infection in babies was difficult although the introduction of an IgM capture ELISA was of some help. IgM was detected by ELISA in 3/13 congenitally infected babies but by immunofluorescence tests in only one of these. In two antenatal studies, involving 16,000 women, an additional 32 women were identified whose babies might have benefited from treatment or the availability of fetoscopy for early detection of congenital infection.