The prevalence of anti-Toxoplasma antibodies among women of child-bearing age varies geographically, ranging from none to more than 90%. A seroprevalence of 39% was found in a large study of 22,845 pregnancies from diverse parts of the United States conducted between 1959 and 1966 (1083). Another survey of 17,658 sera from persons 12 years of age or older conducted in the United States between 1988 and 1994 showed that the overall age-adjusted prevalence was 22.5%. The overall prevalence was highest in the Northeast (29%) and lowest in the West (17.5%). Among women between 15 and 44 years of age, 15% had antibodies to T. gondii. Risk factors for seropositivity were increased age, birth in a foreign country, lower educational level, living in crowded conditions, and working in soil-related occupations. Current cat ownership did not increase the overall likelihood of having antibodies to this parasite (1084). A study of 4,234 persons 12 years of age or older conducted in 1999 and 2000 in the United States found that 15% of women had antibodies to T. gondii, with a higher prevalence among African Americans (1085). Seventh Day Adventists, who follow a diet that contains no meat, have a likelihood of being seropositive for T. gondii that is only 20% of that for a control group of other healthy adults living in the same area (1086). The average seroprevalence among HIV-1-infected women from New York, California, Washington, D.C., and Illinois is about 15%. HIV-1-infected women 50 years of age or older and those born outside the United States have seroprevalence rates of 32% to 41% (1087). Examples of seroprevalence figures for women of child-bearing age from other countries are India (Delhi), 2%; Australia (Melbourne), 4%; Taiwan, 9%; China (Lanzhou), 7%; China (Chengdu), 39%; southern Finland, 20%; Saudi Arabia, 30%; France (Strasbourg), 36%; Poland, 36%; Italy, 40%; Belgium, 46%; Germany, 46% to 68%; Greece, 52%; Panama, 63%; and Ethiopia, more than 75% (15,1082).

The incidence of acute T. gondii infection during pregnancy varies by locale. Reported frequencies per 1,000 pregnancies include Alabama with 0.6, Finland with 2.4, Australia with 5, New Zealand with 6, Germany with 7.5, and Belgium with 14.3 (15,1088-1090). The risk of infection is greatest for pregnant women leaving a region with a low incidence of toxoplasmosis to reside in an area in which the infection is prevalent (1088). Specific risk factors for acquisition of T. gondii infection in pregnancy may include consumption of cured pork and raw meat (including tasting raw meat while preparing food), eating unwashed raw vegetables or fruits, infrequent washing of hands or kitchen utensils after preparation of raw meat before handling another food item, contact with soil, and cleaning cat litter boxes (1091,1092,1093,1094).

Acute toxoplasmosis is asymptomatic in 80% to 90% of pregnant women. Individuals with clinically evident illnesses most commonly present with lymphadenopathy, primarily of the head and neck region; a single node is involved in about two-thirds of patients (1095). T. gondii causes about 1% to 5% of infectious mononucleosis cases and should be considered a likely etiologic agent in patients with negative heterophile antibody test results (1096). Complications such as hepatitis, pneumonia, myocarditis, encephalitis, and deafness are rare in immunocompetent women (1082,1097,1098). Ocular involvement manifests mainly as retinochoroiditis and retinochoroidal scars, but lesions such as neuroretinitis, retinal vascular occlusions, and scleritis also occur (1099,1100,1101). It is thought that severe ocular disease will occur in the immunocompetent patient if the infecting parasite is type I (1100). Psychiatric complications such as psychoses with schizophreniform features, anxiety, and depression have been encountered on rare occasions (1102,1103). Fulminating illnesses are common in immunosuppressed patients (1098,1104).

T. gondii spreads transplacentally to involve the developing fetus in 25%, 54%, or 65% of pregnant women with untreated primary toxoplasmosis during the first, second, or third trimester, respectively (1082). Proper maternal therapy reduces the overall incidence of fetal infection by more than 50%, and fewer infected infants manifest with severe congenital toxoplasmosis (1105,1106). These data underscore the importance of accurate and timely diagnosis of acute toxoplasmosis in pregnant women.

A clinical diagnosis of toxoplasmosis should be considered dubious unless supported by appropriate laboratory test results. T. gondii can be isolated from infected blood, CSF, aqueous humor, amniotic fluid, or homogenized tissues (e.g., placenta, brain, muscle) by inoculating these specimens into the peritoneal cavities of mice or onto tissue cultures. Tissue culture techniques are faster but less sensitive than intraperitoneal inoculation methods, but neither method is practical for clinical purposes. Tachyzoites can be visualized in tissue sections or smears of body fluids, especially if labeled specific anti-Toxoplasma antibodies are used for staining; their presence denotes acute infection. The demonstration of tissue cysts on histopathology can be consistent with an acute or a chronic Toxoplasma infection (619).

Other immunologic techniques that have been used for the diagnosis of toxoplasmosis include intradermal skin tests and transformation of lymphocytes on exposure to
Toxoplasma antigens, both of which connote chronic infection (1082). Toxoplasma antigens can be detected in CSF, urine, serum, or amniotic fluid using enzyme-linked immunosorbent assays (ELISA) or immunoblotting methods. A positive antigen test result indicates that the infection is recent. This is particularly helpful in newborns and immunodeficient persons in whom antibody responses to infection may be absent or unpredictable (15,1107). The antigen test is not available commercially. The PCR has been used successfully for the direct detection of T. gondii DNA in clinical samples by amplification of the P30 or B1 genes of the parasite; amplification of TGR1E (a repetitive DNA sequence) and a part of the small subunit ribosomal DNA (rDNA) also have been used (1108,1109,1110,1111,1112,1113,1114).

The diagnosis of toxoplasmosis most often rests on serologic confirmation. The Sabin-Feldman dye test, traditionally the reference test against which newer methods are compared, requires the use of live parasites. Positive titers are usually in the 1:256 to 1:128,000 range. Most laboratories have abandoned the dye test in favor of simpler techniques that use killed antigens, such as the IFA, ELISA, agglutination, and indirect hemagglutination (IHA) tests.

Toxoplasma-specific IgM antibodies can be measured by IFA, ELISA, or IgM immunosorbent agglutination assay (IgM-ISAGA). False-positive IgM-IFA or IgM-ELISA are encountered in sera containing rheumatoid factor or the IgM can be directed against sphingolipids or ceramides that co-migrate with a low molecular weight antigen of the parasite during lipid extraction; these problem are circumvented if a double-sandwich IgM-ELISA (DS-IgM-ELISA) is performed (1111,1115). Specific IgM antibodies usually become positive within 1 to 2 weeks of infection and continue to be detectable for months or years, especially when measured by very sensitive assays such as DS-IgM-ELISA or IgM-ISAGA. The detection of specific IgM antibodies should not be considered proof that an infection is acute. In a British study of 446 Toxoplasma-infected women, specific IgM antibodies were detected for median periods of 13 and 10 months using ISAGA or IFA methods, respectively (1116). Persistent IgM responses for longer than 2 years were seen in 27% of women using ISAGA and in 9% using IFA (1116). IgM antibodies have been detected for as long as 12 years after acute infection in rare instances (1111).

High specific IgM titers suggest acute infection, especially if accompanied by high specific IgG titers of about 1:1,000 or greater as measured by IFA or the Sabin-Feldman dye test. Low specific IgM titers measured by DS-IgM-ELISA or IgM-ISAGA generally are encountered in patients whose infections occurred several months earlier. IgM-IFA tests are considerably less sensitive than DS-IgM-ELISA or IgM-ISAGA and are positive in only 60% to 70% of patients with acute infections and 25% to 50% of infants with congenital toxoplasmosis. It is important to know the type of assay used to measure specific IgM antibodies in pregnant women to interpret correctly the significance of positive and negative results (15,619). Commercially available test kits for Toxoplasma IgM measurements unfortunately are unreliable as the sole determinant of recent T. gondii infection during pregnancy, and reliance on a single test result can lead to misdiagnoses and possibly inappropriate interventions (FDA Public Health Advisory: Limitations of Toxoplasma IgM Commercial Test Kits, July 25, 1997).

Anti-Toxoplasma IgG antibodies usually appear early during an infection. IgG titers peak at about 2 months, gradually drop thereafter, but remain detectable for years. The predominant IgG antibody response is of the IgG1 subclass (1117). A single high specific IgG titer is considered only suggestive of an acute infection. The strength of the binding of specific IgG to multivalent Toxoplasma antigens (IgG avidity) has been shown to be low in acute infections and high in chronic infections. Low IgG avidity can persist for as long as a year (1113). This test is being refined with the use of recombinant Toxoplasma antigens instead of lysed whole cell antigen, because rates of affinity maturation of IgG antibodies differ for specific parasitic antigens (1118). IgG avidity greater than 20% indicates that the infection occurred at least 20 weeks earlier. Thus, detection of high-avidity IgG antibodies to Toxoplasma antigens in pregnant women during the first half of their pregnancies can provide reassurance that their infections were remote in relation to the pregnancies (1119,1120).

Anti-Toxoplasma IgA antibodies directed against the major surface protein of tachyzoites (P30) are present in more than 95% of patients with acute infections (619). These antibodies are detected at the end of the first month of infection and usually disappear within 4 to 7 months, but can last up to 12 months or occasionally longer (619,1111,1121). Specific IgA antibodies are found rarely in patients with chronic infections (1121).

Specific serum IgE antibodies are present in about 86% of women who seroconvert during pregnancy. They appear shortly before or concomitantly with specific IgA antibodies. Specific IgE antibodies usually persist for less than 4 months, but are occasionally detectable for up to 8 months (619,1122).

Antibodies detected by IHA are different from those measured by the dye test, ELISA, or IFA. The titers take several weeks before becoming positive, making the test unsuitable for diagnosis of acute toxoplasmosis during pregnancy (15).

The differential agglutination test (AC/HS test) compares IgG titers obtained using acetone- or methanol-fixed (AC) tachyzoite antigens with those measured using formalin-fixed (HS) tachyzoite antigens. AC antigens detect acute phase-specific IgG antibodies. The AC-to-HS ratio is used to determine the timing of the primary infection. The test can show an acute pattern for up to 14 months (666,1123).

Acute infection in immunologically normal women can be diagnosed if seroconversion or a fourfold or greater rise in antibody titers occurs when serum samples are collected 3
to 6 weeks apart (1082). The absence of Toxoplasma-specific IgM antibodies as measured by the DS-IgM-ELISA or IgM-ISAGA essentially excludes the diagnosis of acute toxoplasmosis. Elevated titers obtained by these assays are considered suggestive of the diagnosis, especially if specific IgG titers are high as well. Positive IgM-IFA results are more likely to represent recent infection. Specific antibodies detected by IgM-ISAGA or DS-IgM-ELISA usually persist at low levels for months or years after infection. High levels measured many years after the acute infection are rare (1111). The detection of Toxoplasma-specific serum IgA or IgE antibodies indicates a recent infection; conversely, absence of these antibodies in a seropositive woman suggests that the infection is old (619,1121,1124).

A panel of tests referred to as the Toxoplasma Serological Profile (TSP) is available from the Toxoplasma Serology Reference Laboratory of the Palo Alto Medical Foundation (http://www.pamf.org/serology/). TSP includes DS-IgM-ELISA, ELISA IgA, ELISA IgE, the Sabin-Feldman dye test, and the differential agglutination test (AC/HS test). Of 811 women who had positive Toxoplasma IgM test results performed at hospital or commercial laboratories, only 40% were shown to have a recent illness using TSP (1111,1113, 1125).