New Blood Test for Strains of Toxoplasmosis

A new enzyme-linked immunosorbent assay, which was developed by scientists at the National Institute of Allergy and Infectious Diseases, was used to distinguish specific types of Toxoplasma gondii parasite that children acquired in the womb from their acutely infected mothers. The blood tests that are currently available can show whether a person has ever been infected with any strain of the Toxoplasma parasite. The new still-experimental test can detect antibodies in human sera that recognize allelic peptide motifs of distinct T gondii parasite types.¹

Worldwide, at least 15 distinct strains of T gondii have been discovered. A strain called type II predominates in France. Type II parasites are distinguishable from all other strains, which are called not exclusively type II strains (NE-II). T gondii infection can occur through contact with infected cat feces or by consuming undercooked meats.

Using blood samples collected between 1981 and 2009, scientists found evidence of type II or NE-II infections in 183 mother-child pairs.² NE-II parasites were more likely to be associated with premature births. Infants infected with NE-II strains had more severe manifestations of disease than infants infected with type II strains. Of 88 cases involving the NE-II strain, 59 (67%) had severe eye damage. In comparison, 18 (39%) of 46 cases of type II strains involved severe eye damage. However, this association is not absolute; mild, moderate, or severe disease can occur regardless of the infecting strain.

In the United States, NE-II parasites comprise 61% of the strains detected. Specifically, NE-II strains were more common than type II strains along the Gulf Coast, the Pacific coast, and in Hawaii. Among lower-income and rural populations, NE-II strains also predominated.

Worldwide, experts disagree about whether pregnant women should be screened routinely for toxoplasmosis. In the United States, toxoplasmosis is uncommon. The American College of Obstetricians and Gynecologists recommends against screening for toxoplasma antibodies unless the patient has HIV infection or unless T gondii infection is suspected.³ In France, however, the infection is more common, and all women are screened. The main argument against widespread screening in the United States is concerns about false-positive test results, which would lead to unnecessary anxiety in patients and costly follow-up testing.

Pertinent Points:

- A new blood test can identify specific types of Toxoplasma parasite, which is important because certain strains are more strongly associated with premature birth and severe birth defects.
- Although toxoplasmosis is rare, the effects of congenital toxoplasmosis can be devastating. Identifying all patients who would benefit from treatment is reason enough for universal screening of all pregnant women, according to some experts.

References:

References
1. U.S. Department of Health and Human Services, National Institutes of Health. Test links strains of common parasite to severe illness in U.S. newborns. Accessed April 23, 2012.
2. McLeod R, Boyer KM, Lee D, et al, for the Toxoplasmosis Study Group. Prematurity and severity are associated with Toxoplasma gondii alleles (NCCCTS, 1981-2009). Clin Infect Dis. 2012;54:1595-1605.
3. American College of Obstetricians and Gynecologists. Practice bulletin number 20: perinatal viral and parasitic infections. Int J Gynaecol Obstet. 2002;76:95-107.