To our knowledge, this is the largest study of the prevalence of latent Toxoplasma
infection in HIV-infected individuals in the United States. Previous studies involving HIV-infected individuals have reported wide variations in seroprevalence (3%–22%) [4
]. We found that the prevalence of positive tests for Toxoplasma
antibody in this large cohort was 15.1%. HIV-infected women did not differ from HIV-uninfected women with regard to seroprevalence, which is consistent with the findings of previous investigators.
The largest studies to date of Toxoplasma
seroprevalence in the US population of HIV-uninfected people or those of unknown HIV infection status are US military studies and the National Health and Nutritional Exam Survey (NHANES) study [11
]. A large US study of 2862 military recruits conducted in 1989 [11
] reported a prevalence of 9.5%, which was lower than the 14% prevalence reported in a similar study from 1965 [12
]. The lower prevalence of latent Toxoplasma
infection seen in the US military studies may be because the subjects in those studies were younger (80% were aged 17–20 years) than the subjects in ours. Also, only 9% of the military study population was Hispanic, and very few subjects were likely to be of non-US origin, although this was not specified. This prevalence of 9.5% is similar to the 10% seroprevalence noted for the 1637 US-born women in our study. The recently published study of 17,658 participants in the NHANES study from 1988–1994 found a Toxoplasma
seroprevalence of 22.5% with an age-adjusted prevalence of 15% among women aged 15–44 years [13
Although a number of factors, such as ethnicity and residence in New York or Los Angeles, were found to be associated with higher Toxoplasma seropositivity on univariate analysis, these factors were likely markers for birth outside of the United States, which was the strongest predictor of Toxoplasma seropositivity in this cohort, and, after age, the strongest predictor in the NHANES cohort.
We also found that women for whom injection drug use was the risk factor for HIV infection were less likely to have serological test results positive for Toxoplasma infection. This discrepancy may be attributable to the high Toxoplasma seroprevalence among women born outside the United States, who are less likely than US-born women to have acquired HIV infection through injection drug use. We are unable to explain the relationship between low CD4+ T lymphocyte count and serological test results that are positive for Toxoplasma infection noted in this study. It is possible that women who presented with low CD4+ T lymphocyte counts were more likely to be foreign born, but this association remained significant on multivariate analysis after adjusting for other variables.
Toxoplasmosis is an infection for which the prevalence in any population increases with age. Our study showed that women aged ≥50 years were more likely to be seropositive than were younger women. It could be argued that this was a cohort effect and that these women acquired disease during childhood. Women ≥50 years of age had a markedly higher seroprevalence than did women aged 40–49 years (). This age-adjusted increase in prevalence was also demonstrated in the NHANES study, which recruited patients aged 1 to 170 years. These findings suggest that, in the United States, soil exposure, which occurs most frequently during childhood, may not be the principal mode of Toxoplasma
acquisition. There were no significant differences between black and white women from the United States, which supports earlier findings that, in a given population, race does not affect seroprevalence [13
The findings from this study should be interpreted in light of the study’s limitations. Information on cat ownership, dietary habits, soil exposure, and soil-related occupations—important risk factors for disease acquisition—were not obtained during the WIHS interview. However, Toxoplasma
seroprevalence did not vary significantly by the amount meat in the diet or cat ownership in the US NHANES cohort [13
]. Another limitation was that, because this was a point prevalence study, we were unable to ascertain whether the higher prevalence associated with age of ≥50 years was a result of disease acquired during childhood or a result of continuous exposure throughout life.
Despite these limitations, we can draw the following conclusions. The prevalence of latent Toxoplasma infection in a large cohort of HIV-infected women and HIV-uninfected at-risk women was 15%, and the seroprevalence did not differ by HIV status. The prevalence among women born in the United States was 10% and did not differ by race when adjusted for age and CD4+ T lymphocyte count. In the US population of HIV-infected women, women aged ≥50 years, women with lower CD4+ T lymphocyte counts, and women born in other countries were more likely to have latent Toxoplasma infection.