Detection of Latin American strains of Histoplasma in a murine model by use of a commercially available antigen test

Abstract

During a Histoplasma outbreak in a colony of fruit bats at a southern United States zoo, it was observed that although Histoplasma was recovered in culture from multiple sites at necropsy, none of the samples collected from those bats tested positive for Histoplasma antigen (HAg). Five of the Histoplasma isolates from the bats were subsequently identified as Latin American (LA) clade A, restriction fragment length polymorphism (RFLP) class 6. These observations raised concern as to whether the commercially available HAg test could detect Histoplasma antigen not of the North American clade upon which the HAg test had been developed. To evaluate this concern, a murine model of disseminated histoplasmosis was established, and mice were infected with multiple LA Histoplasma isolates, including clinical isolates recovered from Brazilian AIDS patients (RFLP class 5 and class 6) and isolates recovered from the bats during the outbreak (RFLP class 6). Histoplasma antigen was detected in all infected mice in our experiments, even when Histoplasma was not recovered in culture. Because the currently available HAg test is able to detect Histoplasma antigen in mice infected with Latin American isolates, this suggests that bat host factors rather than differences among Histoplasma RFLP classes were responsible for the inability to detect HAg in infected bats.

FIG. 1.

Correlation of lung and spleen homogenates (CFU/g) and HAg in mice infected with a North American (NA) isolate (open circles) or a Latin American (LA) isolate (black circles). Data are from 36 animals divided into groups of 6 mice infected with three inocula (10⁵, 10⁶, and 10⁷) of each isolate. The regression line reads to the left from the LA data.

FIG. 2.

Quantitative culture and antigen level in mouse lung and spleen produced by 6 isolates used to produce infection with a 10⁷-CFU inoculum (groups of 6 mice). The dashed lines on the antigen (upper) and culture (lower) graphs represent the cutoff for a positive antigen value (1 EU) and the lower limit of quantitative culture detection (20 CFU/g), respectively. Note that the line and the points beneath the line of detection are not to scale and have been separated so they may be easily seen. NA, North American isolate; LA, Latin American isolate.