Natural infection of guinea pigs exposed to patients with highly drug-resistant tuberculosis

Abstract

A natural TB infection model using guinea pigs may provide useful information for investigating differences in transmission efficiency and establishment of active disease by clinical TB strains in a highly susceptible host under controlled environmental conditions. We sought to examine the capacity of naturally transmitted multidrug-resistant Mycobacterium tuberculosis to establish infection and produce active disease in guinea pigs. Guinea pigs were continuously exposed for 4 months to the exhaust air of a 6-bed multidrug-resistant tuberculosis inpatient hospital ward in South Africa. Serial tuberculin skin test reactions were measured to determine infection. All animals were subsequently evaluated for histologic disease progression at necropsy. Although 75% of the 362 exposed guinea pigs had positive skin test reactions [≥6 mm], only 12% had histopathologic evidence of active disease. Reversions (≥6 mm change) in skin test reactivity were seen in 22% of animals, exclusively among those with reactions of 6-13 mm. Only two of 86 guinea pigs with reversion had histological evidence of disease compared to 47% (31/66) of guinea pigs with large, non-reverting reactions. Immunosuppression of half the guinea pigs across all skin test categories did not significantly accelerate disease progression. In guinea pigs that reverted a skin test, a second positive reaction in 27 (33%) of them strongly suggested re-infection due to ongoing exposure. These results show that a large majority of guinea pigs naturally exposed to human-source strains of multidrug-resistant tuberculosis became infected, but that many resolved their infection and a large majority failed to progress to detectable disease.

Figure 1

Design and time course of study

Figure 2

Cumulative proportions of guinea pigs with positive (≥ 6 mm) TST results on each skin test.

Figure 3

Panel A (top): Histogram showing the distribution of TST induration diameters among guinea pigs on TST 2. X-axis shows induration diameters and y-axis shows number of guinea pigs. Vertical dashed line illustrates cut-point for positive TST reaction (≥ 6 mm) Panel B (bottom): Histogram showing the distribution of TST induration diameters among guinea pigs on TST 5. X-axis shows induration diameters and y-axis shows number of guinea pigs. Vertical dashed line illustrates cut-point for positive TST reaction (≥ 6 mm)

Figure 4

Examples of TST reversion in 2 guinea pigs. X axis shows time in weeks and corresponds to timing of TST 1 through 5. Y axis shows TST induration size in millimeters.

Figure 5

Disease Severity Among Guinea Pigs with Large TSTs.

Figure 6

Panel A (top left): Low magnification view of lung tissue from a guinea pig with cavity formation (1.3×, H&E). Panel B (top right): Low magnification view of lung tissue from a guinea pig with limited disease progression (4×, H&E). Panel C (bottom left): Low magnification view of mineralization within a granuloma from a guinea pig with minimal disease progression (4×, H&E). Panel D (bottom right): low magnification view of multiple granulomata seen in splenic section from a guinea pig (4×, H&E).

Figure 7

Plots of correlation between TST diameter and Disease Score, among GPs with large TSTs. Panel A (left) shows relationship in GPs that received steroids. Panel B (right) shows relationship in GPs that did not receive steroids.

Figure 8

Box plot of Disease Score Among Large Reactors on TST 5 grouped by whether GP first developed its positive TST early or late in the study. Sample means are significantly different (p<0.001).