Experimental Fasciola hepatica infection alters responses to tests used for diagnosis of bovine tuberculosis

Abstract

Fasciola hepatica is a prevalent helminth parasite of livestock. Infection results in polarization of the host's immune response and generation of type 2 helper (Th2) immune responses, which are known to be inhibitory to Th1 responses. Bovine tuberculosis (BTB) is a bacterial disease of economic and zoonotic importance. Control polices for this disease rely on extensive annual testing and a test-and-slaughter policy. The correct diagnosis of BTB relies on cell-mediated immune responses. We established a model of coinfection of F. hepatica and Mycobacterium bovis BCG to examine the impact of helminth infection on correct diagnosis. We found the predictive capacity of tests to be compromised in coinfected animals and that F. hepatica infection altered macrophage function. Interleukin-4 and gamma interferon expression in whole-blood lymphocytes restimulated in vitro with M. bovis antigen was also altered in coinfected animals. These results raise the question of whether F. hepatica infection can affect the predictive capacity of tests for the diagnosis of BTB and possibly also influence susceptibility to BTB and other bacterial diseases. Further studies on the interplay between helminth infection and BTB are warranted.

FIG. 1.

F. hepatica-specific IgG1 responses (±SEM) in groups 1, 2, 3, and 4. All animals in groups 1, 2, and 3 seroconverted, while those in group 4 did not. No significant difference in titers among groups 1, 2, and 3 were found. Parasite-specific IgG2 levels did not rise above the background during the course of the experiment.

FIG. 2.

(a) Levels of arginase production in blood monocyte-derived macrophages (± SEM) at 24 h postisolation from each group. Measurements were made 14 weeks post BCG inoculation and 18 weeks post F. hepatica infection, except in the case of group 3, when the sampling was 12 weeks post F. hepatica infection. Arginase production by cells from groups 1, 2, and 3 was significantly greater (P < 0.05) than that by cells from group 4. The latter did not produce levels significantly greater than those of cells from negative control animals. (b) NO production in blood monocyte-derived macrophages from naive (uninfected) calves stimulated with PPD-B and F. hepatica ES products at 10 μg/ml. The values are means (±SEM) from three experiments with cells from two animals. (c) Arginase production by macrophages under the conditions described for panel b.

FIG. 3.

IL-4 (a) and IFN-γ (b) production by individual animals (F. hepatica and BCG coinfected) in group 2. Cytokine levels were measured in plasma recovered from cultures of whole blood taken at 1 week and 13 weeks post BCG immunization and stimulated in vitro with PPD-B. Results are presented as the mean of triplicate wells ± the SEM. The response of each animal in the SCITT and its whole-blood IFN-γ BTB test results are also indicated in each panel as follows: N:N, negative in both assays; N:P, SCITT negative, whole-blood assay positive; P:N, SCITT positive, whole-blood assay negative; P:P, positive in both assays. OD, optical density.

FIG. 4.

IL-4 (a) and IFN-γ (b) production by individual animals (BCG and F. hepatica coinfected) in group 3. IL-4 was measured in animals at weeks 1, 5, and 13 post BCG immunization, and IFN-γ was measured at weeks 1, 3, 5, and 13 post BCG immunization, as described for Fig. 3. Responses to BTB diagnostic tests are also indicated as in Fig. 3. OD, optical density.

FIG. 5.

IL-4 (a) and IFN-γ (b) production by individual animals (BCG immunized) in group 4. Sampling times and other conditions were as described for Fig. 4. OD, optical density.