Glutathione S-transferases as markers of salicylanilide resistance in isolates of Fasciola hepatica

A possible link between the level of glutathione S-transferase (GST, E.C. 2.5.1.18) activity and the development of salicylanilide resistance in Fasciola hepatica was investigated. Various isolates of F. hepatica with varying susceptibilities to salicylanilides were isolated and maintained in the laboratory. Individual flukes of these isolates were surveyed for their level of GST activity and a correlation between the level of GST activity and drug efficacy was found. In contrast to most other studies, a decrease in GST activity was associated with an increase in drug resistance. Evidence was collected to show that this may be a selective process since flukes which had survived exposure to rafoxanide and closantel in vivo (in sheep) had lower activity levels of GST than flukes from untreated sheep. Treatment with other flukicides (oxyclozanide, luxabendazole and triclabendazole) did not have this effect. Furthermore, in vivo treatment with closantel induced selection of particular isoenzymes in different isolates of F. hepatica having different degrees of susceptibility to closantel. However, no single isoenzyme or isoenzyme profile was associated with resistance and, in total, up to 8 different isoenzymes could be present in a single isolate. Thus, GST has some potential as a marker enzyme for salicylanilide resistance in F. hepatica. However, the precise role of GST in resistance is unclear and the extensive inter- and intra-isolate variation in activity levels and isoenzyme characteristics of this enzyme indicate the need for considerably more study before application in field situations

Host effects on glutathione S-transferase activity in Fasciola hepatica

Glutathione S-transferases (GST, E.G. 2.5.1.18) in Fasciola hepatica from sheep were previously found to be extremely variable with regard to specific GST activity and isoenzyme profile within and between parasite isolates. The effect of the host on GST activity and isoenzyme profile was examined by infecting mice, rats and cattle as well as sheep with one or the other of two isolates—either salicylanilide-resistant or salicylanilide-susceptible F. hepatica. In the case of both isolates, GST activity in hosts relatively resistant to reinfection—rats and cattle—was lower and more restricted in range compared with hosts susceptible to multiple infection—mice and sheep. In the case of the rat flukes, there was little variation in isozyme profiles whereas cattle flukes appeared to exhibit more variation than sheep flukes. In mice, despite the apparent variability in GST activity, only one GST band was found in the isoenzyme profiles. Therefore, the host appears to exert a pronounced effect on the activity and expression of GSTs in F. hepatica which may be related to variation in the immune responses of the different hosts during infection