I think in vivo passage of parasite is the only and important method to recover the virulence. If you do not want to use BALB/c mice you can use hamester for this purpose and it is better to use them as they get infection quite earlier as compared to mice.
And if you do not wish to use animals at all for this purpose then, you can try using any cell line for this purpose.
try to follow the suggestions you may get benefited.