This might be a really stupid suggestion but wouldnt common rat poison work..the reason I am stating this is that one of my neighbors was on vacation..his wife was taking care of the "family pets" ..--

they had mouse/rat poison in boxes sitting around in various places in the garage to take care of hte occassional mouse/rat coming in to nest in the fall/early winter..

anyway--the dog somehow got into a box from behind the freezer---he ended up dying from it a few days later--she didnt realize he was sick in time..byt the time she figured it out it was too late...

it seems it would be too weak--but it killed a 60lb dog...

Ripp