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I'm sure the zapper has NO effect on the well-being of the heart,
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You don't know, no serious experiment has been done yet (or please show me)
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and actually helps it if you have an infestation of heart worms
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It can be dangerous because the electric wave produced by a zapper seem to only disturbs the big (more than 1mm) parasitic worms, not kill them, and they apparently try to avoid the current, so they move and this can cause irritations, cuts, holes, blocages, inflammations, increases of pressures in the parts of the body where they are (and in the heart this is not good)