[MyFairLacy]

You can read a little about how Ivermectin (what's used in Heartgard) works and why it only harms the worms: http://en.wikipedia.org/wiki/Ivermectin

Ivermectin kills by interfering with nervous system and muscle function, in particular by enhancing inhibitory neurotransmission. The drug binds and activates glutamate-gated chloride channels (GluCls) present in neurons and myocytes, resulting in neuro-muscular paralysis and death.

Although similar in structure to vertebrate ionotropic glycine receptors, glutamate-gated chloride channels are specific to invertebrates (Dent, 2006). The absence of glutamate-gated chloride channels from mammals appears to account in part for the specificity of ivermectin for invertebrate parasites and its relative lack of side effects in mammalian hosts