I sent my father some advice on introducing a dog to new cats with no bloodshed. This is actually quite straightforward to accomplish, and you can get the general gist from my abstract:
The cat's unshakeable belief in its own inherent superiority, and ability to convince the dog of the same in the face of overwhelming evidence, is one of the reasons it is such a successful parasitic lifeform. The cat seeks to displace the dog's benign mutualism for its own ends without the host's knowledge.
I recommend Hillaire Belloc's exhaustive treatment, a classic for anyone interested in the subject. Meanwhile, in the realm of speculative fiction, many of the terrifying parasitic alien lifeforms in Neal Asher's novels are rather reminiscent of the cat or its passenger-cum-co-conspirator, Toxoplasma gondii.