If a buddhist should not kill a mouse living in their home, what justification do they have to rid themselves of a parasite such tapeworms

The way I decide these things is thus:

We are all One being. We are all One God, sharing the same Buddahood. There is no real individuality. Let us leave behind the world with as much good as possible.

Sometimes, the way a Bodhisattva works in the world is a mystery, especially because those of us without jhana and prajna powers cannot penetrate into the ultimate causes and conditions. They frequently do not stick to rules and conventions.

But always, a Bodhisattva acts doing what is best for everyone.

Similarly, even though we do not have the omniscience of a Bodhisattva, we can act intuitively within our own understanding that a tapeworm is a lower being and you can do much more good for the universe than a tapeworm. Think of the cyclic reincarnation and realize that there is not much good that it is doing in the world anyway..

In this way, a being's worth is measured by its ultimate service in helping Us achieve the Goal: for all to realize Complete Buddahood, complete Trikaya. Go ahead and kill the tapeworm, but do not raise any seeds or poisons while doing so. Do it with love and generosity for the highest good of all.

Of course, a measure of moderation is needed and when one experiences doubt one should not do such a "bad action." (Doubt is one of the 6 root afflictions and a good indicator of a wrong pathway, just like the other afflictions, when they arise are also a useful indicator.)

Killing a mouse is for such petty reasons. Still at a meditation center I have observed mouse traps outside in the forest. Mice trapped and died slowly for being outside in the forest. The management blamed the short term servers for not checking the traps. Non posoinous snakes get their holes covered, perhaps with babies in there. A parasite however, perhaps if focusing on making yourself healthy rather than harming the beings. I observed the mercy killing of an injured ant to be a great atrocity for a meditator. This action is 100% better left undone. Aversion associates one with the unwanted situation. In the long run, I question weather or not killing beings will solve the so called problem, more likely will be worse.

There are many guidelines and conventions and rules and fornalities. But what is the cost of violating said conventions?! And to whom is there a cost?

Some thoughts based on my beginner's level knowledge of Buddhism and biology:

The prohibition on killing is based on the premise that we should not cause suffering. For something to suffer, it needs the capacity to suffer. In modern biological terms, that means an organism needs to have a nervous system and brain.

Currently it seems biologists don't agree about whether tapeworms have something that could be considered a brain (although they do have a nervous system). Thus, if you think they don't have brains, you might decide they do not suffer.

Another way of looking at this is that if tapeworms don't have brains they are not sentient, and non-sentient organisms do not suffer.

The original poster asks: "Where to draw the line..."

Therein is the key to the answer: there is noplace you can draw the line, thin and bright, therefore you cannot draw it.

There is, however, an answer. The mouse and the tapeworm situations have similarities and also disparities. The mouse will infiltrate and foul your food, and carry disease and vermin (fleas, etc) which can harm you, but practicing adequate cleanliness and good practices will mitigate that. The mouse may find advantage not in your pantry, but simply in the safe, warm shelter. The tapeworm is itself a disease, which will in time kill the host, and by doing so, itself perish.

Therein lies the difference.

If by your action a creature dies, and also by your inaction that creature also dies, then your action is not ultimately of significance to the death. In that case, the course which prevents your own death also, is to be preferred.

Theravada Buddhist Answer.

Whichever way you spin it, killing(intentionally) is bad Karma which you will have to pay for at some point in Samsara unless it becomes defunct. You can draw the line anywhere you like, but Karma is Karma. If the tapeworm can be removed without killing, you won't break the precept. Otherwise, you can use it to becomes dispassionate about the body and develop Vipassana. Meditate on your strong desire to live. Killing it will not solve your problem. You can have worse bodily conditions if you were born as an animal.

Once there was a farmer who, after having taken his precepts from a respected monk, went to look for his buffalo that had strayed into the forest. Along the way, he was caught by a python. His first thought was to use his axe to kill the snake coiled around him. Then he remembered that he had taken his precepts from a respected monk. The thought came to him for a second time and again he refrained from killing. The third time he was prompted to kill, he threw away his axe. The snake uncoiled itself and freed him!

Where does one draw the line for which forms of life are ok to destroy, and which ones are not?

That (i.e. "which forms of life?") might be not the right question.

If you're describing the situation based on a premise of violence versus non-violence, then another way to look at it might be aggression versus non-aggression, and/or aggression versus self-defence.

For example Buddhists are typically forbidden to tell lies, but there's the following Q&A from "Good Question Good Answer" by Bhante Dhammika. In the chapter about the five precepts, one of the question is whether you should lie to a killer to prevent them from killing. His answer was that,

"If you were sitting in a park and a terrified man ran past you and then a few minutes later another man carrying a knife ran up to you and asked you if you had seen which way the first man had gone, would you tell him the truth or would you lie to him?"

"If I had good reason to suspect that the second man was going to do serious harm to the first I would, as an intelligent caring Buddhist, have no hesitation in lying. We said before that one of the factors determining whether a deed is good or bad is intention. The intention to save a life is many times more positive than telling a lie is negative in circumstances such as these. If lying, drinking, or even stealing meant that I saved a life I should do it. I can always make amends for breaking these, but I can never bring a life back once it is gone. However, as I said before, please do not take this as a license to break the Precepts whenever it is convenient. The Precepts should be practiced with great care and only infringed in extreme cases."

I could try to argue that the difference between the killing-the-mouse and killing-the-tapeworm scenarios isn't the "form of life" but rather the difference between aggression (against the mouse) and self-defence (against the tape-worm).

Killing the mouse is a form of aggression ("I hate you, I kill you"). Killing the tape-worm (or, more specifically, medicating your own body for its malady) is arguably a form of self-defence rather than aggression -- maybe it's that which makes the difference.

You might argue that mouse-in-house is self-defence too, however IMO that (attacking-within-house) is straying ever further from defence-of-self: perhaps you ought to be thinking, of your house, "This is not mine, this is not me, this is not my self." and therefore not trying to defend it as if it were yourself.

The fact that the mouse is in a "house" implies you're a house-holder or layperson and therefore have many social responsibilities. What if it wasn't a tapeworm in you but in your parent, child, friend, etc.?

I don't want to say that it's clever to kill even tape-worms but maybe non-aggression (don't kill mice) versus self-defence (take medicine) is a way to distinguish the two scenarios.

Another consideration is, if you kill a mouse today then what will you do tomorrow? Instead you might prefer to make your house mouse-proof, or perhaps store your food and clothing in mouse-proof containers.

Similarly, if you eliminate a tapeworm today then what will you do tomorrow? I looked in the introduction to the Vinaya to see whether I could see there any instructions about parasites: and didn't find them. A "purge" is a permitted type of medicine for what that's worth. There are however several instructions in the Vinaya: about keeping yourself clean, and about eating cooked food.