This is an attempt to discredit veganism by showing that it's internally inconsistent with killing mosquitos, ticks, or other parasites, and therefore vegans are just cherry-picking some animals to defend.
Just because you advocate for the rights of some group doesn't mean that any action by that party is now automatically justified. Self-defense is still self-defense even if it's against a member of some group that deserves defending. And just because someone who is gay, or of a particular race, or some other group committed a crime, it would not diminish advocacy for that group as a whole. This is no different for animals. Just because some particular animals might carry diseases, attack you or your loved ones, or otherwise (unknowingly) violate your rights, that does not diminish fighting for animal rights as a whole.
It's not as though if you accept the position that we should not be needlessly harming dogs, as a general rule, that this somehow rules out defending yourself in the case of a dog attack. You would need to consider the ethics of that particular situation. But defending yourself against a dog isn't now suddenly a good argument for participating in dog fighting. So it's no different from defending yourself against any animal and protecting your own health and well-being against those of the animal. Specific instances of harming animals might be justified, but generally, harming animals is still wrong.
This is equal to the protections that we enforce against other humans. If for instance, a mentally handicapped person attacked you, you would be justified in protecting yourself. This doesn't make all human murder now justified. You would need to consider the ethics of that particular event. Specific instances of harming humans might be justified, but generally, harming humans is still wrong.
We so easily recognize that in cases when it is necessary to prevent some much greater form of harm, this changes the calculation of what is ethical or not. For instance, it normally is unethical to steal a car. But "What if you had to steal a car to save someone from dying by driving them to a hospital" isn't a great argument for now absolving all car thieves. Specific instances of theft might be justified, but generally, theft is still wrong.