Why is diagnosing parasites in animals hard?

I believe it is because you often have to do tests such as blood and stool tests to find them, and things like heartworm have to be in the right cycle and must have females present(other wise be in the right circumstances )

In some parasites a diagnosis is made by observation of the mature parasites,so during an autopsy in your pets intestines. This is especially true for Tapeworms. Tapeworm eggs are difficult to detect during microscopic fecal analysis, so observation of the actual worm is how they are routinely diagnosed.other parasites found in the blood are also hard to detect

Difference between normal and abnormal tissue is much clearer on mri than on ct and mri much suitable to view soft tissues