The parasites in their thousands or millions then disappear leaving no trace of themselves. They may become air born (eaten by an air born predator), however it is most likely that they burrow back in to the ground where a new part of its life cycle begins. Eventually they will transform back into the form that the animal consumed in the first place, ready for the next unfortunate mammal to come along and consume them. This stage of the life cycle may involve another animal or plant host. This stage of the life cycle may lay dormant for very long periods of time in a near indestructible egg or cyst form that may be microscopic in size.  

When you look at the wide variety of complex life cycles amongst the Platyhelminthes this is plausible, but just a hypothesis.

The collection of Antibodies by either aliens or humans Theory - This explanation is to me totally unsatisfactory, there is no reason for this to be done in an uncontrolled environment, this is not how scientific research is done, it is done in a controlled environment, they would do it in private, on private land with animals they owned and controlled, there is no reason that I can see for them doing this to wild animals or to animals that belong to some random farmer or for them to leave the mess behind to be found. In addition, there is no reason when collecting antibodies to remove organs from holes that are often smaller than the organs being removed, or for the skeletonization of parts of or all of the skull. 

The Platyhelminthes hypothesis cannot explain animals being dropped from great heights, broken legs or animals being dragged or being found in inexplicable places such as in trees or on the top of telegraph poles. However, this can be easily explained by the military trying to encourage the notion of alien involvement and in them trying cause researchers into this phenomena confusion. The method by which this happen is the same in both theories. 

The military may well be responsible for some, (if not all) of the blood removal particularly where the loss of blood is almost total. Just as they may be researching and collecting antibodies they may well be researching and collecting this parasite. 

I suggest that the military are still investigating the parasite which occurs naturally probably in far more cases than we know about. Mostly it will happen in the wilderness and no one will ever see it. Most of the cases we know about are more likely to have military involvement, this is sometimes because they got there first, sometimes because they infected an animal or an area and have been waiting for let’s call it “A Carcass Break Out” or some times because reports are sent to them as a matter of priority.

 

It would be useful if possible to separate the cases where there is obvious human involvement (lifting and moving and dropping of the animal) from those where there is only the injures themselves. I do not have this information to know what percentage each group would make up.
I would suggest the reason they show it to the public is because on rare occasion inexplicable to them, humans have encountered this occurring naturally, it then becomes necessary to create a rational explanation (testing/ psychopaths /Satanists), whilst advantageous perhaps to suggest the notion of aliens, but not officially. But with obvious moving and dropping of the animals the public have few other directions to think.
 They deliberately muddle up the evidence and make it look like its aliens without saying it is / whilst researching and collecting samples for potential weaponization from both natural cases where they get the reports first and cases where they have deliberately infected an animal or an area of land and have been waiting for a carcass break out. It may be that they invented this thing from scratch or adapted it from an existing parasite. But I believe that this is a naturally occurring group of flat worms, that has evolved to parasitize mammals in this way, there are already many we know about that have adapted to parasitize mammals in many various ways. I think it is likely that the military only know the first part of the life cycle. They know and have isolated the element that goes in to the mammal, probably because they killed a mammal and found it in the gut before it had entered the blood stream or because they killed a mammal and found it in the blood stream unchanged into its next form. This stage if having a basic Flat worm physiology would be very easily cloneable, you could literally do it with a sharp knife. http://awesci.com/planarian-worm-cloning-a-creature-was  
It is very possibly that they have no idea of the rest of the life cycle and would not really need to know to weaponize it, but if they wanted to try and get it to evolve under control it would be useful.
But maybe they have watched it happen and they have seen what happens, may be an owl comes or some other night time predator (probably with wings, but may be just light footed) and eats them. Many predators will continue to return to a food sauce until it is exhausted, particularly when feeding young, these parasites will provide a nutritious and easily carryable meal. They will be presumably full of eggs which will become the next stage and the life cycle will continue. The birds, bats or insects excrete them out and eventually another mammal will eat them, there may be more stages in-between.

It may be that the military know or do not know what causes the mass asexual reproduction in the blood. This may not happen in most infected animals, it may require very specific circumstances and may require the mammal to be infected with two parasites (possibly one of them an insect) at the same time or may require some environmental change inside or outside the body of the mammal. 

The Platyhelminthes Hypothesis, is just a hypothesis to be proven or disproven, I dare to be wrong in the hope that it might advance us toward truth on this matter, If I am wrong prove me so, but do not condemn me for having an idea.

Furthermore, I recommend that people watch the film Dreamcatcher (2003) though this a Hollywood fantasy sci fi movie, I believe it contains many subtle and not so subtle hints as to the true nature of this phenomena.