This is a question regarding my little critters that hijack of their victim. By doing so they get direct access to a steady flow of nutrients through the bloodstream and get shelter with optimal temperature and humidity with the added protection of a ribcage. What's terrifying about these little bugs is that you can't get rid of them, because they've highjacked the one organ that keeps you alive.

What kind of physiology would a heart parasite have?

This question is not asking "would this evolve" as this creature is already very complex, it performs a risky procedure as part of its life cycle on mere instinct, which is not something your typical leech or tick does. A more pertinent question is what kind of body they should have to make this work? They must first burrow into a host without inflicting a life threatening injury. Then they must attach to a still beating heart and drain sustenance through the blood. They might go through multiple life stages like a larva for burrowing and a crab for latching on the heart.

What do you think?

Edit: The definition of hijack is "seizing" something, in this case that would be the heart. The parasite does not replace the heart itself, but laches onto it.

  • $\begingroup$ You might look up heartworms. $\endgroup$
    – jamesqf
    Jan 27, 2020 at 18:42
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    $\begingroup$ Replacing whole heart is an extremely different procedure that seemingly offers no benefit for a parasite. Leeching blood from major vessels offers the same benefit for a fraction of a cost. $\endgroup$
    – Alexander
    Jan 27, 2020 at 19:06
  • $\begingroup$ Why would the parasite "hijack" the heart? I see no real benefit for it long-term. It absolutely harms the host. Replacing the heart means that the parasite has to do what the heart did which is not easy to replicate and harder still to counter balance against doing the opposite and leeching even a portion of the blood. Anything a parasite does to the heart will harm the host. Very, very likely kill the host. I fail to see anything but magic or sufficiently advanced technology that produces such creatures and they are specifically tailored towards this task. $\endgroup$
    – VLAZ
    Jan 28, 2020 at 11:57

5 Answers 5


Burrowing through safely isn't impossible (though god help the person feeling it). It just has to miss major arteries and release chemicals that will mitigate the damage (blood parasites tend to use anticoagulants which would be a nono here).

Once it reaches the heart though, that's when the challenge develops. It needs to anesthetize/sedate its host-victim so that their heart stops completely. It probably only has 2 minutes at most to then consume the heart and attach itself to the large arteries and veins without significant leakage. At this point it can start pumping blood again (perhaps now with some anticoagulants to help with some of the clots that might have started to develop... don't want the host-victim having a stroke).

It can also probably wait for the sedation to wear off, there's no need to immediately wake the host-victim.

Given the size of a human heart (roughly the size of that person's fist), this still a pretty big hole for the host-victim to heal. They may die of sepsis within the week. It might be a better adaptation if it makes ingress while small... say, the size of a dime or smaller. It then would gestate internally until it was large enough to take over.

I would imagine that this is going to be a hellish sensation. If it were to happen to you, the "heart" would no longer obey your nervous system. If you wanted or needed to run, and it decided that it wanted you to commit to a walking pace, too bad. It might even infiltrate the Vagus nerve and alter your apetite or sense of stomach-fullness. It could use it to cause the host-victim intense pain if it wanted to punish it (not necessarily intelligently, just as an instinct).

And given how other parasites worked, its method of reproduction would only be more horrific still.

All of this is highly implausible from an evolutionary standpoint. Just like all other real parasites.

  • $\begingroup$ "its method of reproduction would only be more horrific" *Insert footage of chestburster here* $\endgroup$ Jan 28, 2020 at 14:28
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    $\begingroup$ @thescribe-ReinstateMonica I was thinking more like "larval parasites burst from the host-victim's fingertips whenever he comes near someone else to attack them immediately. The chewed tunnels through the arms would make those useless, swollen, and agonizing. For a bonus, psychotropic compounds leaked into the host-victim cause them to actually want to spread the parasite. $\endgroup$
    – John O
    Jan 28, 2020 at 15:37

As @vodolaz095 mentioned, this isn't something that could happen quickly - @John O's answer points out a lot of the issues that show up there.

There is a parasite that does something marginally similar - Cymothoa exigua. This lovely parasite will sever the blood vessels in the fish's tongue, causing it to die and fall off, at which point it attaches itself to the nub and becomes the fish's new tongue.

Obviously, this approach can't work 100% with a heart. Sever the blood vessels to it and you get death. However, a similar approach could work.

The parasite, small-ish, enters the host's body, where it finds its way to the heart. At that point, it attaches itself to one or more blood vessels, where it grows. At some point, it is attached to all of the blood vessels, and begins constricting the blood vessels of the heart, which will then atrophy and die while the parasite takes over.

This would be a gradual process - I'm guessing weeks, maybe months, instead of days or hours. And it would likely be highly painful - A modern society would have someone go to the hospital for heart pains, get a CT scan or MRI, and it would likely be early enough to remove it. Earlier medicine probably wouldn't be able to do much about it - And eventually, the pain would probably go away as the heart died.

I'm not a biologist nor a doctor, but I think that the creature might even be able to react and adjust heart rate depending on things like oxygen levels in the blood, removing or reducing the need for interacting with the nervous system.

Additionally, the way this creature pumps blood could be entirely different to how the human heart works, leading to a different sounding heartbeat or even a different-feeling pulse.

In the end, once the creature is there, it's in the interest of both it and the host to maintain the host in a healthy way, so beyond the initial pains of heart implantation, it would likely not be noticed and the person would be able to live a normal life. Depending on how you want to go, it could even offer a boost to the host's immune system against things like other parasites and, specifically, blood-borne pathogens.


First, i recommend to read about real world parasites, but, reading it can be quite creepy.

If your creature is something like real world worms, it can inject itself into muscle tissue, consume muscle cells and than act itself as muscle fibre. So, this worm have to link itself to nervous system and blood vessel systems of host, to fully emulate muscles in terms of acting on nervous system signals (constricting, like real muscle fibre) and consuming nutrients in a way muscle fibres does. After some time, this parasite can inject itself in all muscles (including heart) and become main component of muscular system of host. Or, probably, this parasite directly goes into heart.

I think, if we took any helminths, and apply some forced artificial evolution for them, its possible to make them evolve into parasites, that can highjack and act as heart after few years they entered host. But highjacking process will be far from dramatic. Possible symptoms - high temperature (immune system tries to deal with parasites), weakness (because parasite eat peripheral muscles' fibres and it have not fully integrated itself into muscles - acting as temporary shelter before parasite enters heart), muscle shackles (parasite is trying to properly bind to nervous system), than - Arrhythmia and various issues with heart ( because parasites are slowly consuming and integrating into heard muscle tissue). So, infection process for this parasite is not dramatic, but once its finished, its nearly impossible to remove it from human - >80% of muscle tissues are parasits, if you kill it, human will become paralysed for best outcome, or, simple die of heart malfunction.

UPD1: imho, if parasite replaced >80% muscle tissue in heart, and its host organism is still living, we can say parasite "hijacked" or "seized" heart, yes?

UPD2: parasite can release various hormones, or act as better muscle tissues giving it host greater strength and prolonging livespan, so, there will be few people who would want to become infected with it, like Olympic athletes in search for new, undetectable doping?


There's a species of eel that's been known to burrow inside fish and nestle in their heart as a parasite, long enough that the blood vessels remodeled themselves to pass blood around the fish. They are thought to do so by tunneling into the ciculatory system or entering through the gills and making their way to the heart. Their gut contents suggest they had been living on blood the entire time, and that's how they get nutrients and oxygen. What killed the fish was being caught, not the presence of parasites within them, and there didn't seem to be evidence of an immune response. However, there's evidence that the fish in question were starting to get poisoned by the eel's waste products, so it's debatable how long they could live. It's thought that these eels more frequently do it to dying fish than healthy ones. A more specialized parasite may have a way to get rid of waste and stay attached to the host longer.


I don't think the energy equations make sense for replacing the whole heart and doing all it's work. You're basically taking up the most stressful job in the body. I'm imagining something like the mistletoe haustorium. Have it creep along the nerves of the heart, slowly devouring and replacing the myelin around them and eventually becoming the hearts nerve structure completely.

From there it could use a technique cancer cells use to induce angiogenesis to cause arteries to grow around it, giving it a huge supply of oxygen rich blood to either chow down on directly or to absorb the nutrients from and re-circulate.

Many parasites are extremely good at hiding from the hosts immune system and this one has the advantage that, hypothetically, antibodies towards to might cause a horrible demyelinating autoimmune disease like ALS.

As a defense mechanism the angiogenesis could create an impossible to untangle mass of blood vessels around the heart that would result in certain death if severed.

Other ideas:

  • It could hypothetically offer some kind of advantage. Maybe it's got better conductance than normal nerves and offers improved reaction time or a heart less prone to ischemic attack.

  • It could also render its host frail and often breathless or anything in between.

  • It has the horrible possibility to spread through blood-blood contact, like the contagious cancers that Tasmanian Devils get.


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