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A world has grassy plains infested with larger-than-normal (perhaps even giant) blood-sucking ticks. Ignoring their main diet for the time being: They should be dangerous enough to threaten humans specifically by blood loss (not just by disease/immune reaction).

How many ticks - and of what size - would need to attach to a person so that blood loss would start to be a real issue?

Is this feasible with known biology, or do these need to be magic ticks?

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    $\begingroup$ Ticks routinely threaten the life of livestock, though it requires an infestation of more than just a few. You'll find sheep or cattle with hundreds, sometimes. If they were larger, fewer would be needed. $\endgroup$ – John O Jan 16 at 14:47
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    $\begingroup$ Blood loss comes in stages, where over 40% blood loss requires urgent care to prevent death. Assuming average of 5L of blood in an average adult, they would need to suck 2000ml blood. This gives you the magic formula: $$\text{No. ticks to cause death} = \frac{\text{2000}}{\text{Capacity of blood sucking per tick}}$$ $\endgroup$ – Vepir Jan 16 at 14:53
  • $\begingroup$ I don't think it was mosquito. Maybe a tiger? For any blood-sucking animal to be dangerous they need to attack human with anticoagulant $\endgroup$ – SZCZERZO KŁY Jan 16 at 16:02
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    $\begingroup$ I'm pretty certain that making ticks bigger makes them less of a threat, because it'll be obvious when they're on you. They're such effective disease vectors because they're often unnoticeable. Consider leeches: they're much bigger, but they're kinda obvious and there aren't a whole lot of cases of people being killed by leeches except via disease and allergic reaction. $\endgroup$ – Starfish Prime Jan 16 at 16:40
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    $\begingroup$ There is a reason why ticks and other bloodsucking bugs are small - they can do their bloodsucking unnoticed. "Giant ticks" would be quickly noticed and removed by humans. $\endgroup$ – Alexander Jan 16 at 19:16
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2.5 to 4 liters of blood loss is lethal (depending on factors like weight) so let's say 3.25 is the magic number. A tick can drink about 0.005 liters of blood before it is full. So 650 average ticks with no interruption and all the time of the world and you are dead in 1 long sitting (about 7/10 days).

Factor to consider, depending on the type of tick it can hold 200 to 600 times it's own body weight in blood (sounds like much but seeing how small they are it is not much compared to how much blood we have in our bodies).

Another thing is that Ticks are parasites (not predators), they prefer to keep their prey alive as long as possible so they don't have "hunt".

Also people will not be that happy about having ticks on them and remove them, such a high number/size that blood loss is lethal will be noticed and removed. So if you are willing to go deeper in to the Sci-fi/magic i would suggest the ability to intoxicate their victims so they can feast on them without interruption.

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    $\begingroup$ Also consider that the body normally replaces about 1% of the red blood cells every day, and can produce several times that if needed due to blood loss. Over the course of a week, it's conceivable that the body could replace ~20% of its blood (~1L), so your net blood loss rate is only about half the rate at which the ticks are sucking it out. $\endgroup$ – Nuclear Hoagie Jan 16 at 15:05
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    $\begingroup$ @NuclearWang first of, great name. Second it would e a nightmare to be awake with all those things feeding on you constantly but no enough to kill you... iving hell $\endgroup$ – A.bakker Jan 16 at 17:02
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    $\begingroup$ I think that the intoxication route is the most promising. If one tick is no problem but 50 will send an adult human to sleep, allowing 500 more to start feeding... Now we're talking! $\endgroup$ – Whitehot Jan 17 at 15:25
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    $\begingroup$ "inticksication"? $\endgroup$ – David Hambling Jan 17 at 15:58
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If you just want to make the area uninhabitable, consider the impact on babies, children, people with the flu, livestock, etc.

Sure, a strong healthy adult can take a trip to the ticklands and come back drained but alive, but it's no place to raise a family let alone settle a town.

And even a strong healthy adult may be weakened to the point where they can't handle anything else that goes wrong. A weak horse and a weak driver can't ford that slightly flooded river that would be a minor obstacle elsewhere.

@Starfish Prime has mentioned two weaknesses in this model: (1) Competent people know how to dress to protect themselves against normal ticks; (2) Ticks are more dangerous because they are small, and thus harder to notice and dislodge. For a person with hands, giant ticks might be less of a problem. Perhaps instead of giant ticks there should be swarms of smaller ones, or ones that feed faster.

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    $\begingroup$ A strong healthy adult can visit the ticklands and remember to dress for the occasion. Children, the elderly, livestock, etc etc, might not. $\endgroup$ – Starfish Prime Jan 16 at 16:41

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