Tarantulas are often portrayed as merciless killers in movies, it bites you and your life is over. This is fortunatly not true at all. All  tarantulas are venomous (almost all spiders have venom glands and venom) but its venom is not dangerous to humans in general. Its been told that Poecilotheria and Stromatopelma can be dangerous even for humans but so far there is no documented case of a human fatality from a tarantula bite regardless of genus. Old world species is believed to have a more potent venom on mammals then new world species and the general effects are a bit worse then for most new world species with intensive pain, cramping and palpitation. The mechanical damage after a bite is moderate. Sometimes you hear that ”if you are allergic you should be extra careful” or the comparison ”a bite is like a bee sting”.
Neither statement are correct. I have not come across any documentation that suggest severe allergic reactions such as anaphylaxis¹ from a tarantula bite. On the other hand after asking a reknowned toxinologist, Brian G. Fry, if theraphosid venom could cause serious allergic reactions and he explained that spider venom are based on peptides and snake- and bee venom are not. They are based on larger proteins and therefor more prone to cause severe allergic reactions – especially bee- and ant venom are highly allergenic.

Above: The venom apparatus. Illustration from Baerg W.J. ”The Tarantula”
University of Kansas Press—Lawrence, 1958

However, almost all new world genus have urticating hair, setae, that can be very unpleasant and could cause damage to a cornea if you are unlucky to get the setae into the eye².
This hair do cause (mild) allergic reactions such as rash, swelling and intensive itching. It seems that the symptoms increases with exposure – a person might not feel any discomfort while handling the tarantula but with time the rash and intching increases and gets worse. (more on urticating hair)
The urticating hair can be very unpleasant and might cause damage to the cornea if you are unlucky to get the setae into the eye².

Regardless if tarantulas have the capability to kill a person or not they are venomous and should be treated with respect. The infamous Australian funnel web spiders are not tarantulas (they belong to the Hexathelidae family) despite what some popular books are saying.

Below a list of symptoms from tarantula bites:

Grammostola rosea – – No visible symptoms
Pterinochilus murinus – Swelling and pain lasting several days
Poecilotheria fasciata – Severe pain, numbness, respitory difficulties and (sometimes) increased or decreased heart beat. Local or total paralysys of the bitten limb. The venom attacks neuromusculare and causes tetanus. These symptoms lasts for 1-2 days but can return under 2-4 weeks
Theraphosa blondi – Intensive pain, miscoloration at the bite. The pain lasted 1-2 days.

External links

¹serious allergic reaction – Wikipedia

² Ophthalmia nodosa secondary to tarantula hairs
J T L Choi and A Rauf article

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