Friday, 16 December 2011

Nasty little spiders

The prime time news on Brazilian channel Globo recently showed a report on spiders*, specifically "Aranhas marroms", or "brown spiders" (translations never sound so impressive!). These little spiders might not look much, perhaps a bit larger than a normal spider, but they are a big problem.

"Brown spiders", (mostly Loxosceles similis) are found through out Brazil, from Para down through Minas and Sao Paulo to Rio Grande de Sul, with the southern state of Parana being something of a hotbed. However, unlike many spiders, the young don't make silk parachutes to travel on the wind and they've only got little legs, so they don't travel very far - colonies tend to be very dense, but localised. They're timid animals and will generally try to hide from humans, but 5,000 people are bitten each year, leading to fevers, skin lesions, and even death. So why is such a little spider so dangerous?

Well, it's all to do with their venom, which is incredibly toxic. Unlike some snake venoms, it's not neurotoxic as such, so victims don't go into paralysis or spasms, it is necrotic, which means that cells just die. Depending on how good a bite it got in, this can cause a little lesion the size of a bottle top, to one up to 40 cm wide. If this gets infected, or the toxin is carried in the blood stream to other organs (which is rare) the consequences can be very serious. The bizarre thing about their venom is that whilst some mammals such as rabbits and humans are very susceptible, mice and rats aren't, goodness knows why.

So what to do?


Virtually all cases of bites have been from spiders forced into close contact, by someone picking them up, or they were in clothing or shoes, so shake out any clothing. Don't, what ever you do, spray them with insecticide! It causes a nervous reaction making them very aggressive and much more likely to bite!


If you suspect you've been bitten, wash the site with soap and water, but don't try to squeeze or suck out the venom, you'll just spread it around. Go straight to a doctor, with, if possible, the spider - treatment is by anti-venom and it helps to know who bit you!

Every year, specialised (and very brave!) teams go out hunting around the southern city of Curitiba and catch thousands of spiders, which are "milked" for their poison. This is used to make the anti-venom. about 20,000 doses per year, to be distributed to hospitals all over Brazil.The actual technique used to make the anti-venom is explained nicely in the Globo piece.

Be nice to geckos

The principle predators of brown spiders are geckos, which presumably are immune to their toxin.

* The news report can be found here....

1 comment:

Anonymous said...

This is not fair! Why are rats not susceptible? "Rats!"