The redback spider is dangerous, but unlikely to kill a healthy adult, even if the anti-venom is not administered. They are an invasive species and dangerous, especially to children, the elderly and anyone with a weakened immune system. Small pets (cat size) are likely to die if bitten and not given the anti-venom, for perspective.
If you see one of these spiders, avoid it or kill it.
Like most folks, I am not a fan of spiders. Like significantly fewer folks, I take that dislike to extreme lengths, preferring to burn down and rebuild if a particularly nasty spider is found within sight of me. I know it is not a logical fear, but… spiders! It makes no difference. Most of the time those fears are unfounded. The vast majority of spiders can do little more than make me uncomfortable and give me a bite little more painful than a mosquito bite.
In the case of the Australian redback spider, however, there is actually some cause for concern. This breed of spider is a “widow” spider, similar to the “black widow” spider common to my native Seattle. The “widow” refers to the female killing and eating the male in order to obtain their sperm, which can be stored for up to two years… shudder… Besides being a deviant practicing “sexual cannibalism,” the spider is venomous, and has a tendency to make its home inside human occupied areas, to my utter dismay.
The venom of the redback spider is rarely fatal, but it is extremely painful and bites should be taken seriously. To put it into perspective, in its native Australia, everything can kill you, and this spider is considered among the most dangerous spiders in that environment. Crocodiles and drop-bears alike tremble in its presence. Ok, so I made that up, but the “dangerous” part is real.
An adult female redback is easily distinguished by her round black body with prominent red stripe on the top of her abdomen and an hourglass-shaped red or orange streak on the bottom. Females have a body length of about 10 millimeters (0.4 in), while the male is much smaller, being only 3–4 mm (0.12–0.16 in) long. These spiders are nocturnal, and they build their “untidy” webs in warm sheltered locations, very often inside of human occupied areas.
There are more poisonous spiders out there, but a preference for living near humans makes up for any deficiencies, and the bite can regardless cause serious problems, including death. If you are lucky, you will suffer only a “dry bite,” in which no venom is injected and minor pain will follow. A “wet” bite is accompanied by a hot load of venom, and the amount of venom injected, the size of the victim, and their general health at the time will have a lot to do with what happens next.
The initial bite will feel like a pinprick followed by a burning sensation. If venom was introduced, the pain will worsen over the next hour, developing into localized pain (a small area surrounding the wound) sweating and goose-flesh. The pain may continue to spread and become generalized (your whole body will hurt), followed by stomach cramps, nausea and vomiting. The typical duration is three to six days. Victims unable to receive the anti-venom may feel sickly, weak, and endure muscle pain for weeks.
Even without the anti-venom, the bite is rarely fatal to a healthy adult, but by way of comparison, a cat or small dog will likely die if bitten and not given the anti-venom. Children, the elderly, and anyone with a weakened immune system are at the most risk from bites.
The redback spider is Japan’s most recent spider immigrant, though I think we all wish they had caught that one at the border. While there is no way to “protect yourself” against these spiders, you should know that while they live near humans, they are nocturnal, and very shy. They wont be living where you are most of the time, so the best defense is to be careful in areas of the home that are rarely used. It is unlikely you will see one, they are still uncommon, but you should be careful anyway.
Photo "Latrodectus hasseltii close" by Toby Hudson - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.