web spider

Australasian funnel-web spider

Australian funnel-web spiders are very venomous spiders of the family Hexathelidae. These spiders are found in two genera of the family: Hadronyche (which is not associated with any known human fatalities) and Atrax (which probably have killed 13 people) They are notorious for the inclusion of the Sydney funnel-web spider (Atrax robustus), native to eastern Australia. There are other genera in family Hexathelidae, but these do not have the notorious reputation of the Australasian funnel-webs.

Description

These spiders are medium-to-large in size, with body lengths ranging from 1 cm to 5 cm (0.4" to 2"). They are darkly coloured, ranging from black to brown, with a glossy carapace covering the front part of the body. Some of these spiders greatly resemble tarantulas. Like the related diplurid spiders, the hexathelids generally have long spinnerets; this is especially true of A. robustus.

Like other Mygalomorphae, (also called the Orthognatha an infraorder of spiders that includes the so-called tarantulas- (Mygalomorph tarantulas are more correctly termed "bird-eating spiders"- the true 'tarantula' is an araneomorph Lycosid from Europe )) , these spiders have fangs which point straight down and do not cross each other (cf Araneomorphae). They have ample venom glands that lie entirely within their chelicerae. Their fangs are large and powerful. Although they are fairly small compared to the Mygalomorph tarantulas, they should not be handled without taking substantial precautions, because their fangs have been known to penetrate fingernails and soft shoes, resulting in a dangerous bite.

Habitat

The primary range of the Australasian funnel-web spiders is the eastern coast of Australia, with specimens found in New South Wales, South Australia, Victoria, and Queensland. In addition, some specimens have been found in other islands in the south Pacific. The only Australian state that lacks a funnel-web population is Western Australia.

Medical significance

Funnel-webs are probably one of the three most dangerous spiders in the world and are regarded by some to be the most dangerous.

Examination of bite records seems to indicate that wandering males have caused a large majority of fatal bites to humans, however this has not been proven. Males, recognised by the modified terminal segment of the palp, are aggressive and tend to wander during the warmer months of the year looking for receptive females of their kind for mating. They are attracted to water and hence are often found in swimming pools where they have fallen while wandering. The spiders can survive such immersion for several hours and can deliver a bite when removed from the water, so this should not be done by hand.. They also show up in garages and yards in suburban Sydney. Contrary to the commonly held belief, no funnel-web spider is able to jump, although they can run swiftly.

While some very venomous spiders may give dry bites, these spiders do so much less frequently. It appears that approximately 10% to 25% of bites will produce toxicity but the likelihood cannot be predicted and all should be treated as potentially life-threatening.

There have been 26 recorded deaths in Australia in the last 100 years from spider bites. Bites from funnel web spiders have caused 13 deaths (seven in children). In all cases where the sex of the biting spider could be determined, it was found to be the male of the species. Most victims were young, ill or infirm. Members of the genus Hadronyche may also cause fatal envenomations although none has been recorded. Assays of venom from several Hadronyche species have shown it to be similar to Atrax venom.

Toxins

There are a large number of different toxins in the venom of Atrax and Hadronyche spiders. Collectively, the toxins are given the name atracotoxins (ACTX), as all these spiders belong to the subfamily Atracinae. The first toxins isolated were the δ-ACTX toxins present in the venom of both A. robustus (δ-ACTX-Ar1, formerly known as robustoxin) and H. versuta (δ-ACTX-Hv1a, formerly known as versutoxin). Both these toxins produce the same funnel-web envenoming syndrome in monkeys as that seen in humans, suggesting that they are responsible for the physiological effects seen with crude venom.

These toxins are thought to operate by opening sodium channels. They are presynaptic neurotoxins that (via sodium channels) induce spontaneous, repetitive firing of action potentials in autonomic and motor neurons and inhibit neurally mediated transmitted release resulting in a surge of endogenous acetylcholine, noradrenaline and adrenaline.

Although extremely toxic to primates, the venom appears to be fairly harmless to many other animals, including dogs, cats, horses, rabbits, guinea-pigs, chickens and even cane toads. It has been suggested that these animals may be resistant to the venom's effects due to the presence of IgG, and possibly cross-linked IgG and IgM inactivating factors in their blood plasma that bind to the toxins responsible and neutralise them, or it may involve a non-specific reaction due to the highly basic nature of the toxins.

The female venom was thought to be only about a sixth as potent to humans as that of the male but recent research has proven that false. The bite of a female or juvenile may be serious; however, considerable variability occurs in venom toxicity between species.

Symptoms

The envenoming syndromes observed following bites by all these spiders are identical. The bite is initially very painful due to the acidity of the venom and the size of the fangs penetrating the skin. Systemic envenoming may follow the local effects. Early symptoms of systemic envenoming include tingling around the mouth and tongue, facial muscle twitching, nausea, vomiting, profuse sweating, salivation, and shortness of breath. Patients may rapidly develop agitation, confusion and coma associated with hypertension, metabolic acidosis, dilation of the pupils, generalised muscle twitching and pulmonary oedema. Death results from progressive hypotension or possibly raised intracranial pressure resulting from cerebral oedema.

The onset of severe envenoming is rapid; in one prospective study, the median time to onset of envenoming was 28 minutes, with only two cases having onset after 2 hours (both had pressure immobilization bandages applied). Deaths may occur within a period ranging from 15 minutes (this occurred when a small child was bitten) to three days.

Treatment

First aid for funnel-web bites consists of applying a pressure immobilization bandage. Pressure immobilization is the wrapping of the bitten limb with a crepe bandage and splint. It was originally developed for snakebites but has been shown to be effective at slowing venom movement in funnel web bites and may also slowly inactivate the venom.

Further supportive care may be necessary, but the mainstay of treatment is antivenom. Antivenom is raised against male Atrax robustus venom but appears to be effective for all species of funnel web spiders. Funnel-web antivenom has also been shown to reverse the in vitro effects of Eastern Mouse spider (Missulena bradleyi) venom.

Prior to the introduction of antivenom, envenoming resulted in significant morbidity and mortality. The purified rabbit IgG antivenom was developed in 1981 through a team effort led by Dr Struan Sutherland, head of immunology at the Australian Commonwealth Serum Laboratories in Melbourne. Fortunately, the antivenom is fast-acting and highly and globally effective. Antivenom therapy has shortened the course of envenoming: prior to antivenom availability, the average length of hospital treatment for severe bites was about 14 days. Today, antivenom-treated patients are commonly discharged from hospital within 1 to 3 days. There have been no deaths since it became available.

Species

The following funnel-web species have been identified:

  • Atrax robustus, Sydney funnel-web spider
  • Hadronyche adelaidensis, Adelaide funnel-web spider
  • Hadronyche cerberea, Tree funnel-web spider or Southern funnel-web spider
  • Hadronyche eyrei, Eyre Peninsula funnel-web spider
  • Hadronyche flindersi, Flinders funnel-web spider
  • Hadronyche formidabilis, Northern Rivers funnel-web spider or Northern funnel-web spider
  • Hadronyche infensa, Toowoomba funnel-web spider or Fraser Island funnel-web spider.
  • Hadronyche modesta, Victorian funnel-web spider
  • Hadronyche pulvinator, Cascade funnel-web spider
  • Hadronyche venenata, Tasmanian funnel-web spider
  • Hadronyche versuta, Blue Mountains funnel-web spider

See List of Hexathelidae species for a complete list of species within this family (about 90 species in 11 genera).

Footnotes

References

External links

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