Atracinae, commonly known as Australian funnel-web spiders, is a subfamily of spiders in the funnel-web spider family Hexathelidae. Atracinae consists of three genera: Atrax, Hadronyche, and Illawarra. The subfamily includes species with medically significant venom, six species of which have severely envenomed human victims. The bite of the Sydney funnel-web spider (Atrax robustus) is potentially deadly, but there have been no fatalities since the introduction of modern first aid techniques and antivenom. The subfamily contains 35 species. All members of the subfamily are native to Australia.
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 blue-black to plum to brown, with a glossy, hairless carapace covering the front part of the body. Like the related diplurid spiders, some hexathelids have relatively long spinnerets; this is especially true of A. robustus. Males have a large mating spur projecting from the middle of their second pair of legs.
Like other Mygalomorphae (also incorrectly called “Orthognatha”) —an infraorder of spiders that includes the tropical tarantulas —these spiders have fangs which point straight down the body and do not point towards each other (cf Araneomorphae). They have ample venom glands that lie entirely within their chelicerae. Their fangs are large and powerful, capable of penetrating fingernails and soft shoes.
Funnel-webs make their burrows in moist, cool, sheltered habitats—under rocks, in and under rotting logs, some in rough-barked trees (occasionally metres above ground). They are commonly found in suburban rockeries and shrubberies, rarely in lawns or other open terrain. A funnel-web’s burrow characteristically has irregular silk trip-lines radiating from the entrance. Unlike some related trapdoor spiders, funnel-webs do not build lids to their burrows.
The primary range of the Australian funnel-web spiders is the eastern coast of Australia, with specimens found in New South Wales, South Australia, Victoria, and Queensland. The only Australian states or territories without funnel-webs are Western Australia and the Northern Territory.
Six species have severely envenomed human victims, including the C funnel web, northern tree funnel web (Hadronyche formidabilis), southern tree funnel web (H. cerberea), Blue Mountains funnel web (H. versuta), Darling Downs funnel web (H. infensa) and the Port Macquarie funnel web (H. macquariensis). Australian funnel-webs are one of the most dangerous groups of spiders in the world and are regarded by some to be the most deadly, both in terms of clinical cases and venom toxicity.
Examination of bite records has implicated wandering males in most if not all fatal funnel-web bites to humans. Adult males, recognised by the modified terminal segment of the palp, will defend themselves vigorously if they feel threatened and tend to wander during the warmer months of the year looking for receptive females for mating. They are attracted to water and hence are often found in swimming pools, into which they often fall while wandering. The spiders can survive such immersion for several hours and can deliver a bite when removed from the water. They also show up in garages and yards in suburban Sydney. Contrary to a commonly held belief, no funnel-web spider is able to jump, although they can run quickly.
While some very venomous spiders may give dry bites, these spiders do so much less frequently. An inefficient mode of delivery (to large animals) and interrupted contact are also possible causes of low volume delivery. It has been claimed that approximately 10% to 25% of bites will produce significant toxicity but the likelihood cannot be predicted and all should be treated as potentially life-threatening. Funnel-webs are not normally aggressive but will defend themselves vigorously if frightened or threatened. During an attack the funnel-web spider generally maintains a tight grip on its victim and bites repeatedly, making it an especially traumatic experience for humans who are bitten and increasing the risk of severe envenomation.
There have been 27 recorded deaths in Australia in the last 100 years from spider bites. Bites from Sydney funnel-web spiders have caused thirteen 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. One member of the genus Hadronyche, the northern tree funnel-web has also been claimed to cause fatal envenomation but, to date, this lacks the support of a specific medical report. Assays of venom from several Hadronyche species have shown it to be similar to Atrax venom.
There are many different toxins in the venom of Atrax and Hadronyche spiders. Collectively, these spider 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 or atracotoxin) and H. versuta (δ-ACTX-Hv1a, formerly known as versutotoxin). 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. 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.
The female venom was thought to be only about a sixth as potent to humans as that of the male. The bite of a female or juvenile may still be serious; however, considerable variability occurs in venom toxicity between species, together with assumable degrees of inefficiency in the method of venom delivery.
Envenomation symptoms observed following bites by these spiders are very similar. The bite is initially very painful, due to the size of the fangspenetrating the skin. Puncture marks and local bleeding are also usually visible. If substantial envenomation occurs, symptoms will generally occur within minutes and progress rapidly.
Early symptoms of systemic envenomation include goose bumps, sweating, tingling around the mouth and tongue, twitching (initially facial and intercostal), salivation, watery eyes, elevated heart rate and elevated blood pressure. As systemic envenomation progresses symptoms include nausea, vomiting, shortness of breath (caused by airway obstruction), agitation, confusion, writhing, grimacing, muscle spasms, pulmonary oedema (of neurogenic or hypertensive origin), metabolic acidosis and extreme hypertension. The final stages of severe envenomation include dilation of the pupils (often fixed), uncontrolled generalised muscle twitching, unconsciousness, elevated intracranial pressure and death. When death occurs it is generally as a result of progressive hypotension or possibly elevated intracranial pressure consequent on cerebral edema.
The onset of severe envenomation can be rapid. In one prospective study, the median time to onset of envenomation was 28 minutes, with only two cases having onset after 2 hours (both had pressure immobilisation bandages applied). Death may occur within a period ranging from 15 minutes (this occurred when a small child was bitten) to three days.
Due to the severity of symptoms, and the speed with which they progress, in areas where these spiders are known to live all bites from large black spiders should be treated as though they were caused by a funnel-web spider. First aid treatment for a suspected funnel-web spider bite consists of immediately applying a pressure immobilisation bandage; a technique which consists of wrapping the bitten limb with a crepe bandage, as well as applying a splint to limit movement of the limb. This technique was originally developed for snakebites but has also been shown to be effective at slowing venom movement and preventing systemic envenomation in case of funnel-web spider bite; there is evidence to suggest that periods of prolonged localisation may slowly inactivate the venom. Emergency medical treatment should be sought as soon as possible. It is important that the pressure immobilisation bandage not be removed prematurely; it should remain in place until intravenous access has been established, adequate medical monitoring is in place and a supply of antivenom is available. A patient who is not displaying any symptoms of systemic envenomation may deteriorate rapidly when the bandage is removed.
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, envenomation 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 envenomation: 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 known deaths since it became available.
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