Redback spider

The redback spider (Latrodectus hasselti), also known as the Australian black widow, is a species of highly venomous spider believed to originate in Australia, but which is now found in Southeast Asia, Japan and New Zealand. It is a member of the cosmopolitan genus Latrodectus, the widow spiders. The adult female is easily recognised by her spherical black body with a prominent red stripe on the upper side of her abdomen and an hourglass-shaped red/orange streak on the underside. Females usually have a body length of about , while the male is much smaller, being only long.

Mainly nocturnal, the female redback lives in an untidy web in a warm sheltered location, commonly near or inside human residences. It preys on insects, spiders and small vertebrates that become ensnared in its web. It kills its prey by injecting a complex venom through its two fangs when it bites, before wrapping them in silk and sucking out the liquefied insides. Often, it first squirts its victim with what resembles 'superglue' from its spinnerets, immobilising the prey by sticking the victim's limbs and appendages to its own body. The redback spider then trusses the victim with silk. Once its prey is restrained, it is bitten repeatedly on the head, body and leg segments and is then hauled back to the redback spider's retreat. Sometimes a potentially dangerous victim can be left to struggle for hours until it is exhausted enough to approach safely.

The redback is one of the few spider species that can be seriously harmful to humans, and its liking for habitats in built structures has led it to being responsible for a large number of serious spider bites in Australia. Predominantly neurotoxic to vertebrates, the venom gives rise to the syndrome of latrodectism in humans; this starts with pain around the bite site, which typically becomes severe and progresses up the bitten limb and persists for over 24 hours. Sweating in localised patches of skin occasionally occurs and is highly indicative of latrodectism. Generalised symptoms of nausea, vomiting, headache, and agitation may also occur and indicate severe envenomation. An antivenom has been available since 1956.

Taxonomy and naming

Common names

The common name "redback" is derived from the distinctive red stripe along the dorsal aspect of its abdomen. Other common names include red-striped spider, Murra-ngura spider, Kapara spider and the Kanna-jeri spider.

History

Before DNA analysis, the taxonomy of the widow spider genus Latrodectus had been unclear—changes in the number of species reflect the difficulty of using morphology to determine subdivisions within the genus. from specimens collected in Rockhampton and Bowen in central Queensland. He named it Latrodectus hasseltii in honour of colleague A.W.M. van Hasselt. In the same paper, he named a female from Cape York with an all-black abdomen L. scelio, which was later regarded as a subspecies of the redback. Another subspecies, L. h. aruensis, was described by Norwegian entomologist Embrik Strand in 1911. Subspecies indica (of L. scelio) had been described by Eugène Simon in 1897, but its origin is unclear. The redback was also considered by some to be conspecific with the katipō (L. katipo), which is native to New Zealand, though Koch regarded them as distinct. Subsequently, more reliable genetic studies have split the genus into about 30 species, and the redback has no recognised subspecies in modern classifications.

Placement

A member of the genus Latrodectus in the family Theridiidae, the redback belongs in a clade with the black widow spider, There is evidence of interbreeding between female katipō and male redbacks in the wild.

Description

thumb|right|A juvenile female, showing typical white banding

The adult female redback has a body around long, with slender legs, the first pair of which are longer than the rest. The round abdomen is a deep black (occasionally brownish), with a red (sometimes orange) longitudinal stripe on the upper surface and an hourglass-shaped scarlet streak on the underside. Juvenile females have additional white markings on the abdomen. with very small fangs. Small compared to the female, the male redback is long and is light brown, with white markings on the upper side of the abdomen and a pale hourglass marking on the underside.

Another species in Australia with a similar physique, Steatoda capensis, has been termed the "false redback spider", but it is uniformly black (or plum), and does not display the red stripe.

Behaviour

thumb|right|Female in its web

Web

The redback is mainly nocturnal; These webs are usually placed between two flat surfaces, one beneath the other. The female spends more time in the funnel and less time moving around during cooler weather.

The individual web filaments are quite strong, able to entangle and hold small reptiles.

Prey

thumb|left|Female with a lizard it has captured

Redbacks usually prey on insects, but can capture larger animals that become entangled in the web, including trapdoor spiders, small lizards, and even on rare occasion snakes. One web was recorded as containing a dead mouse. The woodlouse (Porcellio scaber) is a particularly common food item. Developing spiderlings need size-appropriate prey, and laboratory studies show that they are willing to consume common fruit flies (Drosophila melanogaster), mealworm larvae (Tenebrio molitor), muscoid flies and early nymphs of cockroaches. After hatching they spend about a week inside the egg sac, feeding on the yolk and molting once. Baby spiders appear from September to January (spring to early summer). Male spiders mature through five instars in about 45–90 days. Redbacks can survive temperatures from below freezing point to , though they do need relatively warm summers, with temperatures of for two to three months, to survive and breed. They are attracted by pheromones, which are secreted by unmated sexually mature female redback spiders onto their webs and include a serine derivative (N-3-methylbutyryl-O-(S)-2-methylbutyryl-L-serine). This is thought to be the sole method by which males assess a female's reproductive status, and their courtship dismantles much of the pheremone-marked web.

During mating, the male redback attempts to copulate by inserting one of its palps into one of the female's two spermathecae, each of which has its own insemination orifice. It then tries and often succeeds in inserting the other palp into the female's second orifice. The redback spider is one of only two animals known where the male has been found to actively assist the female in sexual cannibalism. In the process of mating, the much smaller male somersaults to place his abdomen over the female's mouthparts. In about two of three cases, the female fully consumes the male while mating continues. Males which are not eaten die of their injuries soon after mating. Sacrifice during mating is thought to confer two advantages to the species. The first is the eating process allows for a longer period of copulation and thus fertilisation of more eggs. The second is females which have eaten a male are more likely to reject subsequent males. Although this prohibits future mating for the males, this is not a serious disadvantage, because the spiders are sufficiently sparse that less than 20% of males ever find a potential mate during their lifetimes, and in any case, the male is functionally sterile if he has used the contents of both of his palps in the first mating.

Once the female has mated, the sperm is stored in one or both of her spermathecae. The sperm can be used to fertilise several batches of eggs, over a period of up to two years (estimated from observations of closely related species), but typically restarts the female's pheromone production advertising her sexual availability about three months after mating. A female spider may lay four to ten egg sacs, England, Belgium, the United Arab Emirates and Iran. However, an earlier informal description (1850) from the Adelaide Hills is now known, and names in Australian Aboriginal languages also show that it was present well before European settlement. Its original range is thought to be a relatively small arid part of South Australia and Western Australia. Its spread has been inadvertently aided by modern buildings, which often provide habitats conducive to redback populations. The close relationship between the redback and the New Zealand katipō also supports the native status of both in their respective countries. It is wide spread throughout urban Australia, with most suburban backyards in the city of Canberra (for instance) having one or more nesting females in such places as firewood piles, stored brick stacks and around unused or restoring motor vehicles as well as generally behind the shed - as observed since at least the 1970s and probably earlier. The redback spider is commonly found in close proximity to human residences. Webs are usually built in dry, dark, sheltered sites, such as among rocks, in logs, tree hollows, shrubs, old tyres, sheds, outhouses, empty tins and boxes, children's toys or under rubbish or litter. Letterboxes and the undersurface of toilet seats are common sites. Populations can be controlled by clearing these habitats, The CSIRO Division of Entomology recommends against the use of spider pesticides due to their toxicity, and because redbacks are rapid recolonists anyway.

Introductions

The redback spider's affinity for human-modified habitat has enabled it to spread to several countries via international shipping and trade. Furthermore, its tolerance to cold means that it has the ability to colonise many temperate countries with a winter climate cooler than Australia. This is concerning due to the risks to people being bitten who are unaware of its venomous nature, and also to the conservation of local threatened insect species that the redback might prey upon. They were introduced into New Zealand in the early 1980s, and now are found around Central Otago (including Alexandra, Bannockburn and near Wānaka) in the South Island and New Plymouth in the North Island. Authorities in the United Arab Emirates warn residents and visitors of redback spiders, which have been present since 1990. Colonies have also been established in greenhouses in Belgium, and isolated observations indicate possible presence in New Guinea, the Philippines, and India. One redback was found in a back garden in Dartford in Kent. Two females were discovered in the Iranian port city of Bandar Abbas in 2010.

There is an established population of redback spiders in Osaka, Japan, thought to have arrived in cargoes of wood chips.

In 2008, redback spiders were found in Fukuoka, Japan. Over 700 have been found near the container terminal in Hakata Bay, Fukuoka City. Dispersal mechanisms within Japan are unclear, but redbacks are thought to have spread by walking or by being carried on vehicles. Signs warning about redback spiders have been posted in parks around the city. and redbacks are often absent if these species are present in significant numbers. Agenioideus nigricornis, a spider wasp, is a parasitoid of the adult redback.

Bites to humans

Incidence

The redback spider has been historically responsible for more envenomations requiring antivenom than any other creature in Australia. However, by 2017 the spider was blamed for only 250 envenomations requiring antivenom annually. Estimates of the number of people thought to be bitten by redback spiders each year across Australia range from 2,000 to 10,000. As the female redback is slow-moving and rarely leaves her web, bites generally occur as a result of placing a hand or other body part too close to the spider, such as when reaching into dark holes or wall cavities. Bites often also occur when a hidden spider is disturbed in items such as clothes, shoes, gloves, building materials, garden tools or children's outdoor toys. Historically, victims were often bitten on the genitalia, though this phenomenon disappeared as outhouses were superseded by plumbed indoor toilets. Conversely, bites on the head and neck have increased with use of safety helmets and ear muffs. Precautions to avoid being bitten include wearing gloves and shoes while gardening, not leaving clothes on the floor, and shaking out gloves or shoes before putting them on. Also, children can be educated not to touch spiders.

Venom

thumb|right|The distinctive red stripe of the adult female

The redback and its relatives in the genus Latrodectus are considered dangerous, alongside funnel-web spiders (Atrax and Hadronyche), mouse spiders (Missulena), wandering spiders (Phoneutria) and recluse spiders (Loxosceles). Venom is produced by holocrine glands in the spider's chelicerae (mouth parts). Venom accumulates in the lumen of the glands and passes through paired ducts into the spider's two hollow fangs.

In vertebrates, alpha-latrotoxin produces its effect through destabilisation of cell membranes and degranulation of nerve terminals, resulting in excessive release of neurotransmitters, namely acetylcholine, norepinephrine and GABA. Excess neurotransmitter activity leads to clinical manifestations of envenomation, although the precise mechanisms are not well understood. Female redbacks have an average of around 0.08–0.10 mg of venom, and experiments indicate that the median lethal dose (LD50) for mice at room temperature is 10–20% of this quantity (0.27–0.91 mg/kg based on the mass of the mice used), but that it is considerably deadlier for mice kept at lower or higher temperatures. Pure alpha-latrotoxin has an LD50 in mice of 20–40 μg/kg.

The specific variant of the vertebrate toxin found in the redback was cloned and sequenced in 2012, and was found to be a sequence of 1180 amino acids, with a strong similarity to the equivalent molecule across the Latrodectus mactans clade. The syndromes caused by bites from any spiders of the genus Latrodectus have similarities; there is some evidence there is a higher incidence of sweating, and local and radiating pain with the redback, while black widow envenomation results in more back and abdominal pain, and abdominal rigidity is a feature common with bites from the west coast button spider (Latrodectus indistinctus) of South Africa. Angiotensin-converting-enzyme inhibitors, or ACE inhibitors, are a class of widely-prescribed medications used in hypertension and heart failure.

the venom of the redback, although little-studied, likely has similar agents.]]

Redback antivenom was developed by Commonwealth Serum Laboratories, then a government body involved with discovering antivenoms for many venomous Australian creatures. Production involves the milking of venom from redbacks and repeatedly inoculating horses with non-lethal doses. The horse immune systems makes polyclonal antibodies. Blood plasma, containing the antibodies, is extracted by plasmapheresis. The plasma is treated with pepsin, and the active F(ab')2 fragments are separated and purified. Each vial contains 500 units of redback antivenom in approximately 1.5 ml, which is enough to inactivate 5 mg of redback spider venom in a test tube. The antivenom has been safely administered to women in various stages of pregnancy. however, as these cases are often mild and the evidence of its effectiveness is limited, this treatment is not recommended. Animal studies also support its use against envenomation from other widow spiders, having successfully been tested against venom from L. mactans, L. hesperus, and L. tredecimguttatus (synonym L. lugubris).

Signs and symptoms

Envenomation from a redback spider bite produces a syndrome known as latrodectism. A small but significant percentage of people bitten develop significant pain or systemic symptoms. Symptoms typically include nausea, vomiting, abdominal or chest pain, agitation, headache, generalised sweating and hypertension.

Children and infants may be unable to report being bitten, making it difficult to associate their symptoms with a spider bite. Symptoms seen in infants include inconsolable crying, refusing to feed and a general erythematous rash. Muscle aches and pains, and neck spasm are often seen in children over four years of age.  Latrodectism has been misdiagnosed as various medical conditions including acute hepatitis, sepsis, testicular torsion or an acute abdomen.

Hospital assessment is recommended if simple pain relief does not resolve local pain, or systemic symptoms occur. Opioid analgesics may be necessary to relieve pain. In an Australian study of 750 emergency hospital admissions for spider bites where the spider was definitively identified, 56 were from redbacks. Of these, 37 had significant pain lasting over 24 hours. Only six were treated with the antivenom.

The antivenom manufacturer's product information recommends one vial, although more has been used. Past guidelines indicated two vials, with a further two vials recommended if symptoms did not resolve within two hours, however recent guidelines state "antivenom is sometimes given if there is a history, symptoms and signs consistent with systemic envenoming, and severe pain unresponsive to oral analgesics ... however recent trials show antivenom has a low response rate little better than placebo, and any effect is less than might be achieved with optimal use of standard analgesics." In January 2008 toxicologist Geoffrey Isbister suggested IM antivenom was not as effective as IV antivenom, after proposing that IM antivenom took longer to reach the blood serum. Isbister subsequently found the difference between IV and IM routes of administration was, at best, small and did not justify routinely choosing one route over the other.

These concerns led two handbooks to recommend IV in preference to IM administration in Australian practice. Despite a long history of usage and anecdotal evidence of effectiveness, there is a lack of data from controlled studies confirming the antivenom's benefits. The question of abandoning the antivenom on the basis of this and previous studies came up in the Annals of Emergency Medicine in 2015 where White and Weinstein argued that if the recommendations in the 2014 Isbister et al. paper were followed it would lead to abandonment of antivenom as a treatment option, an outcome White and Weinstein considered undesirable. Authors of the 2014 Isbister et al. paper responded in the same issue by suggesting patients for whom antivenom is considered should be fully informed "there is considerable weight of evidence to suggest it is no better than placebo", and in light of a risk of anaphylaxis and serum sickness, "routine use of the antivenom is therefore not recommended".

Before the introduction of antivenom, benzodiazepines and intravenous calcium gluconate were used to relieve symptoms of pain and distress, although calcium is not recommended as its benefit has not been shown in clinical trials.

Studies support the safety of antivenom, with around a 5% chance of an acute reaction, 1–2% of anaphylaxis and 10% chance of a delayed reaction due to serum sickness. However, in the vast majority of cases, it is administered within 24 hours. In almost all cases, symptoms resolve within a week. Apart from that, there have been no deaths due to redback bite since the introduction of antivenom.