Trevor T. Zachariah, DVM, MS
All spiders belong to Class Arachnida, Order Aranae. Within this order, the paraphyletic group Mygalomorphae includes the evolutionarily primitive spider families. Family Theraphosidae contains all the spiders known as "tarantulas", of which there are 120 genera and greater than 900 species.16
Theraphosids are often divided into two informal groups based on geography, as well as generalized anatomical and behavioral characteristics. New World spiders originate in the Americas, many species have urticating hairs as a defense mechanism, and are often more docile; examples include the Mexican redknee (Brachypelma smithi) and Chilean rose (Grammostola rosea). Old World spiders originate in Africa, Asia, and Australia, do not have urticating hairs, and are often more aggressive and quicker to defend themselves; examples include the king baboon (Pelinobius muticus) and ornamentals (Poecilotheria spp.).
Anatomy and Physiology
Like all spiders, the theraphosid body plan is divided into two segments: the prosoma (cephalothorax) and the opisthosoma (abdomen). Connecting the two segments is a narrow area called the pedicel.
The prosoma contains the paired venom glands, primitive central nervous system, extensive musculature, anterior digestive tract, and portions of the intestinal tract. It also serves as the attachment for most of the appendages. In order from anterior to posterior, these include a pair of chelicerae (to which the fangs attach), a pair of pedipalps, and four pairs of legs. The dorsal and ventral aspects of the prosoma are known as the carapace and sternum, respectively, which move in relation to each other due to a pliable membrane connecting them. The eight small eyes are tightly clustered on the anterior midline of the carapace.
The majority of the internal organs are present within the opisthosoma. These include the tubular heart (along the dorsal midline), extensive diverticula of the intestinal tract, Malpighian tubules and stercoral pocket (excretory organs), two pairs of book lungs (in the anterioventral aspect), gonads, and various silk glands. The only appendages associated with the opisthosoma are the spinnerets at the caudal end.
The appendages all contain striated muscles for contraction, though none for extension, which is accomplished by hemolymph pressure from compression within the prosoma. The legs contain seven segments (proximal to distal): coxa, trochanter, femur, patella, tibia, metatarsus, tarsus; the pedipalps are similar, though lack the metatarsus.8 All the appendages can be autotomized and regenerated during subsequent molts.
Spiders have a relatively low metabolic rate. This is due to a variety of factors, including: respiration by passive diffusion across the book lung membrane; inefficiency of oxygen transport by the copper-containing protein hemocyanin; poikilothermy; semi-open circulatory system (no capillaries or veins).
Numerous hairs cover the exoskeleton of a theraphosid. Many of these have a sensory function, especially detection of chemical and tactile stimuli. Theraphosid eyesight is poor13 and largely limited to detection of movement and light.
A number of methods of physical restraint of theraphosids have been described. However, only a few of these are practical for examination or other medical purposes. The easiest is to gently prod a spider into a clear glass or plastic container, though this impedes access to the patient for close inspection or procedures. Stretching clear plastic wrap over an animal can also be done.
Handling theraphosids requires a calm and steady hand to avoid startling them, leading to traumatic falls. This can be minimized by holding them as near to the surface and as far from the edge of a table as possible. Whenever handling a spider, it is best to wear latex examination gloves.
Spiders may be prodded onto an open palm placed in front of them. Gentle pressure on the carapace can be used pin an animal for brief periods, though struggling often occurs. "Pinching" between the second and third pairs of legs with the thumb and index finger allows for a spider to be lifted with minimal resistance. A cupped hand placed over a theraphosid dorsally can be used to "scoop" it up.19
A gauze sling restraint has been described.9 However, the author does not recommend this method due to the potential for injury from over-tightening and the possibility of the animal catching its claws in the gauze. Hypothermia as a method of restraint is also not recommended, except in extreme circumstances.
Anesthesia of spiders has a mostly anecdotal history.3,9,13 However, recent studies have more rigorously examined the parameters of inhalation anesthesia of theraphosids. Chamber induction of G. rosea with carbon dioxide (100%) or isoflurane (5%) resulted in 3.32 ± 1.47 and 5.3 ± 2.67 min to loss of movement, and 4.75 ± 3.25 and 8.1 ± 4.65 min to recovery, respectively.6 Another study of 5% isoflurane (in 1 L/min oxygen) anesthesia using G. rosea and goliath birdeater spiders (Theraphosa blondi) found chamber median induction times of 10 min, and recovery times of 12.5 and 30 min, respectively.23 A similar study of G. rosea using 5% sevoflurane in 1 L/min oxygen resulted in median induction times of 16 min and recovery times of 29 min.26
A description of an invertebrate surgery chamber has been published.11 More direct application of inhalation anesthetics via a cone placed over the opisthosoma can also be performed.12
Monitoring an anesthetized spider is difficult. The only reliable method identified so far is the righting reflex.23
Anamnesis and Physical Examination
As with any patient, a thorough history should include signalment, husbandry conditions, feeding and excretion patterns, and information about past and present medical conditions. It should also include any information on the source of the animal, as well as the ecdysis cycle. Clients should be encouraged to keep records to help provide this information.9 Physical examination should assess for abnormalities in physical condition, movement, and behavior.
Standard survey radiography of spiders is often unrewarding. Elucidation of the gastrointestinal tract can be achieved by ingestion of radiopaque contrast material, though oral administration of such can be difficult.5 Experimental use of magnetic resonance imaging (MRI) has produced images of internal structures of theraphosids that are of sufficient detail for diagnostic purposes.17 However, this imaging modality is not typically available on a commercial basis.
Hemolymph can be analyzed for various biochemical and blood gas parameters.21,24,26 Hemograms are difficult to perform due to various nomenclatures for the cells and lack of interpretive information. To collect a hemolymph sample, it is best to use anesthesia. Intracardiac samples provide the most volume and are quickest to perform. The spider can be restrained with one hand (index finger on the carapace, thumb stabilizing the caudal aspect of the opisthosoma), while the other hand handles the syringe. A needle of small gauge (25–30) is inserted through the central aspect of the dorsal midline of the opisthosoma at an approximately 30–45° angle.20 The needle bevel should only just pierce the exoskeleton, as the heart or pericardial sac is entered immediately. Hemolymph may also be collected at the arthrodial membranes of the legs.
Hemolymph samples should be immediately placed in an anticoagulant (e.g., heparin, sodium citrate), though coagulation often occurs despite this. Clots should be removed, or the supernatant can be extracted after centrifugation. Digital pressure or tissue glue may need to be applied to the puncture site in the opisthosoma to facilitate hemolymphstasis.
Discharge from any orifice should be collected for cytology and culture and sensitivity. Fecal samples may be analyzed for microflora and evidence of parasites. Interpretation of culture results can be difficult, as the normal internal and external microbial flora of spiders have not been fully discerned.
Fluid therapy may be administered through a number of routes to theraphosid spiders. In animals that are able to locomote, providing them with a readily accessible source of standing water from which to drink is often sufficient. For animals that are not mobile, they can be placed in a shallow dish of water, provided their orientation keeps the book lungs from being submerged. Water may also be administered directly to the oral opening with a syringe.
Parenteral fluid administration is also possible, though more complicated. Theraphosid hemolymph has a relatively high osmolality (> 350 mOsm/L),25 so the type of fluid chosen should match this as closely as possible. Among commercially available solutions, 0.9% saline (308 mOSm/L) is the best match. Injections can be made directly into the heart or opisthosomal cavity; however, they must be made very slowly so as not to cause damage to the exoskeleton or internal structures.
Anecdotal accounts of topical and parenteral antimicrobial therapy appear in the literature,4,9,13 but no studies of efficacy or safety have been conducted.
Amputation of an appendage is often necessary due to trauma or deformation. The loss of an appendage allows for its replacement with a spider's regeneration abilities. The most common amputations involve the legs, and are performed by inducing autotomy. The leg is firmly grasped with hemostats at the level of the femur near its articulation with the trochanter. A sharp upward tug is then employed to excise the limb, and hemorrhage is often minimal. Unfortunately, since autotomy is a voluntary action on the part of the spider, amputations must be performed without anesthesia to prevent excessive hemorrhage.
Various methods have been used to restore integrity to the theraphosid exoskeleton after a traumatic event. Tissue glue, other adhesives and sealants, and fine suture material have all been used with varying success. The outcome often depends on the extent of the injuries.
Elective surgery in theraphosids is rare. The only reported description is the implantation of passive integrated transponders for monitoring a group of spiders for experimental purposes.18 However, the technique was performed without complication and could be applied to other situations (e.g., pet animals).
Diseases and Conditions
Traumatic lesions most frequently occur when a spider is dropped or falls from a distance greater than its legspan. Any injury that destroys the integrity of the exoskeleton causes hemorrhage and has the potential to lead to death from hypovolemia. Thus, it should be treated as a medical emergency. The common sites of injury are the appendages and the opisthosoma, due to its lack of exocuticle and thinner exoskeleton.
Proper theraphosid husbandry includes allowing access to a pool of standing water from which to drink. Without this, dehydration can occur. Mild to moderate dehydration can be difficult to detect due to the rigid nature of the exoskeleton. However, clinical signs become apparent when it is severe enough to cause hypovolemia. These include paresis, contraction of appendages, and decreased turgor of the opisthosoma.
Improper or incomplete molt is a serious problem for a spider. It can lead to traumatic injury, deformation, and even death. The exact cause is unclear, but the etiology is likely multifactorial. Age, poor physical condition, disease, injury, and low enclosure humidity have all been postulated as causes.19 If a spider has not completed a molt within 24 hours, intervention is likely needed. Application of glycerin to soften the old exoskeleton or soap solution for lubrication may be used in combination with gentle traction. The old exoskeleton may actually need to be cut off of the animal. Whatever the strategy, great care must be taken to avoid injury to the fragile new exoskeleton.
Many captive New World theraphosids are found to develop alopecia on the dorsal aspect of the opisthosoma. This is a consequence of flicking urticating hairs in excess, most likely caused by stress.
Efforts should be directed towards identifying and eliminating stimuli responsible for the animal's disturbance.
It is unclear whether mite infestations should be considered parasitic infections in theraphosid spiders.1 At the least, they are aesthetically unappealing. To eradicate them, close attention to cleanliness of enclosures, along with repeated removal of the mites from the spider with a damp swab or fine artist's brush, is needed. The use of predatory mites (Hypoaspis sp.) has also been described.1
A newer described disease is infection of theraphosid colonies with an oral nematode of the family Panagrolaimidae. Infections spread rapidly through colonies and do not respond to treatment, causing high morbidity and mortality. Quarantine and euthanasia of affected individuals are recommended to counter such infections.14,15 So far, this disease has not been reported outside of Europe.
Many clients do not realize that theraphosid spiders normally lie in dorsal recumbency during ecdysis, from which position they push the old exoskeleton off of themselves. Clients should be made aware that the animal is not dead, and should be left undisturbed due to the fragile nature of the new, unhardened exoskeleton. The typical death pose of most spiders is sternal recumbency, with all legs curled underneath the body and a somewhat shriveled overall appearance.
Human Health Hazards
Theraphosids have fangs commensurate with their size, up to one inch in length for the largest species. Thus, they are able to inflict significant mechanical damage and pain with a bite. Also of concern are secondary infections from such puncture wounds. Bacteria that have been isolated from the mouthparts of captive theraphosids include Staphylococcus, Bacillus, Pseudomonas, Aeromonas, Acinetobacter, Micrococcus, Coryneforms, and other Enterobacteriaceae species, some of which are known human pathogens.10
Spiders are able to voluntarily control the expulsion of venom during a bite. Dry bites likely do occur, though the frequency of this in defensive situations is not known. No confirmed case of death due to envenomation from a theraphosid has ever been recorded.7 In vertebrates, theraphosid venoms typically cause reactions similar to other common arthropod envenomations, such as wasp or bee stings. Though uncommon, more serious effects can occur, including edema, joint stiffness and swollen limbs, muscle cramps, and temporary paralysis.7
New World theraphosid species commonly have a special set of approximately 1–2 mm, barbed hairs for defense. These urticating hairs are most often found on the dorsal aspects of the opisthosoma, and can number more than one million immediately after a molt.2 A disturbed spider will use the fourth pair of legs to kick them off, making them airborne and thus able to settle on and imbed in sensitive tissues (i.e., skin, mucous membranes) of threatening animals. If the hairs embed in the cornea, it produces a painful, chronic condition known as ophthalmia nodosa.23
Urticating hairs are so named for the pruritic inflammatory reaction they cause, which can last up to two weeks and be particularly intense in sensitive individuals. The hairs also settle in the environment, so care should be taken whenever working in a New World theraphosid's enclosure.
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25. Zachariah TT, Mitchell MA, Langan JN, Acierno MJ. Hemolymph osmolality of the Chilean rose spider (Grammostola rosea). Proc Rept Amphib Vet 2010:117. (Abstr.)
26. Zachariah TT, Woodworth S, Clark-Price SC, McMichael M, Watson M, Mitchell MA. Sevoflurane anesthesia and hemolymph gas analysis of wild-caught Chilean rose spiders (Grammostola rosea). Proc Rept Amphib Vet. 2010:116. (Abstr.)